lib.rs

// This file is part of Substrate.

// Copyright (C) 2017-2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! # Staking Pallet
//!
//! The Staking pallet is used to manage funds at stake by network maintainers.
//!
//! - [`Config`]
//! - [`Call`]
//! - [`Pallet`]
//!
//! ## Overview
//!
//! The Staking pallet is the means by which a set of network maintainers (known as _authorities_ in
//! some contexts and _validators_ in others) are chosen based upon those who voluntarily place
//! funds under deposit. Under deposit, those funds are rewarded under normal operation but are held
//! at pain of _slash_ (expropriation) should the staked maintainer be found not to be discharging
//! its duties properly.
//!
//! ### Terminology
//! <!-- Original author of paragraph: @gavofyork -->
//!
//! - Staking: The process of locking up funds for some time, placing them at risk of slashing
//!   (loss) in order to become a rewarded maintainer of the network.
//! - Validating: The process of running a node to actively maintain the network, either by
//!   producing blocks or guaranteeing finality of the chain.
//! - Nominating: The process of placing staked funds behind one or more validators in order to
//!   share in any reward, and punishment, they take.
//! - Stash account: The account holding an owner's funds used for staking.
//! - Controller account: The account that controls an owner's funds for staking.
//! - Era: A (whole) number of sessions, which is the period that the validator set (and each
//!   validator's active nominator set) is recalculated and where rewards are paid out.
//! - Slash: The punishment of a staker by reducing its funds.
//!
//! ### Goals
//! <!-- Original author of paragraph: @gavofyork -->
//!
//! The staking system in Substrate NPoS is designed to make the following possible:
//!
//! - Stake funds that are controlled by a cold wallet.
//! - Withdraw some, or deposit more, funds without interrupting the role of an entity.
//! - Switch between roles (nominator, validator, idle) with minimal overhead.
//!
//! ### Scenarios
//!
//! #### Staking
//!
//! Almost any interaction with the Staking pallet requires a process of _**bonding**_ (also known
//! as being a _staker_). To become *bonded*, a fund-holding account known as the _stash account_,
//! which holds some or all of the funds that become frozen in place as part of the staking process,
//! is paired with an active **controller** account, which issues instructions on how they shall be
//! used.
//!
//! An account pair can become bonded using the [`bond`](Call::bond) call.
//!
//! Stash accounts can change their associated controller using the
//! [`set_controller`](Call::set_controller) call.
//!
//! There are three possible roles that any staked account pair can be in: `Validator`, `Nominator`
//! and `Idle` (defined in [`StakerStatus`]). There are three
//! corresponding instructions to change between roles, namely:
//! [`validate`](Call::validate),
//! [`nominate`](Call::nominate), and [`chill`](Call::chill).
//!
//! #### Validating
//!
//! A **validator** takes the role of either validating blocks or ensuring their finality,
//! maintaining the veracity of the network. A validator should avoid both any sort of malicious
//! misbehavior and going offline. Bonded accounts that state interest in being a validator do NOT
//! get immediately chosen as a validator. Instead, they are declared as a _candidate_ and they
//! _might_ get elected at the _next era_ as a validator. The result of the election is determined
//! by nominators and their votes.
//!
//! An account can become a validator candidate via the
//! [`validate`](Call::validate) call.
//!
//! #### Nomination
//!
//! A **nominator** does not take any _direct_ role in maintaining the network, instead, it votes on
//! a set of validators  to be elected. Once interest in nomination is stated by an account, it
//! takes effect at the next election round. The funds in the nominator's stash account indicate the
//! _weight_ of its vote. Both the rewards and any punishment that a validator earns are shared
//! between the validator and its nominators. This rule incentivizes the nominators to NOT vote for
//! the misbehaving/offline validators as much as possible, simply because the nominators will also
//! lose funds if they vote poorly.
//!
//! An account can become a nominator via the [`nominate`](Call::nominate) call.
//!
//! #### Voting
//!
//! Staking is closely related to elections; actual validators are chosen from among all potential
//! validators by election by the potential validators and nominators. To reduce use of the phrase
//! "potential validators and nominators", we often use the term **voters**, who are simply
//! the union of potential validators and nominators.
//!
//! #### Rewards and Slash
//!
//! The **reward and slashing** procedure is the core of the Staking pallet, attempting to _embrace
//! valid behavior_ while _punishing any misbehavior or lack of availability_.
//!
//! Rewards must be claimed for each era before it gets too old by `$HISTORY_DEPTH` using the
//! `payout_stakers` call. Any account can call `payout_stakers`, which pays the reward to the
//! validator as well as its nominators. Only the [`Config::MaxNominatorRewardedPerValidator`]
//! biggest stakers can claim their reward. This is to limit the i/o cost to mutate storage for each
//! nominator's account.
//!
//! Slashing can occur at any point in time, once misbehavior is reported. Once slashing is
//! determined, a value is deducted from the balance of the validator and all the nominators who
//! voted for this validator (values are deducted from the _stash_ account of the slashed entity).
//!
//! Slashing logic is further described in the documentation of the `slashing` pallet.
//!
//! Similar to slashing, rewards are also shared among a validator and its associated nominators.
//! Yet, the reward funds are not always transferred to the stash account and can be configured. See
//! [Reward Calculation](#reward-calculation) for more details.
//!
//! #### Chilling
//!
//! Finally, any of the roles above can choose to step back temporarily and just chill for a while.
//! This means that if they are a nominator, they will not be considered as voters anymore and if
//! they are validators, they will no longer be a candidate for the next election.
//!
//! An account can step back via the [`chill`](Call::chill) call.
//!
//! ### Session managing
//!
//! The pallet implement the trait `SessionManager`. Which is the only API to query new validator
//! set and allowing these validator set to be rewarded once their era is ended.
//!
//! ## Interface
//!
//! ### Dispatchable Functions
//!
//! The dispatchable functions of the Staking pallet enable the steps needed for entities to accept
//! and change their role, alongside some helper functions to get/set the metadata of the pallet.
//!
//! ### Public Functions
//!
//! The Staking pallet contains many public storage items and (im)mutable functions.
//!
//! ## Usage
//!
//! ### Example: Rewarding a validator by id.
//!
//! ```
//! use frame_support::{decl_module, dispatch};
//! use frame_system::ensure_signed;
//! use pallet_staking::{self as staking};
//!
//! pub trait Config: staking::Config {}
//!
//! decl_module! {
//!     pub struct Module<T: Config> for enum Call where origin: T::Origin {
//!         /// Reward a validator.
//!         #[weight = 0]
//!         pub fn reward_myself(origin) -> dispatch::DispatchResult {
//!             let reported = ensure_signed(origin)?;
//!             <staking::Pallet<T>>::reward_by_ids(vec![(reported, 10)]);
//!             Ok(())
//!         }
//!     }
//! }
//! # fn main() { }
//! ```
//!
//! ## Implementation Details
//!
//! ### Era payout
//!
//! The era payout is computed using yearly inflation curve defined at
//! [`Config::EraPayout`] as such:
//!
//! ```nocompile
//! staker_payout = yearly_inflation(npos_token_staked / total_tokens) * total_tokens / era_per_year
//! ```
//! This payout is used to reward stakers as defined in next section
//!
//! ```nocompile
//! remaining_payout = max_yearly_inflation * total_tokens / era_per_year - staker_payout
//! ```
//! The remaining reward is send to the configurable end-point
//! [`Config::RewardRemainder`].
//!
//! ### Reward Calculation
//!
//! Validators and nominators are rewarded at the end of each era. The total reward of an era is
//! calculated using the era duration and the staking rate (the total amount of tokens staked by
//! nominators and validators, divided by the total token supply). It aims to incentivize toward a
//! defined staking rate. The full specification can be found
//! [here](https://research.web3.foundation/en/latest/polkadot/Token%20Economics.html#inflation-model).
//!
//! Total reward is split among validators and their nominators depending on the number of points
//! they received during the era. Points are added to a validator using
//! [`reward_by_ids`](Pallet::reward_by_ids).
//!
//! [`Pallet`] implements
//! [`pallet_authorship::EventHandler`] to add reward
//! points to block producer and block producer of referenced uncles.
//!
//! The validator and its nominator split their reward as following:
//!
//! The validator can declare an amount, named
//! [`commission`](ValidatorPrefs::commission), that does not get shared
//! with the nominators at each reward payout through its
//! [`ValidatorPrefs`]. This value gets deducted from the total reward
//! that is paid to the validator and its nominators. The remaining portion is split among the
//! validator and all of the nominators that nominated the validator, proportional to the value
//! staked behind this validator (_i.e._ dividing the
//! [`own`](Exposure::own) or
//! [`others`](Exposure::others) by
//! [`total`](Exposure::total) in [`Exposure`]).
//!
//! All entities who receive a reward have the option to choose their reward destination through the
//! [`Payee`] storage item (see
//! [`set_payee`](Call::set_payee)), to be one of the following:
//!
//! - Controller account, (obviously) not increasing the staked value.
//! - Stash account, not increasing the staked value.
//! - Stash account, also increasing the staked value.
//!
//! ### Additional Fund Management Operations
//!
//! Any funds already placed into stash can be the target of the following operations:
//!
//! The controller account can free a portion (or all) of the funds using the
//! [`unbond`](Call::unbond) call. Note that the funds are not immediately
//! accessible. Instead, a duration denoted by
//! [`Config::BondingDuration`] (in number of eras) must
//! pass until the funds can actually be removed. Once the `BondingDuration` is over, the
//! [`withdraw_unbonded`](Call::withdraw_unbonded) call can be used to actually
//! withdraw the funds.
//!
//! Note that there is a limitation to the number of fund-chunks that can be scheduled to be
//! unlocked in the future via [`unbond`](Call::unbond). In case this maximum
//! (`MAX_UNLOCKING_CHUNKS`) is reached, the bonded account _must_ first wait until a successful
//! call to `withdraw_unbonded` to remove some of the chunks.
//!
//! ### Election Algorithm
//!
//! The current election algorithm is implemented based on Phragmén. The reference implementation
//! can be found [here](https://github.com/w3f/consensus/tree/master/NPoS).
//!
//! The election algorithm, aside from electing the validators with the most stake value and votes,
//! tries to divide the nominator votes among candidates in an equal manner. To further assure this,
//! an optional post-processing can be applied that iteratively normalizes the nominator staked
//! values until the total difference among votes of a particular nominator are less than a
//! threshold.
//!
//! ## GenesisConfig
//!
//! The Staking pallet depends on the [`GenesisConfig`]. The
//! `GenesisConfig` is optional and allow to set some initial stakers.
//!
//! ## Related Modules
//!
//! - [Balances](../pallet_balances/index.html): Used to manage values at stake.
//! - [Session](../pallet_session/index.html): Used to manage sessions. Also, a list of new
//!   validators is stored in the Session pallet's `Validators` at the end of each era.

#![recursion_limit = "128"]
#![cfg_attr(not(feature = "std"), no_std)]

#[cfg(any(test, feature = "make-bags"))]
pub mod mock;
#[cfg(test)]
mod tests;
#[cfg(any(feature = "runtime-benchmarks", test))]
pub mod testing_utils;
#[cfg(any(feature = "runtime-benchmarks", test))]
pub mod benchmarking;

pub mod inflation;
pub mod slashing;
pub mod voter_bags;
pub mod weights;

use sp_std::{
	result,
	prelude::*,
	collections::btree_map::BTreeMap,
	convert::From,
};
use codec::{HasCompact, Encode, Decode};
use frame_support::{
	pallet_prelude::*,
	weights::{
		Weight, WithPostDispatchInfo,
		constants::{WEIGHT_PER_MICROS, WEIGHT_PER_NANOS},
	},
	traits::{
		Currency, LockIdentifier, LockableCurrency, WithdrawReasons, OnUnbalanced, Imbalance, Get,
		UnixTime, EstimateNextNewSession, EnsureOrigin, CurrencyToVote,
	},
};
use pallet_session::historical;
use sp_runtime::{
	DispatchError, Perbill, Percent, RuntimeDebug,
	curve::PiecewiseLinear,
	traits::{
		AtLeast32BitUnsigned, Bounded, CheckedSub, Convert, SaturatedConversion, Saturating,
		StaticLookup, Zero,
	},
};
use sp_staking::{
	SessionIndex,
	offence::{OnOffenceHandler, OffenceDetails, Offence, ReportOffence, OffenceError},
};
use frame_system::{ensure_signed, ensure_root, pallet_prelude::*, offchain::SendTransactionTypes};
use frame_election_provider_support::{ElectionProvider, VoteWeight, Supports, data_provider};
use voter_bags::{VoterList, VoterType};
pub use weights::WeightInfo;
pub use pallet::*;

const STAKING_ID: LockIdentifier = *b"staking ";
pub(crate) const LOG_TARGET: &'static str = "runtime::staking";

// syntactic sugar for logging.
#[macro_export]
macro_rules! log {
	($level:tt, $patter:expr $(, $values:expr)* $(,)?) => {
		log::$level!(
			target: crate::LOG_TARGET,
			concat!("[{:?}] 💸 ", $patter), <frame_system::Pallet<T>>::block_number() $(, $values)*
		)
	};
}

pub const MAX_UNLOCKING_CHUNKS: usize = 32;

/// Counter for the number of eras that have passed.
pub type EraIndex = u32;

/// Counter for the number of "reward" points earned by a given validator.
pub type RewardPoint = u32;

/// The balance type of this pallet.
pub type BalanceOf<T> =
	<<T as Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;

type PositiveImbalanceOf<T> = <<T as Config>::Currency as Currency<
	<T as frame_system::Config>::AccountId,
>>::PositiveImbalance;
type NegativeImbalanceOf<T> = <<T as Config>::Currency as Currency<
	<T as frame_system::Config>::AccountId,
>>::NegativeImbalance;

type AccountIdOf<T> = <T as frame_system::Config>::AccountId;
type VotingDataOf<T> = (AccountIdOf<T>, VoteWeight, Vec<AccountIdOf<T>>);

/// Information regarding the active era (era in used in session).
#[derive(Encode, Decode, RuntimeDebug)]
pub struct ActiveEraInfo {
	/// Index of era.
	pub index: EraIndex,
	/// Moment of start expressed as millisecond from `$UNIX_EPOCH`.
	///
	/// Start can be none if start hasn't been set for the era yet,
	/// Start is set on the first on_finalize of the era to guarantee usage of `Time`.
	start: Option<u64>,
}

/// Reward points of an era. Used to split era total payout between validators.
///
/// This points will be used to reward validators and their respective nominators.
#[derive(PartialEq, Encode, Decode, Default, RuntimeDebug)]
pub struct EraRewardPoints<AccountId: Ord> {
	/// Total number of points. Equals the sum of reward points for each validator.
	total: RewardPoint,
	/// The reward points earned by a given validator.
	individual: BTreeMap<AccountId, RewardPoint>,
}

/// Indicates the initial status of the staker.
#[derive(RuntimeDebug)]
#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
pub enum StakerStatus<AccountId> {
	/// Chilling.
	Idle,
	/// Declared desire in validating or already participating in it.
	Validator,
	/// Nominating for a group of other stakers.
	Nominator(Vec<AccountId>),
}

/// A destination account for payment.
#[derive(PartialEq, Eq, Copy, Clone, Encode, Decode, RuntimeDebug)]
pub enum RewardDestination<AccountId> {
	/// Pay into the stash account, increasing the amount at stake accordingly.
	Staked,
	/// Pay into the stash account, not increasing the amount at stake.
	Stash,
	/// Pay into the controller account.
	Controller,
	/// Pay into a specified account.
	Account(AccountId),
	/// Receive no reward.
	None,
}

impl<AccountId> Default for RewardDestination<AccountId> {
	fn default() -> Self {
		RewardDestination::Staked
	}
}

/// Preference of what happens regarding validation.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct ValidatorPrefs {
	/// Reward that validator takes up-front; only the rest is split between themselves and
	/// nominators.
	#[codec(compact)]
	pub commission: Perbill,
	/// Whether or not this validator is accepting more nominations. If `true`, then no nominator
	/// who is not already nominating this validator may nominate them. By default, validators
	/// are accepting nominations.
	pub blocked: bool,
}

impl Default for ValidatorPrefs {
	fn default() -> Self {
		ValidatorPrefs {
			commission: Default::default(),
			blocked: false,
		}
	}
}

/// Just a Balance/BlockNumber tuple to encode when a chunk of funds will be unlocked.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct UnlockChunk<Balance: HasCompact> {
	/// Amount of funds to be unlocked.
	#[codec(compact)]
	value: Balance,
	/// Era number at which point it'll be unlocked.
	#[codec(compact)]
	era: EraIndex,
}

/// The ledger of a (bonded) stash.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct StakingLedger<AccountId, Balance: HasCompact> {
	/// The stash account whose balance is actually locked and at stake.
	pub stash: AccountId,
	/// The total amount of the stash's balance that we are currently accounting for.
	/// It's just `active` plus all the `unlocking` balances.
	#[codec(compact)]
	pub total: Balance,
	/// The total amount of the stash's balance that will be at stake in any forthcoming
	/// rounds.
	#[codec(compact)]
	pub active: Balance,
	/// Any balance that is becoming free, which may eventually be transferred out
	/// of the stash (assuming it doesn't get slashed first).
	pub unlocking: Vec<UnlockChunk<Balance>>,
	/// List of eras for which the stakers behind a validator have claimed rewards. Only updated
	/// for validators.
	pub claimed_rewards: Vec<EraIndex>,
}

impl<
	AccountId,
	Balance: HasCompact + Copy + Saturating + AtLeast32BitUnsigned,
> StakingLedger<AccountId, Balance> {
	/// Remove entries from `unlocking` that are sufficiently old and reduce the
	/// total by the sum of their balances.
	fn consolidate_unlocked(self, current_era: EraIndex) -> Self {
		let mut total = self.total;
		let unlocking = self.unlocking.into_iter()
			.filter(|chunk| if chunk.era > current_era {
				true
			} else {
				total = total.saturating_sub(chunk.value);
				false
			})
			.collect();

		Self {
			stash: self.stash,
			total,
			active: self.active,
			unlocking,
			claimed_rewards: self.claimed_rewards
		}
	}

	/// Re-bond funds that were scheduled for unlocking.
	fn rebond(mut self, value: Balance) -> Self {
		let mut unlocking_balance: Balance = Zero::zero();

		while let Some(last) = self.unlocking.last_mut() {
			if unlocking_balance + last.value <= value {
				unlocking_balance += last.value;
				self.active += last.value;
				self.unlocking.pop();
			} else {
				let diff = value - unlocking_balance;

				unlocking_balance += diff;
				self.active += diff;
				last.value -= diff;
			}

			if unlocking_balance >= value {
				break
			}
		}

		self
	}
}

impl<AccountId, Balance> StakingLedger<AccountId, Balance> where
	Balance: AtLeast32BitUnsigned + Saturating + Copy,
{
	/// Slash the validator for a given amount of balance. This can grow the value
	/// of the slash in the case that the validator has less than `minimum_balance`
	/// active funds. Returns the amount of funds actually slashed.
	///
	/// Slashes from `active` funds first, and then `unlocking`, starting with the
	/// chunks that are closest to unlocking.
	fn slash(
		&mut self,
		mut value: Balance,
		minimum_balance: Balance,
	) -> Balance {
		let pre_total = self.total;
		let total = &mut self.total;
		let active = &mut self.active;

		let slash_out_of = |
			total_remaining: &mut Balance,
			target: &mut Balance,
			value: &mut Balance,
		| {
			let mut slash_from_target = (*value).min(*target);

			if !slash_from_target.is_zero() {
				*target -= slash_from_target;

				// Don't leave a dust balance in the staking system.
				if *target <= minimum_balance {
					slash_from_target += *target;
					*value += sp_std::mem::replace(target, Zero::zero());
				}

				*total_remaining = total_remaining.saturating_sub(slash_from_target);
				*value -= slash_from_target;
			}
		};

		slash_out_of(total, active, &mut value);

		let i = self.unlocking.iter_mut()
			.map(|chunk| {
				slash_out_of(total, &mut chunk.value, &mut value);
				chunk.value
			})
			.take_while(|value| value.is_zero()) // Take all fully-consumed chunks out.
			.count();

		// Kill all drained chunks.
		let _ = self.unlocking.drain(..i);

		pre_total.saturating_sub(*total)
	}
}

/// A record of the nominations made by a specific account.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct Nominations<AccountId> {
	/// The targets of nomination.
	pub targets: Vec<AccountId>,
	/// The era the nominations were submitted.
	///
	/// Except for initial nominations which are considered submitted at era 0.
	pub submitted_in: EraIndex,
	/// Whether the nominations have been suppressed. This can happen due to slashing of the
	/// validators, or other events that might invalidate the nomination.
	///
	/// NOTE: this for future proofing and is thus far not used.
	pub suppressed: bool,
}

/// The amount of exposure (to slashing) than an individual nominator has.
#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Encode, Decode, RuntimeDebug)]
pub struct IndividualExposure<AccountId, Balance: HasCompact> {
	/// The stash account of the nominator in question.
	pub who: AccountId,
	/// Amount of funds exposed.
	#[codec(compact)]
	pub value: Balance,
}

/// A snapshot of the stake backing a single validator in the system.
#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Encode, Decode, Default, RuntimeDebug)]
pub struct Exposure<AccountId, Balance: HasCompact> {
	/// The total balance backing this validator.
	#[codec(compact)]
	pub total: Balance,
	/// The validator's own stash that is exposed.
	#[codec(compact)]
	pub own: Balance,
	/// The portions of nominators stashes that are exposed.
	pub others: Vec<IndividualExposure<AccountId, Balance>>,
}

/// A pending slash record. The value of the slash has been computed but not applied yet,
/// rather deferred for several eras.
#[derive(Encode, Decode, Default, RuntimeDebug)]
pub struct UnappliedSlash<AccountId, Balance: HasCompact> {
	/// The stash ID of the offending validator.
	validator: AccountId,
	/// The validator's own slash.
	own: Balance,
	/// All other slashed stakers and amounts.
	others: Vec<(AccountId, Balance)>,
	/// Reporters of the offence; bounty payout recipients.
	reporters: Vec<AccountId>,
	/// The amount of payout.
	payout: Balance,
}

/// Means for interacting with a specialized version of the `session` trait.
///
/// This is needed because `Staking` sets the `ValidatorIdOf` of the `pallet_session::Config`
pub trait SessionInterface<AccountId>: frame_system::Config {
	/// Disable a given validator by stash ID.
	///
	/// Returns `true` if new era should be forced at the end of this session.
	/// This allows preventing a situation where there is too many validators
	/// disabled and block production stalls.
	fn disable_validator(validator: &AccountId) -> Result<bool, ()>;
	/// Get the validators from session.
	fn validators() -> Vec<AccountId>;
	/// Prune historical session tries up to but not including the given index.
	fn prune_historical_up_to(up_to: SessionIndex);
}

impl<T: Config> SessionInterface<<T as frame_system::Config>::AccountId> for T where
	T: pallet_session::Config<ValidatorId = <T as frame_system::Config>::AccountId>,
	T: pallet_session::historical::Config<
		FullIdentification = Exposure<<T as frame_system::Config>::AccountId, BalanceOf<T>>,
		FullIdentificationOf = ExposureOf<T>,
	>,
	T::SessionHandler: pallet_session::SessionHandler<<T as frame_system::Config>::AccountId>,
	T::SessionManager: pallet_session::SessionManager<<T as frame_system::Config>::AccountId>,
	T::ValidatorIdOf:
		Convert<<T as frame_system::Config>::AccountId, Option<<T as frame_system::Config>::AccountId>>,
{
	fn disable_validator(validator: &<T as frame_system::Config>::AccountId) -> Result<bool, ()> {
		<pallet_session::Pallet<T>>::disable(validator)
	}

	fn validators() -> Vec<<T as frame_system::Config>::AccountId> {
		<pallet_session::Pallet<T>>::validators()
	}

	fn prune_historical_up_to(up_to: SessionIndex) {
		<pallet_session::historical::Pallet<T>>::prune_up_to(up_to);
	}
}

/// Handler for determining how much of a balance should be paid out on the current era.
pub trait EraPayout<Balance> {
	/// Determine the payout for this era.
	///
	/// Returns the amount to be paid to stakers in this era, as well as whatever else should be
	/// paid out ("the rest").
	fn era_payout(
		total_staked: Balance,
		total_issuance: Balance,
		era_duration_millis: u64,
	) -> (Balance, Balance);
}

impl<Balance: Default> EraPayout<Balance> for () {
	fn era_payout(
		_total_staked: Balance,
		_total_issuance: Balance,
		_era_duration_millis: u64,
	) -> (Balance, Balance) {
		(Default::default(), Default::default())
	}
}

/// Adaptor to turn a `PiecewiseLinear` curve definition into an `EraPayout` impl, used for
/// backwards compatibility.
pub struct ConvertCurve<T>(sp_std::marker::PhantomData<T>);
impl<
	Balance: AtLeast32BitUnsigned + Clone,
	T: Get<&'static PiecewiseLinear<'static>>,
> EraPayout<Balance> for ConvertCurve<T> {
	fn era_payout(
		total_staked: Balance,
		total_issuance: Balance,
		era_duration_millis: u64,
	) -> (Balance, Balance) {
		let (validator_payout, max_payout) = inflation::compute_total_payout(
			&T::get(),
			total_staked,
			total_issuance,
			// Duration of era; more than u64::MAX is rewarded as u64::MAX.
			era_duration_millis,
		);
		let rest = max_payout.saturating_sub(validator_payout.clone());
		(validator_payout, rest)
	}
}

/// Mode of era-forcing.
#[derive(Copy, Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug)]
#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
pub enum Forcing {
	/// Not forcing anything - just let whatever happen.
	NotForcing,
	/// Force a new era, then reset to `NotForcing` as soon as it is done.
	/// Note that this will force to trigger an election until a new era is triggered, if the
	/// election failed, the next session end will trigger a new election again, until success.
	ForceNew,
	/// Avoid a new era indefinitely.
	ForceNone,
	/// Force a new era at the end of all sessions indefinitely.
	ForceAlways,
}

impl Default for Forcing {
	fn default() -> Self {
		Forcing::NotForcing
	}
}

// A value placed in storage that represents the current version of the Staking storage. This value
// is used by the `on_runtime_upgrade` logic to determine whether we run storage migration logic.
// This should match directly with the semantic versions of the Rust crate.
#[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug)]
enum Releases {
	V1_0_0Ancient,
	V2_0_0,
	V3_0_0,
	V4_0_0,
	V5_0_0, // blockable validators.
	V6_0_0, // removal of all storage associated with offchain phragmen.
	V7_0_0, // keep track of number of nominators / validators in map
	V8_0_0, // VoterList and efficient semi-sorted iteration
}

impl Default for Releases {
	fn default() -> Self {
		Releases::V8_0_0
	}
}

pub mod migrations {
	use super::*;

	pub mod v8 {
		use super::*;

		pub fn pre_migrate<T: Config>() -> Result<(), &'static str> {
			ensure!(StorageVersion::<T>::get() == Releases::V7_0_0, "must upgrade linearly");
			ensure!(VoterList::<T>::decode_len().unwrap_or_default() == 0, "voter list already exists");
			Ok(())
		}

		pub fn migrate<T: Config>() -> Weight {
			log!(info, "Migrating staking to Releases::V8_0_0");

			let migrated = VoterList::<T>::regenerate();

			StorageVersion::<T>::put(Releases::V8_0_0);
			log!(
				info,
				"Completed staking migration to Releases::V8_0_0 with {} voters migrated",
				migrated,
			);

			T::WeightInfo::regenerate(
				CounterForValidators::<T>::get(),
				CounterForNominators::<T>::get(),
			).saturating_add(T::DbWeight::get().reads(2))
		}
	}

	pub mod v7 {
		use super::*;

		pub fn pre_migrate<T: Config>() -> Result<(), &'static str> {
			assert!(CounterForValidators::<T>::get().is_zero(), "CounterForValidators already set.");
			assert!(CounterForNominators::<T>::get().is_zero(), "CounterForNominators already set.");
			assert!(StorageVersion::<T>::get() == Releases::V6_0_0);
			Ok(())
		}

		pub fn migrate<T: Config>() -> Weight {
			log!(info, "Migrating staking to Releases::V7_0_0");
			let validator_count = Validators::<T>::iter().count() as u32;
			let nominator_count = Nominators::<T>::iter().count() as u32;

			CounterForValidators::<T>::put(validator_count);
			CounterForNominators::<T>::put(nominator_count);

			StorageVersion::<T>::put(Releases::V7_0_0);
			log!(info, "Completed staking migration to Releases::V7_0_0");

			T::DbWeight::get().reads_writes(
				validator_count.saturating_add(nominator_count).into(),
				2,
			)
		}
	}

	pub mod v6 {
		use super::*;
		use frame_support::{traits::Get, weights::Weight, generate_storage_alias};

		// NOTE: value type doesn't matter, we just set it to () here.
		generate_storage_alias!(Staking, SnapshotValidators => Value<()>);
		generate_storage_alias!(Staking, SnapshotNominators => Value<()>);
		generate_storage_alias!(Staking, QueuedElected => Value<()>);
		generate_storage_alias!(Staking, QueuedScore => Value<()>);
		generate_storage_alias!(Staking, EraElectionStatus => Value<()>);
		generate_storage_alias!(Staking, IsCurrentSessionFinal => Value<()>);

		/// check to execute prior to migration.
		pub fn pre_migrate<T: Config>() -> Result<(), &'static str> {
			// these may or may not exist.
			log!(info, "SnapshotValidators.exits()? {:?}", SnapshotValidators::exists());
			log!(info, "SnapshotNominators.exits()? {:?}", SnapshotNominators::exists());
			log!(info, "QueuedElected.exits()? {:?}", QueuedElected::exists());
			log!(info, "QueuedScore.exits()? {:?}", QueuedScore::exists());
			// these must exist.
			assert!(IsCurrentSessionFinal::exists(), "IsCurrentSessionFinal storage item not found!");
			assert!(EraElectionStatus::exists(), "EraElectionStatus storage item not found!");
			Ok(())
		}

		/// Migrate storage to v6.
		pub fn migrate<T: Config>() -> Weight {
			log!(info, "Migrating staking to Releases::V6_0_0");

			SnapshotValidators::kill();
			SnapshotNominators::kill();
			QueuedElected::kill();
			QueuedScore::kill();
			EraElectionStatus::kill();
			IsCurrentSessionFinal::kill();

			StorageVersion::<T>::put(Releases::V6_0_0);
			log!(info, "Done.");
			T::DbWeight::get().writes(6 + 1)
		}
	}
}

#[frame_support::pallet]
pub mod pallet {
	use super::*;

	#[pallet::pallet]
	#[pallet::generate_store(pub(super) trait Store)]
	pub struct Pallet<T>(_);

	#[pallet::config]
	pub trait Config: frame_system::Config + SendTransactionTypes<Call<Self>> {
		/// The staking balance.
		type Currency: LockableCurrency<Self::AccountId, Moment = Self::BlockNumber>;

		/// Time used for computing era duration.
		///
		/// It is guaranteed to start being called from the first `on_finalize`. Thus value at genesis
		/// is not used.
		type UnixTime: UnixTime;

		/// Convert a balance into a number used for election calculation. This must fit into a `u64`
		/// but is allowed to be sensibly lossy. The `u64` is used to communicate with the
		/// [`sp_npos_elections`] crate which accepts u64 numbers and does operations in 128.
		/// Consequently, the backward convert is used convert the u128s from sp-elections back to a
		/// [`BalanceOf`].
		type CurrencyToVote: CurrencyToVote<BalanceOf<Self>>;

		/// Something that provides the election functionality.
		type ElectionProvider: frame_election_provider_support::ElectionProvider<
			Self::AccountId,
			Self::BlockNumber,
			// we only accept an election provider that has staking as data provider.
			DataProvider = Pallet<Self>,
		>;

		/// Something that provides the election functionality at genesis.
		type GenesisElectionProvider: frame_election_provider_support::ElectionProvider<
			Self::AccountId,
			Self::BlockNumber,
			DataProvider = Pallet<Self>,
		>;

		/// Maximum number of nominations per nominator.
		const MAX_NOMINATIONS: u32;

		/// Tokens have been minted and are unused for validator-reward.
		/// See [Era payout](./index.html#era-payout).
		type RewardRemainder: OnUnbalanced<NegativeImbalanceOf<Self>>;

		/// The overarching event type.
		type Event: From<Event<Self>> + IsType<<Self as frame_system::Config>::Event>;

		/// Handler for the unbalanced reduction when slashing a staker.
		type Slash: OnUnbalanced<NegativeImbalanceOf<Self>>;

		/// Handler for the unbalanced increment when rewarding a staker.
		type Reward: OnUnbalanced<PositiveImbalanceOf<Self>>;

		/// Number of sessions per era.
		#[pallet::constant]
		type SessionsPerEra: Get<SessionIndex>;

		/// Number of eras that staked funds must remain bonded for.
		#[pallet::constant]
		type BondingDuration: Get<EraIndex>;

		/// Number of eras that slashes are deferred by, after computation.
		///
		/// This should be less than the bonding duration. Set to 0 if slashes
		/// should be applied immediately, without opportunity for intervention.
		#[pallet::constant]
		type SlashDeferDuration: Get<EraIndex>;

		/// The origin which can cancel a deferred slash. Root can always do this.
		type SlashCancelOrigin: EnsureOrigin<Self::Origin>;

		/// Interface for interacting with a session pallet.
		type SessionInterface: self::SessionInterface<Self::AccountId>;

		/// The payout for validators and the system for the current era.
		/// See [Era payout](./index.html#era-payout).
		type EraPayout: EraPayout<BalanceOf<Self>>;

		/// Something that can estimate the next session change, accurately or as a best effort guess.
		type NextNewSession: EstimateNextNewSession<Self::BlockNumber>;

		/// The maximum number of nominators rewarded for each validator.
		///
		/// For each validator only the `$MaxNominatorRewardedPerValidator` biggest stakers can claim
		/// their reward. This used to limit the i/o cost for the nominator payout.
		#[pallet::constant]
		type MaxNominatorRewardedPerValidator: Get<u32>;

		/// Weight information for extrinsics in this pallet.
		type WeightInfo: WeightInfo;

		/// The list of thresholds separating the various voter bags.
		///
		/// Voters are separated into unsorted bags according to their vote weight. This specifies
		/// the thresholds separating the bags. A voter's bag is the largest bag for which the
		/// voter's weight is less than or equal to its upper threshold.
		///
		/// When voters are iterated, higher bags are iterated completely before lower bags. This
		/// means that iteration is _semi-sorted_: voters of higher weight tend to come before
		/// voters of lower weight, but peer voters within a particular bag are sorted in insertion
		/// order.
		///
		/// # Expressing the constant
		///
		/// This constant must be sorted in strictly increasing order. Duplicate items are not
		/// permitted.
		///
		/// There is an implied upper limit of `VoteWeight::MAX`; that value does not need to be
		/// specified within the bag. For any two threshold lists, if one ends with
		/// `VoteWeight::MAX`, the other one does not, and they are otherwise equal, the two lists
		/// will behave identically.
		///
		/// # Calculation
		///
		/// It is recommended to generate the set of thresholds in a geometric series, such that
		/// there exists some constant ratio such that `threshold[k + 1] == (threshold[k] *
		/// constant_ratio).max(threshold[k] + 1)` for all `k`.
		///
		/// The helpers in the `voter_bags::make_bags` module can simplify this calculation. To use
		/// them, the `make-bags` feature must be enabled.
		///
		/// # Examples
		///
		/// - If `VoterBagThresholds::get().is_empty()`, then all voters are put into the same bag,
		///   and iteration is strictly in insertion order.
		/// - If `VoterBagThresholds::get().len() == 64`, and the thresholds are determined
		///   according to the procedure given above, then the constant ratio is equal to 2.
		/// - If `VoterBagThresholds::get().len() == 200`, and the thresholds are determined
		///   according to the procedure given above, then the constant ratio is approximately equal
		///   to 1.248.
		/// - If the threshold list begins `[1, 2, 3, ...]`, then a voter with weight 0 or 1 will
		///   fall into bag 0, a voter with weight 2 will fall into bag 1, etc.
		///
		/// # Migration
		///
		/// In the event that this list ever changes, a copy of the old bags list must be retained.
		/// With that `VoterList::migrate` can be called, which will perform the appropriate
		/// migration.
		#[pallet::constant]
		type VoterBagThresholds: Get<&'static [VoteWeight]>;
	}

	#[pallet::extra_constants]
	impl<T: Config> Pallet<T> {
		//TODO: rename to snake case after https://github.com/paritytech/substrate/issues/8826 fixed.
		#[allow(non_snake_case)]
		fn MaxNominations() -> u32 {
			T::MAX_NOMINATIONS
		}
	}

	#[pallet::type_value]
	pub(crate) fn HistoryDepthOnEmpty() -> u32 { 84u32 }

	/// Number of eras to keep in history.
	///
	/// Information is kept for eras in `[current_era - history_depth; current_era]`.
	///
	/// Must be more than the number of eras delayed by session otherwise. I.e. active era must
	/// always be in history. I.e. `active_era > current_era - history_depth` must be
	/// guaranteed.
	#[pallet::storage]
	#[pallet::getter(fn history_depth)]
	pub(crate) type HistoryDepth<T> = StorageValue<_, u32, ValueQuery, HistoryDepthOnEmpty>;

	/// The ideal number of staking participants.
	#[pallet::storage]
	#[pallet::getter(fn validator_count)]
	pub type ValidatorCount<T> = StorageValue<_, u32, ValueQuery>;

	/// Minimum number of staking participants before emergency conditions are imposed.
	#[pallet::storage]
	#[pallet::getter(fn minimum_validator_count)]
	pub type MinimumValidatorCount<T> = StorageValue<_, u32, ValueQuery>;

	/// Any validators that may never be slashed or forcibly kicked. It's a Vec since they're
	/// easy to initialize and the performance hit is minimal (we expect no more than four
	/// invulnerables) and restricted to testnets.
	#[pallet::storage]
	#[pallet::getter(fn invulnerables)]
	pub type Invulnerables<T: Config> = StorageValue<_, Vec<T::AccountId>, ValueQuery>;

	/// Map from all locked "stash" accounts to the controller account.
	#[pallet::storage]
	#[pallet::getter(fn bonded)]
	pub type Bonded<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, T::AccountId>;

	/// The minimum active bond to become and maintain the role of a nominator.
	#[pallet::storage]
	pub type MinNominatorBond<T: Config> = StorageValue<_, BalanceOf<T>, ValueQuery>;

	/// The minimum active bond to become and maintain the role of a validator.
	#[pallet::storage]
	pub type MinValidatorBond<T: Config> = StorageValue<_, BalanceOf<T>, ValueQuery>;

	/// Map from all (unlocked) "controller" accounts to the info regarding the staking.
	#[pallet::storage]
	#[pallet::getter(fn ledger)]
	pub type Ledger<T: Config> = StorageMap<
		_,
		Blake2_128Concat, T::AccountId,
		StakingLedger<T::AccountId, BalanceOf<T>>,
	>;

	/// Where the reward payment should be made. Keyed by stash.
	#[pallet::storage]
	#[pallet::getter(fn payee)]
	pub type Payee<T: Config> = StorageMap<
		_,
		Twox64Concat, T::AccountId,
		RewardDestination<T::AccountId>,
		ValueQuery,
	>;

	/// The map from (wannabe) validator stash key to the preferences of that validator.
	///
	/// When updating this storage item, you must also update the `CounterForValidators`.
	#[pallet::storage]
	#[pallet::getter(fn validators)]
	pub type Validators<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, ValidatorPrefs, ValueQuery>;

	/// A tracker to keep count of the number of items in the `Validators` map.
	#[pallet::storage]
	pub type CounterForValidators<T> = StorageValue<_, u32, ValueQuery>;

	/// The maximum validator count before we stop allowing new validators to join.
	///
	/// When this value is not set, no limits are enforced.
	#[pallet::storage]
	pub type MaxValidatorsCount<T> = StorageValue<_, u32, OptionQuery>;

	/// The map from nominator stash key to the set of stash keys of all validators to nominate.
	///
	/// When updating this storage item, you must also update the `CounterForNominators`.
	#[pallet::storage]
	#[pallet::getter(fn nominators)]
	pub type Nominators<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, Nominations<T::AccountId>>;

	/// A tracker to keep count of the number of items in the `Nominators` map.
	#[pallet::storage]
	pub type CounterForNominators<T> = StorageValue<_, u32, ValueQuery>;

	/// The maximum nominator count before we stop allowing new validators to join.
	///
	/// When this value is not set, no limits are enforced.
	#[pallet::storage]
	pub type MaxNominatorsCount<T> = StorageValue<_, u32, OptionQuery>;

	/// The current era index.
	///
	/// This is the latest planned era, depending on how the Session pallet queues the validator
	/// set, it might be active or not.
	#[pallet::storage]
	#[pallet::getter(fn current_era)]
	pub type CurrentEra<T> = StorageValue<_, EraIndex>;

	/// The active era information, it holds index and start.
	///
	/// The active era is the era being currently rewarded. Validator set of this era must be
	/// equal to [`SessionInterface::validators`].
	#[pallet::storage]
	#[pallet::getter(fn active_era)]
	pub type ActiveEra<T> = StorageValue<_, ActiveEraInfo>;

	/// The session index at which the era start for the last `HISTORY_DEPTH` eras.
	///
	/// Note: This tracks the starting session (i.e. session index when era start being active)
	/// for the eras in `[CurrentEra - HISTORY_DEPTH, CurrentEra]`.
	#[pallet::storage]
	#[pallet::getter(fn eras_start_session_index)]
	pub type ErasStartSessionIndex<T> = StorageMap<_, Twox64Concat, EraIndex, SessionIndex>;

	/// Exposure of validator at era.
	///
	/// This is keyed first by the era index to allow bulk deletion and then the stash account.
	///
	/// Is it removed after `HISTORY_DEPTH` eras.
	/// If stakers hasn't been set or has been removed then empty exposure is returned.
	#[pallet::storage]
	#[pallet::getter(fn eras_stakers)]
	pub type ErasStakers<T: Config> = StorageDoubleMap<
		_,
		Twox64Concat, EraIndex,
		Twox64Concat, T::AccountId,
		Exposure<T::AccountId, BalanceOf<T>>,
		ValueQuery,
	>;

	/// Clipped Exposure of validator at era.
	///
	/// This is similar to [`ErasStakers`] but number of nominators exposed is reduced to the
	/// `T::MaxNominatorRewardedPerValidator` biggest stakers.
	/// (Note: the field `total` and `own` of the exposure remains unchanged).
	/// This is used to limit the i/o cost for the nominator payout.
	///
	/// This is keyed fist by the era index to allow bulk deletion and then the stash account.
	///
	/// Is it removed after `HISTORY_DEPTH` eras.
	/// If stakers hasn't been set or has been removed then empty exposure is returned.
	#[pallet::storage]
	#[pallet::getter(fn eras_stakers_clipped)]
	pub type ErasStakersClipped<T: Config> = StorageDoubleMap<
		_,
		Twox64Concat, EraIndex,
		Twox64Concat, T::AccountId,
		Exposure<T::AccountId, BalanceOf<T>>,
		ValueQuery,
	>;

	/// Similar to `ErasStakers`, this holds the preferences of validators.
	///
	/// This is keyed first by the era index to allow bulk deletion and then the stash account.
	///
	/// Is it removed after `HISTORY_DEPTH` eras.
	// If prefs hasn't been set or has been removed then 0 commission is returned.
	#[pallet::storage]
	#[pallet::getter(fn eras_validator_prefs)]
	pub type ErasValidatorPrefs<T: Config> = StorageDoubleMap<
		_,
		Twox64Concat, EraIndex,
		Twox64Concat, T::AccountId,
		ValidatorPrefs,
		ValueQuery,
	>;

	/// The total validator era payout for the last `HISTORY_DEPTH` eras.
	///
	/// Eras that haven't finished yet or has been removed doesn't have reward.
	#[pallet::storage]
	#[pallet::getter(fn eras_validator_reward)]
	pub type ErasValidatorReward<T: Config> = StorageMap<_, Twox64Concat, EraIndex, BalanceOf<T>>;

	/// Rewards for the last `HISTORY_DEPTH` eras.
	/// If reward hasn't been set or has been removed then 0 reward is returned.
	#[pallet::storage]
	#[pallet::getter(fn eras_reward_points)]
	pub type ErasRewardPoints<T: Config> = StorageMap<
		_,
		Twox64Concat, EraIndex,
		EraRewardPoints<T::AccountId>,
		ValueQuery,
	>;

	/// The total amount staked for the last `HISTORY_DEPTH` eras.
	/// If total hasn't been set or has been removed then 0 stake is returned.
	#[pallet::storage]
	#[pallet::getter(fn eras_total_stake)]
	pub type ErasTotalStake<T: Config> = StorageMap<_, Twox64Concat, EraIndex, BalanceOf<T>, ValueQuery>;

	/// Mode of era forcing.
	#[pallet::storage]
	#[pallet::getter(fn force_era)]
	pub type ForceEra<T> = StorageValue<_, Forcing, ValueQuery>;

	/// The percentage of the slash that is distributed to reporters.
	///
	/// The rest of the slashed value is handled by the `Slash`.
	#[pallet::storage]
	#[pallet::getter(fn slash_reward_fraction)]
	pub type SlashRewardFraction<T> = StorageValue<_, Perbill, ValueQuery>;

	/// The amount of currency given to reporters of a slash event which was
	/// canceled by extraordinary circumstances (e.g. governance).
	#[pallet::storage]
	#[pallet::getter(fn canceled_payout)]
	pub type CanceledSlashPayout<T: Config> = StorageValue<_, BalanceOf<T>, ValueQuery>;

	/// All unapplied slashes that are queued for later.
	#[pallet::storage]
	pub type UnappliedSlashes<T: Config> = StorageMap<
		_,
		Twox64Concat, EraIndex,
		Vec<UnappliedSlash<T::AccountId, BalanceOf<T>>>,
		ValueQuery,
	>;

	/// A mapping from still-bonded eras to the first session index of that era.
	///
	/// Must contains information for eras for the range:
	/// `[active_era - bounding_duration; active_era]`
	#[pallet::storage]
	pub(crate) type BondedEras<T: Config> = StorageValue<_, Vec<(EraIndex, SessionIndex)>, ValueQuery>;

	/// All slashing events on validators, mapped by era to the highest slash proportion
	/// and slash value of the era.
	#[pallet::storage]
	pub(crate) type ValidatorSlashInEra<T: Config> = StorageDoubleMap<
		_,
		Twox64Concat, EraIndex,
		Twox64Concat, T::AccountId,
		(Perbill, BalanceOf<T>),
	>;

	/// All slashing events on nominators, mapped by era to the highest slash value of the era.
	#[pallet::storage]
	pub(crate) type NominatorSlashInEra<T: Config> = StorageDoubleMap<
		_,
		Twox64Concat, EraIndex,
		Twox64Concat, T::AccountId,
		BalanceOf<T>,
	>;

	/// Slashing spans for stash accounts.
	#[pallet::storage]
	pub(crate) type SlashingSpans<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, slashing::SlashingSpans>;

	/// Records information about the maximum slash of a stash within a slashing span,
	/// as well as how much reward has been paid out.
	#[pallet::storage]
	pub(crate) type SpanSlash<T: Config> = StorageMap<
		_,
		Twox64Concat, (T::AccountId, slashing::SpanIndex),
		slashing::SpanRecord<BalanceOf<T>>,
		ValueQuery,
	>;

	/// The earliest era for which we have a pending, unapplied slash.
	#[pallet::storage]
	pub(crate) type EarliestUnappliedSlash<T> = StorageValue<_, EraIndex>;

	/// The last planned session scheduled by the session pallet.
	///
	/// This is basically in sync with the call to [`SessionManager::new_session`].
	#[pallet::storage]
	#[pallet::getter(fn current_planned_session)]
	pub type CurrentPlannedSession<T> = StorageValue<_, SessionIndex, ValueQuery>;

	/// True if network has been upgraded to this version.
	/// Storage version of the pallet.
	///
	/// This is set to v6.0.0 for new networks.
	#[pallet::storage]
	pub(crate) type StorageVersion<T: Config> = StorageValue<_, Releases, ValueQuery>;


	// The next storage items collectively comprise the voter bags: a composite data structure
	// designed to allow efficient iteration of the top N voters by stake, mostly. See
	// `mod voter_bags` for details.
	//
	// In each of these items, voter bags are indexed by their upper weight threshold.

	/// How many voters are registered.
	#[pallet::storage]
	pub(crate) type VoterCount<T> = StorageValue<_, u32, ValueQuery>;

	/// Which bag currently contains a particular voter.
	///
	/// This may not be the appropriate bag for the voter's weight if they have been rewarded or
	/// slashed.
	#[pallet::storage]
	pub(crate) type VoterBagFor<T: Config> = StorageMap<_, Twox64Concat, AccountIdOf<T>, VoteWeight>;

	/// This storage item maps a bag (identified by its upper threshold) to the `Bag` struct, which
	/// mainly exists to store head and tail pointers to the appropriate nodes.
	#[pallet::storage]
	pub(crate) type VoterBags<T: Config> = StorageMap<
		_,
		Twox64Concat, VoteWeight,
		voter_bags::Bag<T>,
	>;

	/// Voter nodes store links forward and back within their respective bags, the stash id, and
	/// whether the voter is a validator or nominator.
	///
	/// There is nothing in this map directly identifying to which bag a particular node belongs.
	/// However, the `Node` data structure has helpers which can provide that information.
	#[pallet::storage]
	pub(crate) type VoterNodes<T: Config> = StorageMap<
		_,
		Twox64Concat, AccountIdOf<T>,
		voter_bags::Node<T>,
	>;

	// End of voter bags data.

	/// The threshold for when users can start calling `chill_other` for other validators / nominators.
	/// The threshold is compared to the actual number of validators / nominators (`CountFor*`) in
	/// the system compared to the configured max (`Max*Count`).
	#[pallet::storage]
	pub(crate) type ChillThreshold<T: Config> = StorageValue<_, Percent, OptionQuery>;

	#[pallet::genesis_config]
	pub struct GenesisConfig<T: Config> {
		pub history_depth: u32,
		pub validator_count: u32,
		pub minimum_validator_count: u32,
		pub invulnerables: Vec<T::AccountId>,
		pub force_era: Forcing,
		pub slash_reward_fraction: Perbill,
		pub canceled_payout: BalanceOf<T>,
		pub stakers: Vec<(T::AccountId, T::AccountId, BalanceOf<T>, StakerStatus<T::AccountId>)>,
		pub min_nominator_bond: BalanceOf<T>,
		pub min_validator_bond: BalanceOf<T>,
	}

	#[cfg(feature = "std")]
	impl<T: Config> Default for GenesisConfig<T> {
		fn default() -> Self {
			GenesisConfig {
				history_depth: 84u32,
				validator_count: Default::default(),
				minimum_validator_count: Default::default(),
				invulnerables: Default::default(),
				force_era: Default::default(),
				slash_reward_fraction: Default::default(),
				canceled_payout: Default::default(),
				stakers: Default::default(),
				min_nominator_bond: Default::default(),
				min_validator_bond: Default::default(),
			}
		}
	}

	#[pallet::genesis_build]
	impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
		fn build(&self) {
			HistoryDepth::<T>::put(self.history_depth);
			ValidatorCount::<T>::put(self.validator_count);
			MinimumValidatorCount::<T>::put(self.minimum_validator_count);
			Invulnerables::<T>::put(&self.invulnerables);
			ForceEra::<T>::put(self.force_era);
			CanceledSlashPayout::<T>::put(self.canceled_payout);
			SlashRewardFraction::<T>::put(self.slash_reward_fraction);
			StorageVersion::<T>::put(Releases::V6_0_0);
			MinNominatorBond::<T>::put(self.min_nominator_bond);
			MinValidatorBond::<T>::put(self.min_validator_bond);

			for &(ref stash, ref controller, balance, ref status) in &self.stakers {
				assert!(
					T::Currency::free_balance(&stash) >= balance,
					"Stash does not have enough balance to bond."
				);
				let _ = <Pallet<T>>::bond(
					T::Origin::from(Some(stash.clone()).into()),
					T::Lookup::unlookup(controller.clone()),
					balance,
					RewardDestination::Staked,
				);
				let _ = match status {
					StakerStatus::Validator => {
						<Pallet<T>>::validate(
							T::Origin::from(Some(controller.clone()).into()),
							Default::default(),
						)
					},
					StakerStatus::Nominator(votes) => {
						<Pallet<T>>::nominate(
							T::Origin::from(Some(controller.clone()).into()),
							votes.iter().map(|l| T::Lookup::unlookup(l.clone())).collect(),
						)
					}, _ => Ok(())
				};
			}
		}
	}

	#[pallet::event]
	#[pallet::generate_deposit(pub(super) fn deposit_event)]
	#[pallet::metadata(T::AccountId = "AccountId", BalanceOf<T> = "Balance")]
	pub enum Event<T: Config> {
		/// The era payout has been set; the first balance is the validator-payout; the second is
		/// the remainder from the maximum amount of reward.
		/// \[era_index, validator_payout, remainder\]
		EraPayout(EraIndex, BalanceOf<T>, BalanceOf<T>),
		/// The staker has been rewarded by this amount. \[stash, amount\]
		Reward(T::AccountId, BalanceOf<T>),
		/// One validator (and its nominators) has been slashed by the given amount.
		/// \[validator, amount\]
		Slash(T::AccountId, BalanceOf<T>),
		/// An old slashing report from a prior era was discarded because it could
		/// not be processed. \[session_index\]
		OldSlashingReportDiscarded(SessionIndex),
		/// A new set of stakers was elected.
		StakingElection,
		/// An account has bonded this amount. \[stash, amount\]
		///
		/// NOTE: This event is only emitted when funds are bonded via a dispatchable. Notably,
		/// it will not be emitted for staking rewards when they are added to stake.
		Bonded(T::AccountId, BalanceOf<T>),
		/// An account has unbonded this amount. \[stash, amount\]
		Unbonded(T::AccountId, BalanceOf<T>),
		/// An account has called `withdraw_unbonded` and removed unbonding chunks worth `Balance`
		/// from the unlocking queue. \[stash, amount\]
		Withdrawn(T::AccountId, BalanceOf<T>),
		/// A nominator has been kicked from a validator. \[nominator, stash\]
		Kicked(T::AccountId, T::AccountId),
		/// The election failed. No new era is planned.
		StakingElectionFailed,
		/// Moved an account from one bag to another. \[who, from, to\].
		Rebagged(T::AccountId, VoteWeight, VoteWeight),
	}

	#[pallet::error]
	pub enum Error<T> {
		/// Not a controller account.
		NotController,
		/// Not a stash account.
		NotStash,
		/// Stash is already bonded.
		AlreadyBonded,
		/// Controller is already paired.
		AlreadyPaired,
		/// Targets cannot be empty.
		EmptyTargets,
		/// Duplicate index.
		DuplicateIndex,
		/// Slash record index out of bounds.
		InvalidSlashIndex,
		/// Can not bond with value less than minimum required.
		InsufficientBond,
		/// Can not schedule more unlock chunks.
		NoMoreChunks,
		/// Can not rebond without unlocking chunks.
		NoUnlockChunk,
		/// Attempting to target a stash that still has funds.
		FundedTarget,
		/// Invalid era to reward.
		InvalidEraToReward,
		/// Invalid number of nominations.
		InvalidNumberOfNominations,
		/// Items are not sorted and unique.
		NotSortedAndUnique,
		/// Rewards for this era have already been claimed for this validator.
		AlreadyClaimed,
		/// Incorrect previous history depth input provided.
		IncorrectHistoryDepth,
		/// Incorrect number of slashing spans provided.
		IncorrectSlashingSpans,
		/// Internal state has become somehow corrupted and the operation cannot continue.
		BadState,
		/// Too many nomination targets supplied.
		TooManyTargets,
		/// A nomination target was supplied that was blocked or otherwise not a validator.
		BadTarget,
		/// The user has enough bond and thus cannot be chilled forcefully by an external person.
		CannotChillOther,
		/// There are too many nominators in the system. Governance needs to adjust the staking settings
		/// to keep things safe for the runtime.
		TooManyNominators,
		/// There are too many validators in the system. Governance needs to adjust the staking settings
		/// to keep things safe for the runtime.
		TooManyValidators,
	}

	#[pallet::hooks]
	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {
		fn on_runtime_upgrade() -> Weight {
			if StorageVersion::<T>::get() == Releases::V6_0_0 {
				migrations::v7::migrate::<T>()
			} else {
				T::DbWeight::get().reads(1)
			}
		}

		#[cfg(feature = "try-runtime")]
		fn pre_upgrade() -> Result<(), &'static str> {
			if StorageVersion::<T>::get() == Releases::V6_0_0 {
				migrations::v7::pre_migrate::<T>()
			} else {
				Ok(())
			}
		}

		fn on_initialize(_now: BlockNumberFor<T>) -> Weight {
			// just return the weight of the on_finalize.
			T::DbWeight::get().reads(1)
		}

		fn on_finalize(_n: BlockNumberFor<T>) {
			// Set the start of the first era.
			if let Some(mut active_era) = Self::active_era() {
				if active_era.start.is_none() {
					let now_as_millis_u64 = T::UnixTime::now().as_millis().saturated_into::<u64>();
					active_era.start = Some(now_as_millis_u64);
					// This write only ever happens once, we don't include it in the weight in general
					ActiveEra::<T>::put(active_era);
				}
			}
			// `on_finalize` weight is tracked in `on_initialize`
		}

		fn integrity_test() {
			sp_std::if_std! {
				sp_io::TestExternalities::new_empty().execute_with(|| {
					assert!(
						T::SlashDeferDuration::get() < T::BondingDuration::get() || T::BondingDuration::get() == 0,
						"As per documentation, slash defer duration ({}) should be less than bonding duration ({}).",
						T::SlashDeferDuration::get(),
						T::BondingDuration::get(),
					);

					assert!(
						T::VoterBagThresholds::get().windows(2).all(|window| window[1] > window[0]),
						"Voter bag thresholds must strictly increase",
					);
				});
			}
		}
	}

	#[pallet::call]
	impl<T: Config> Pallet<T> {
		/// Take the origin account as a stash and lock up `value` of its balance. `controller` will
		/// be the account that controls it.
		///
		/// `value` must be more than the `minimum_balance` specified by `T::Currency`.
		///
		/// The dispatch origin for this call must be _Signed_ by the stash account.
		///
		/// Emits `Bonded`.
		///
		/// # <weight>
		/// - Independent of the arguments. Moderate complexity.
		/// - O(1).
		/// - Three extra DB entries.
		///
		/// NOTE: Two of the storage writes (`Self::bonded`, `Self::payee`) are _never_ cleaned
		/// unless the `origin` falls below _existential deposit_ and gets removed as dust.
		/// ------------------
		/// Weight: O(1)
		/// DB Weight:
		/// - Read: Bonded, Ledger, [Origin Account], Current Era, History Depth, Locks
		/// - Write: Bonded, Payee, [Origin Account], Locks, Ledger
		/// # </weight>
		#[pallet::weight(T::WeightInfo::bond())]
		pub fn bond(
			origin: OriginFor<T>,
			controller: <T::Lookup as StaticLookup>::Source,
			#[pallet::compact] value: BalanceOf<T>,
			payee: RewardDestination<T::AccountId>,
		) -> DispatchResult {
			let stash = ensure_signed(origin)?;

			if <Bonded<T>>::contains_key(&stash) {
				Err(Error::<T>::AlreadyBonded)?
			}

			let controller = T::Lookup::lookup(controller)?;

			if <Ledger<T>>::contains_key(&controller) {
				Err(Error::<T>::AlreadyPaired)?
			}

			// Reject a bond which is considered to be _dust_.
			if value < T::Currency::minimum_balance() {
				Err(Error::<T>::InsufficientBond)?
			}

			frame_system::Pallet::<T>::inc_consumers(&stash).map_err(|_| Error::<T>::BadState)?;

			// You're auto-bonded forever, here. We might improve this by only bonding when
			// you actually validate/nominate and remove once you unbond __everything__.
			<Bonded<T>>::insert(&stash, &controller);
			<Payee<T>>::insert(&stash, payee);

			let current_era = CurrentEra::<T>::get().unwrap_or(0);
			let history_depth = Self::history_depth();
			let last_reward_era = current_era.saturating_sub(history_depth);

			let stash_balance = T::Currency::free_balance(&stash);
			let value = value.min(stash_balance);
			Self::deposit_event(Event::<T>::Bonded(stash.clone(), value));
			let item = StakingLedger {
				stash,
				total: value,
				active: value,
				unlocking: vec![],
				claimed_rewards: (last_reward_era..current_era).collect(),
			};
			Self::update_ledger(&controller, &item);
			Ok(())
		}

		/// Add some extra amount that have appeared in the stash `free_balance` into the balance up
		/// for staking.
		///
		/// Use this if there are additional funds in your stash account that you wish to bond.
		/// Unlike [`bond`] or [`unbond`] this function does not impose any limitation on the amount
		/// that can be added.
		///
		/// The dispatch origin for this call must be _Signed_ by the stash, not the controller and
		/// it can be only called when [`EraElectionStatus`] is `Closed`.
		///
		/// Emits `Bonded`.
		///
		/// # <weight>
		/// - Independent of the arguments. Insignificant complexity.
		/// - O(1).
		/// - One DB entry.
		/// ------------
		/// DB Weight:
		/// - Read: Era Election Status, Bonded, Ledger, [Origin Account], Locks
		/// - Write: [Origin Account], Locks, Ledger
		/// # </weight>
		#[pallet::weight(T::WeightInfo::bond_extra())]
		pub fn bond_extra(
			origin: OriginFor<T>,
			#[pallet::compact] max_additional: BalanceOf<T>,
		) -> DispatchResult {
			let stash = ensure_signed(origin)?;

			let controller = Self::bonded(&stash).ok_or(Error::<T>::NotStash)?;
			let mut ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;

			let stash_balance = T::Currency::free_balance(&stash);
			if let Some(extra) = stash_balance.checked_sub(&ledger.total) {
				let extra = extra.min(max_additional);
				ledger.total += extra;
				ledger.active += extra;
				// Last check: the new active amount of ledger must be more than ED.
				ensure!(ledger.active >= T::Currency::minimum_balance(), Error::<T>::InsufficientBond);

				Self::deposit_event(Event::<T>::Bonded(stash.clone(), extra));
				Self::update_ledger(&controller, &ledger);
				Self::do_rebag(&stash);
			}
			Ok(())
		}

		/// Schedule a portion of the stash to be unlocked ready for transfer out after the bond
		/// period ends. If this leaves an amount actively bonded less than
		/// T::Currency::minimum_balance(), then it is increased to the full amount.
		///
		/// Once the unlock period is done, you can call `withdraw_unbonded` to actually move
		/// the funds out of management ready for transfer.
		///
		/// No more than a limited number of unlocking chunks (see `MAX_UNLOCKING_CHUNKS`)
		/// can co-exists at the same time. In that case, [`Call::withdraw_unbonded`] need
		/// to be called first to remove some of the chunks (if possible).
		///
		/// If a user encounters the `InsufficientBond` error when calling this extrinsic,
		/// they should call `chill` first in order to free up their bonded funds.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		/// And, it can be only called when [`EraElectionStatus`] is `Closed`.
		///
		/// Emits `Unbonded`.
		///
		/// See also [`Call::withdraw_unbonded`].
		///
		/// # <weight>
		/// - Independent of the arguments. Limited but potentially exploitable complexity.
		/// - Contains a limited number of reads.
		/// - Each call (requires the remainder of the bonded balance to be above `minimum_balance`)
		///   will cause a new entry to be inserted into a vector (`Ledger.unlocking`) kept in storage.
		///   The only way to clean the aforementioned storage item is also user-controlled via
		///   `withdraw_unbonded`.
		/// - One DB entry.
		/// ----------
		/// Weight: O(1)
		/// DB Weight:
		/// - Read: EraElectionStatus, Ledger, CurrentEra, Locks, BalanceOf Stash,
		/// - Write: Locks, Ledger, BalanceOf Stash,
		/// </weight>
		#[pallet::weight(T::WeightInfo::unbond())]
		pub fn unbond(origin: OriginFor<T>, #[pallet::compact] value: BalanceOf<T>) -> DispatchResult {
			let controller = ensure_signed(origin)?;
			let mut ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			ensure!(
				ledger.unlocking.len() < MAX_UNLOCKING_CHUNKS,
				Error::<T>::NoMoreChunks,
			);

			let mut value = value.min(ledger.active);

			if !value.is_zero() {
				ledger.active -= value;

				// Avoid there being a dust balance left in the staking system.
				if ledger.active < T::Currency::minimum_balance() {
					value += ledger.active;
					ledger.active = Zero::zero();
				}

				let min_active_bond = if Nominators::<T>::contains_key(&ledger.stash) {
					MinNominatorBond::<T>::get()
				} else if Validators::<T>::contains_key(&ledger.stash) {
					MinValidatorBond::<T>::get()
				} else {
					Zero::zero()
				};

				// Make sure that the user maintains enough active bond for their role.
				// If a user runs into this error, they should chill first.
				ensure!(ledger.active >= min_active_bond, Error::<T>::InsufficientBond);

				// Note: in case there is no current era it is fine to bond one era more.
				let era = Self::current_era().unwrap_or(0) + T::BondingDuration::get();
				ledger.unlocking.push(UnlockChunk { value, era });
				Self::update_ledger(&controller, &ledger);
				Self::deposit_event(Event::<T>::Unbonded(ledger.stash, value));
			}
			Ok(())
		}

		/// Remove any unlocked chunks from the `unlocking` queue from our management.
		///
		/// This essentially frees up that balance to be used by the stash account to do
		/// whatever it wants.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		/// And, it can be only called when [`EraElectionStatus`] is `Closed`.
		///
		/// Emits `Withdrawn`.
		///
		/// See also [`Call::unbond`].
		///
		/// # <weight>
		/// - Could be dependent on the `origin` argument and how much `unlocking` chunks exist.
		///  It implies `consolidate_unlocked` which loops over `Ledger.unlocking`, which is
		///  indirectly user-controlled. See [`unbond`] for more detail.
		/// - Contains a limited number of reads, yet the size of which could be large based on `ledger`.
		/// - Writes are limited to the `origin` account key.
		/// ---------------
		/// Complexity O(S) where S is the number of slashing spans to remove
		/// Update:
		/// - Reads: EraElectionStatus, Ledger, Current Era, Locks, [Origin Account]
		/// - Writes: [Origin Account], Locks, Ledger
		/// Kill:
		/// - Reads: EraElectionStatus, Ledger, Current Era, Bonded, Slashing Spans, [Origin
		///   Account], Locks, BalanceOf stash
		/// - Writes: Bonded, Slashing Spans (if S > 0), Ledger, Payee, Validators, Nominators,
		///   [Origin Account], Locks, BalanceOf stash.
		/// - Writes Each: SpanSlash * S
		/// NOTE: Weight annotation is the kill scenario, we refund otherwise.
		/// # </weight>
		#[pallet::weight(T::WeightInfo::withdraw_unbonded_kill(*num_slashing_spans))]
		pub fn withdraw_unbonded(
			origin: OriginFor<T>,
			num_slashing_spans: u32,
		) -> DispatchResultWithPostInfo {
			let controller = ensure_signed(origin)?;
			let mut ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			let (stash, old_total) = (ledger.stash.clone(), ledger.total);
			if let Some(current_era) = Self::current_era() {
				ledger = ledger.consolidate_unlocked(current_era)
			}

			let post_info_weight = if ledger.unlocking.is_empty() && ledger.active < T::Currency::minimum_balance() {
				// This account must have called `unbond()` with some value that caused the active
				// portion to fall below existential deposit + will have no more unlocking chunks
				// left. We can now safely remove all staking-related information.
				Self::kill_stash(&stash, num_slashing_spans)?;
				// Remove the lock.
				T::Currency::remove_lock(STAKING_ID, &stash);
				// This is worst case scenario, so we use the full weight and return None
				None
			} else {
				// This was the consequence of a partial unbond. just update the ledger and move on.
				Self::update_ledger(&controller, &ledger);

				// This is only an update, so we use less overall weight.
				Some(T::WeightInfo::withdraw_unbonded_update(num_slashing_spans))
			};

			// `old_total` should never be less than the new total because
			// `consolidate_unlocked` strictly subtracts balance.
			if ledger.total < old_total {
				// Already checked that this won't overflow by entry condition.
				let value = old_total - ledger.total;
				Self::deposit_event(Event::<T>::Withdrawn(stash, value));
			}

			Ok(post_info_weight.into())
		}

		/// Declare the desire to validate for the origin controller.
		///
		/// Effects will be felt at the beginning of the next era.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		/// And, it can be only called when [`EraElectionStatus`] is `Closed`.
		///
		/// # <weight>
		/// - Independent of the arguments. Insignificant complexity.
		/// - Contains a limited number of reads.
		/// - Writes are limited to the `origin` account key.
		/// -----------
		/// Weight: O(1)
		/// DB Weight:
		/// - Read: Era Election Status, Ledger
		/// - Write: Nominators, Validators
		/// # </weight>
		#[pallet::weight(T::WeightInfo::validate())]
		pub fn validate(origin: OriginFor<T>, prefs: ValidatorPrefs) -> DispatchResult {
			let controller = ensure_signed(origin)?;

			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			ensure!(ledger.active >= MinValidatorBond::<T>::get(), Error::<T>::InsufficientBond);
			let stash = &ledger.stash;

			// Only check limits if they are not already a validator.
			if !Validators::<T>::contains_key(stash) {
				// If this error is reached, we need to adjust the `MinValidatorBond` and start calling `chill_other`.
				// Until then, we explicitly block new validators to protect the runtime.
				if let Some(max_validators) = MaxValidatorsCount::<T>::get() {
					ensure!(CounterForValidators::<T>::get() < max_validators, Error::<T>::TooManyValidators);
				}
			}

			Self::do_remove_nominator(stash);
			Self::do_add_validator(stash, prefs);
			Ok(())
		}

		/// Declare the desire to nominate `targets` for the origin controller.
		///
		/// Effects will be felt at the beginning of the next era. This can only be called when
		/// [`EraElectionStatus`] is `Closed`.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		/// And, it can be only called when [`EraElectionStatus`] is `Closed`.
		///
		/// # <weight>
		/// - The transaction's complexity is proportional to the size of `targets` (N)
		/// which is capped at CompactAssignments::LIMIT (MAX_NOMINATIONS).
		/// - Both the reads and writes follow a similar pattern.
		/// ---------
		/// Weight: O(N)
		/// where N is the number of targets
		/// DB Weight:
		/// - Reads: Era Election Status, Ledger, Current Era
		/// - Writes: Validators, Nominators
		/// # </weight>
		#[pallet::weight(T::WeightInfo::nominate(targets.len() as u32))]
		pub fn nominate(
			origin: OriginFor<T>,
			targets: Vec<<T::Lookup as StaticLookup>::Source>,
		) -> DispatchResult {
			let controller = ensure_signed(origin)?;

			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			ensure!(ledger.active >= MinNominatorBond::<T>::get(), Error::<T>::InsufficientBond);
			let stash = &ledger.stash;

			// Only check limits if they are not already a nominator.
			if !Nominators::<T>::contains_key(stash) {
				// If this error is reached, we need to adjust the `MinNominatorBond` and start calling `chill_other`.
				// Until then, we explicitly block new nominators to protect the runtime.
				if let Some(max_nominators) = MaxNominatorsCount::<T>::get() {
					ensure!(CounterForNominators::<T>::get() < max_nominators, Error::<T>::TooManyNominators);
				}
			}

			ensure!(!targets.is_empty(), Error::<T>::EmptyTargets);
			ensure!(targets.len() <= T::MAX_NOMINATIONS as usize, Error::<T>::TooManyTargets);

			let old = Nominators::<T>::get(stash).map_or_else(Vec::new, |x| x.targets);

			let targets = targets.into_iter()
				.map(|t| T::Lookup::lookup(t).map_err(DispatchError::from))
				.map(|n| n.and_then(|n| if old.contains(&n) || !Validators::<T>::get(&n).blocked {
					Ok(n)
				} else {
					Err(Error::<T>::BadTarget.into())
				}))
				.collect::<result::Result<Vec<T::AccountId>, _>>()?;

			let nominations = Nominations {
				targets,
				// Initial nominations are considered submitted at era 0. See `Nominations` doc
				submitted_in: Self::current_era().unwrap_or(0),
				suppressed: false,
			};

			Self::do_remove_validator(stash);
			Self::do_add_nominator(stash, nominations);
			Ok(())
		}

		/// Declare no desire to either validate or nominate.
		///
		/// Effects will be felt at the beginning of the next era.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		/// And, it can be only called when [`EraElectionStatus`] is `Closed`.
		///
		/// # <weight>
		/// - Independent of the arguments. Insignificant complexity.
		/// - Contains one read.
		/// - Writes are limited to the `origin` account key.
		/// --------
		/// Weight: O(1)
		/// DB Weight:
		/// - Read: EraElectionStatus, Ledger
		/// - Write: Validators, Nominators
		/// # </weight>
		#[pallet::weight(T::WeightInfo::chill())]
		pub fn chill(origin: OriginFor<T>) -> DispatchResult {
			let controller = ensure_signed(origin)?;
			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			Self::chill_stash(&ledger.stash);
			VoterList::<T>::remove(&ledger.stash);
			Ok(())
		}

		/// (Re-)set the payment target for a controller.
		///
		/// Effects will be felt at the beginning of the next era.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		///
		/// # <weight>
		/// - Independent of the arguments. Insignificant complexity.
		/// - Contains a limited number of reads.
		/// - Writes are limited to the `origin` account key.
		/// ---------
		/// - Weight: O(1)
		/// - DB Weight:
		///     - Read: Ledger
		///     - Write: Payee
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_payee())]
		pub fn set_payee(
			origin: OriginFor<T>,
			payee: RewardDestination<T::AccountId>,
		) -> DispatchResult {
			let controller = ensure_signed(origin)?;
			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			let stash = &ledger.stash;
			<Payee<T>>::insert(stash, payee);
			Ok(())
		}

		/// (Re-)set the controller of a stash.
		///
		/// Effects will be felt at the beginning of the next era.
		///
		/// The dispatch origin for this call must be _Signed_ by the stash, not the controller.
		///
		/// # <weight>
		/// - Independent of the arguments. Insignificant complexity.
		/// - Contains a limited number of reads.
		/// - Writes are limited to the `origin` account key.
		/// ----------
		/// Weight: O(1)
		/// DB Weight:
		/// - Read: Bonded, Ledger New Controller, Ledger Old Controller
		/// - Write: Bonded, Ledger New Controller, Ledger Old Controller
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_controller())]
		pub fn set_controller(
			origin: OriginFor<T>,
			controller: <T::Lookup as StaticLookup>::Source,
		) -> DispatchResult {
			let stash = ensure_signed(origin)?;
			let old_controller = Self::bonded(&stash).ok_or(Error::<T>::NotStash)?;
			let controller = T::Lookup::lookup(controller)?;
			if <Ledger<T>>::contains_key(&controller) {
				Err(Error::<T>::AlreadyPaired)?
			}
			if controller != old_controller {
				<Bonded<T>>::insert(&stash, &controller);
				if let Some(l) = <Ledger<T>>::take(&old_controller) {
					<Ledger<T>>::insert(&controller, l);
				}
			}
			Ok(())
		}

		/// Sets the ideal number of validators.
		///
		/// The dispatch origin must be Root.
		///
		/// # <weight>
		/// Weight: O(1)
		/// Write: Validator Count
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_validator_count())]
		pub fn set_validator_count(
			origin: OriginFor<T>,
			#[pallet::compact] new: u32,
		) -> DispatchResult {
			ensure_root(origin)?;
			ValidatorCount::<T>::put(new);
			Ok(())
		}

		/// Increments the ideal number of validators.
		///
		/// The dispatch origin must be Root.
		///
		/// # <weight>
		/// Same as [`set_validator_count`].
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_validator_count())]
		pub fn increase_validator_count(
			origin: OriginFor<T>,
			#[pallet::compact] additional: u32,
		) -> DispatchResult {
			ensure_root(origin)?;
			ValidatorCount::<T>::mutate(|n| *n += additional);
			Ok(())
		}

		/// Scale up the ideal number of validators by a factor.
		///
		/// The dispatch origin must be Root.
		///
		/// # <weight>
		/// Same as [`set_validator_count`].
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_validator_count())]
		pub fn scale_validator_count(origin: OriginFor<T>, factor: Percent) -> DispatchResult {
			ensure_root(origin)?;
			ValidatorCount::<T>::mutate(|n| *n += factor * *n);
			Ok(())
		}

		/// Force there to be no new eras indefinitely.
		///
		/// The dispatch origin must be Root.
		///
		/// # Warning
		///
		/// The election process starts multiple blocks before the end of the era.
		/// Thus the election process may be ongoing when this is called. In this case the
		/// election will continue until the next era is triggered.
		///
		/// # <weight>
		/// - No arguments.
		/// - Weight: O(1)
		/// - Write: ForceEra
		/// # </weight>
		#[pallet::weight(T::WeightInfo::force_no_eras())]
		pub fn force_no_eras(origin: OriginFor<T>) -> DispatchResult {
			ensure_root(origin)?;
			ForceEra::<T>::put(Forcing::ForceNone);
			Ok(())
		}

		/// Force there to be a new era at the end of the next session. After this, it will be
		/// reset to normal (non-forced) behaviour.
		///
		/// The dispatch origin must be Root.
		///
		/// # Warning
		///
		/// The election process starts multiple blocks before the end of the era.
		/// If this is called just before a new era is triggered, the election process may not
		/// have enough blocks to get a result.
		///
		/// # <weight>
		/// - No arguments.
		/// - Weight: O(1)
		/// - Write ForceEra
		/// # </weight>
		#[pallet::weight(T::WeightInfo::force_new_era())]
		pub fn force_new_era(origin: OriginFor<T>) -> DispatchResult {
			ensure_root(origin)?;
			ForceEra::<T>::put(Forcing::ForceNew);
			Ok(())
		}

		/// Set the validators who cannot be slashed (if any).
		///
		/// The dispatch origin must be Root.
		///
		/// # <weight>
		/// - O(V)
		/// - Write: Invulnerables
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_invulnerables(invulnerables.len() as u32))]
		pub fn set_invulnerables(
			origin: OriginFor<T>,
			invulnerables: Vec<T::AccountId>,
		) -> DispatchResult {
			ensure_root(origin)?;
			<Invulnerables<T>>::put(invulnerables);
			Ok(())
		}

		/// Force a current staker to become completely unstaked, immediately.
		///
		/// The dispatch origin must be Root.
		///
		/// # <weight>
		/// O(S) where S is the number of slashing spans to be removed
		/// Reads: Bonded, Slashing Spans, Account, Locks
		/// Writes: Bonded, Slashing Spans (if S > 0), Ledger, Payee, Validators, Nominators, Account, Locks
		/// Writes Each: SpanSlash * S
		/// # </weight>
		#[pallet::weight(T::WeightInfo::force_unstake(*num_slashing_spans))]
		pub fn force_unstake(
			origin: OriginFor<T>,
			stash: T::AccountId,
			num_slashing_spans: u32,
		) -> DispatchResult {
			ensure_root(origin)?;

			// Remove all staking-related information.
			Self::kill_stash(&stash, num_slashing_spans)?;

			// Remove the lock.
			T::Currency::remove_lock(STAKING_ID, &stash);
			Ok(())
		}

		/// Force there to be a new era at the end of sessions indefinitely.
		///
		/// The dispatch origin must be Root.
		///
		/// # Warning
		///
		/// The election process starts multiple blocks before the end of the era.
		/// If this is called just before a new era is triggered, the election process may not
		/// have enough blocks to get a result.
		///
		/// # <weight>
		/// - Weight: O(1)
		/// - Write: ForceEra
		/// # </weight>
		#[pallet::weight(T::WeightInfo::force_new_era_always())]
		pub fn force_new_era_always(origin: OriginFor<T>) -> DispatchResult {
			ensure_root(origin)?;
			ForceEra::<T>::put(Forcing::ForceAlways);
			Ok(())
		}

		/// Cancel enactment of a deferred slash.
		///
		/// Can be called by the `T::SlashCancelOrigin`.
		///
		/// Parameters: era and indices of the slashes for that era to kill.
		///
		/// # <weight>
		/// Complexity: O(U + S)
		/// with U unapplied slashes weighted with U=1000
		/// and S is the number of slash indices to be canceled.
		/// - Read: Unapplied Slashes
		/// - Write: Unapplied Slashes
		/// # </weight>
		#[pallet::weight(T::WeightInfo::cancel_deferred_slash(slash_indices.len() as u32))]
		pub fn cancel_deferred_slash(
			origin: OriginFor<T>,
			era: EraIndex,
			slash_indices: Vec<u32>,
		) -> DispatchResult {
			T::SlashCancelOrigin::ensure_origin(origin)?;

			ensure!(!slash_indices.is_empty(), Error::<T>::EmptyTargets);
			ensure!(is_sorted_and_unique(&slash_indices), Error::<T>::NotSortedAndUnique);

			let mut unapplied = <Self as Store>::UnappliedSlashes::get(&era);
			let last_item = slash_indices[slash_indices.len() - 1];
			ensure!((last_item as usize) < unapplied.len(), Error::<T>::InvalidSlashIndex);

			for (removed, index) in slash_indices.into_iter().enumerate() {
				let index = (index as usize) - removed;
				unapplied.remove(index);
			}

			<Self as Store>::UnappliedSlashes::insert(&era, &unapplied);
			Ok(())
		}

		/// Pay out all the stakers behind a single validator for a single era.
		///
		/// - `validator_stash` is the stash account of the validator. Their nominators, up to
		///   `T::MaxNominatorRewardedPerValidator`, will also receive their rewards.
		/// - `era` may be any era between `[current_era - history_depth; current_era]`.
		///
		/// The origin of this call must be _Signed_. Any account can call this function, even if
		/// it is not one of the stakers.
		///
		/// This can only be called when [`EraElectionStatus`] is `Closed`.
		///
		/// # <weight>
		/// - Time complexity: at most O(MaxNominatorRewardedPerValidator).
		/// - Contains a limited number of reads and writes.
		/// -----------
		/// N is the Number of payouts for the validator (including the validator)
		/// Weight:
		/// - Reward Destination Staked: O(N)
		/// - Reward Destination Controller (Creating): O(N)
		/// DB Weight:
		/// - Read: EraElectionStatus, CurrentEra, HistoryDepth, ErasValidatorReward,
		///         ErasStakersClipped, ErasRewardPoints, ErasValidatorPrefs (8 items)
		/// - Read Each: Bonded, Ledger, Payee, Locks, System Account (5 items)
		/// - Write Each: System Account, Locks, Ledger (3 items)
		///
		///   NOTE: weights are assuming that payouts are made to alive stash account (Staked).
		///   Paying even a dead controller is cheaper weight-wise. We don't do any refunds here.
		/// # </weight>
		#[pallet::weight(T::WeightInfo::payout_stakers_alive_staked(T::MaxNominatorRewardedPerValidator::get()))]
		pub fn payout_stakers(
			origin: OriginFor<T>,
			validator_stash: T::AccountId,
			era: EraIndex,
		) -> DispatchResultWithPostInfo {
			ensure_signed(origin)?;
			Self::do_payout_stakers(validator_stash, era)
		}

		/// Rebond a portion of the stash scheduled to be unlocked.
		///
		/// The dispatch origin must be signed by the controller, and it can be only called when
		/// [`EraElectionStatus`] is `Closed`.
		///
		/// # <weight>
		/// - Time complexity: O(L), where L is unlocking chunks
		/// - Bounded by `MAX_UNLOCKING_CHUNKS`.
		/// - Storage changes: Can't increase storage, only decrease it.
		/// ---------------
		/// - DB Weight:
		///     - Reads: EraElectionStatus, Ledger, Locks, [Origin Account]
		///     - Writes: [Origin Account], Locks, Ledger
		/// # </weight>
		#[pallet::weight(T::WeightInfo::rebond(MAX_UNLOCKING_CHUNKS as u32))]
		pub fn rebond(
			origin: OriginFor<T>,
			#[pallet::compact] value: BalanceOf<T>,
		) -> DispatchResultWithPostInfo {
			let controller = ensure_signed(origin)?;
			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			ensure!(!ledger.unlocking.is_empty(), Error::<T>::NoUnlockChunk);

			let ledger = ledger.rebond(value);
			// Last check: the new active amount of ledger must be more than ED.
			ensure!(ledger.active >= T::Currency::minimum_balance(), Error::<T>::InsufficientBond);

			Self::deposit_event(Event::<T>::Bonded(ledger.stash.clone(), value));
			Self::update_ledger(&controller, &ledger);
			Ok(Some(
				35 * WEIGHT_PER_MICROS
				+ 50 * WEIGHT_PER_NANOS * (ledger.unlocking.len() as Weight)
				+ T::DbWeight::get().reads_writes(3, 2)
			).into())
		}

		/// Set `HistoryDepth` value. This function will delete any history information
		/// when `HistoryDepth` is reduced.
		///
		/// Parameters:
		/// - `new_history_depth`: The new history depth you would like to set.
		/// - `era_items_deleted`: The number of items that will be deleted by this dispatch.
		///    This should report all the storage items that will be deleted by clearing old
		///    era history. Needed to report an accurate weight for the dispatch. Trusted by
		///    `Root` to report an accurate number.
		///
		/// Origin must be root.
		///
		/// # <weight>
		/// - E: Number of history depths removed, i.e. 10 -> 7 = 3
		/// - Weight: O(E)
		/// - DB Weight:
		///     - Reads: Current Era, History Depth
		///     - Writes: History Depth
		///     - Clear Prefix Each: Era Stakers, EraStakersClipped, ErasValidatorPrefs
		///     - Writes Each: ErasValidatorReward, ErasRewardPoints, ErasTotalStake, ErasStartSessionIndex
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_history_depth(*_era_items_deleted))]
		pub fn set_history_depth(origin: OriginFor<T>,
			#[pallet::compact] new_history_depth: EraIndex,
			#[pallet::compact] _era_items_deleted: u32,
		) -> DispatchResult {
			ensure_root(origin)?;
			if let Some(current_era) = Self::current_era() {
				HistoryDepth::<T>::mutate(|history_depth| {
					let last_kept = current_era.checked_sub(*history_depth).unwrap_or(0);
					let new_last_kept = current_era.checked_sub(new_history_depth).unwrap_or(0);
					for era_index in last_kept..new_last_kept {
						Self::clear_era_information(era_index);
					}
					*history_depth = new_history_depth
				})
			}
			Ok(())
		}

		/// Remove all data structure concerning a staker/stash once its balance is at the minimum.
		/// This is essentially equivalent to `withdraw_unbonded` except it can be called by anyone
		/// and the target `stash` must have no funds left beyond the ED.
		///
		/// This can be called from any origin.
		///
		/// - `stash`: The stash account to reap. Its balance must be zero.
		///
		/// # <weight>
		/// Complexity: O(S) where S is the number of slashing spans on the account.
		/// DB Weight:
		/// - Reads: Stash Account, Bonded, Slashing Spans, Locks
		/// - Writes: Bonded, Slashing Spans (if S > 0), Ledger, Payee, Validators, Nominators, Stash Account, Locks
		/// - Writes Each: SpanSlash * S
		/// # </weight>
		#[pallet::weight(T::WeightInfo::reap_stash(*num_slashing_spans))]
		pub fn reap_stash(
			_origin: OriginFor<T>,
			stash: T::AccountId,
			num_slashing_spans: u32,
		) -> DispatchResult {
			let at_minimum = T::Currency::total_balance(&stash) == T::Currency::minimum_balance();
			ensure!(at_minimum, Error::<T>::FundedTarget);
			Self::kill_stash(&stash, num_slashing_spans)?;
			T::Currency::remove_lock(STAKING_ID, &stash);
			Ok(())
		}

		/// Remove the given nominations from the calling validator.
		///
		/// Effects will be felt at the beginning of the next era.
		///
		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
		/// And, it can be only called when [`EraElectionStatus`] is `Closed`. The controller
		/// account should represent a validator.
		///
		/// - `who`: A list of nominator stash accounts who are nominating this validator which
		///   should no longer be nominating this validator.
		///
		/// Note: Making this call only makes sense if you first set the validator preferences to
		/// block any further nominations.
		#[pallet::weight(T::WeightInfo::kick(who.len() as u32))]
		pub fn kick(origin: OriginFor<T>, who: Vec<<T::Lookup as StaticLookup>::Source>) -> DispatchResult {
			let controller = ensure_signed(origin)?;
			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			let stash = &ledger.stash;

			for nom_stash in who.into_iter()
				.map(T::Lookup::lookup)
				.collect::<Result<Vec<T::AccountId>, _>>()?
				.into_iter()
			{
				Nominators::<T>::mutate(&nom_stash, |maybe_nom| if let Some(ref mut nom) = maybe_nom {
					if let Some(pos) = nom.targets.iter().position(|v| v == stash) {
						nom.targets.swap_remove(pos);
						Self::deposit_event(Event::<T>::Kicked(nom_stash.clone(), stash.clone()));
					}
				});
			}

			Ok(())
		}

		/// Update the various staking limits this pallet.
		///
		/// * `min_nominator_bond`: The minimum active bond needed to be a nominator.
		/// * `min_validator_bond`: The minimum active bond needed to be a validator.
		/// * `max_nominator_count`: The max number of users who can be a nominator at once.
		///   When set to `None`, no limit is enforced.
		/// * `max_validator_count`: The max number of users who can be a validator at once.
		///   When set to `None`, no limit is enforced.
		///
		/// Origin must be Root to call this function.
		///
		/// NOTE: Existing nominators and validators will not be affected by this update.
		/// to kick people under the new limits, `chill_other` should be called.
		#[pallet::weight(T::WeightInfo::set_staking_limits())]
		pub fn set_staking_limits(
			origin: OriginFor<T>,
			min_nominator_bond: BalanceOf<T>,
			min_validator_bond: BalanceOf<T>,
			max_nominator_count: Option<u32>,
			max_validator_count: Option<u32>,
			threshold: Option<Percent>,
		) -> DispatchResult {
			ensure_root(origin)?;
			MinNominatorBond::<T>::set(min_nominator_bond);
			MinValidatorBond::<T>::set(min_validator_bond);
			MaxNominatorsCount::<T>::set(max_nominator_count);
			MaxValidatorsCount::<T>::set(max_validator_count);
			ChillThreshold::<T>::set(threshold);
			Ok(())
		}

		/// Declare a `controller` to stop participating as either a validator or nominator.
		///
		/// Effects will be felt at the beginning of the next era.
		///
		/// The dispatch origin for this call must be _Signed_, but can be called by anyone.
		///
		/// If the caller is the same as the controller being targeted, then no further checks are
		/// enforced, and this function behaves just like `chill`.
		///
		/// If the caller is different than the controller being targeted, the following conditions
		/// must be met:
		/// * A `ChillThreshold` must be set and checked which defines how close to the max
		///   nominators or validators we must reach before users can start chilling one-another.
		/// * A `MaxNominatorCount` and `MaxValidatorCount` must be set which is used to determine
		///   how close we are to the threshold.
		/// * A `MinNominatorBond` and `MinValidatorBond` must be set and checked, which determines
		///   if this is a person that should be chilled because they have not met the threshold
		///   bond required.
		///
		/// This can be helpful if bond requirements are updated, and we need to remove old users
		/// who do not satisfy these requirements.
		///
		// TODO: Maybe we can deprecate `chill` in the future.
		// https://github.com/paritytech/substrate/issues/9111
		#[pallet::weight(T::WeightInfo::chill_other())]
		pub fn chill_other(
			origin: OriginFor<T>,
			controller: T::AccountId,
		) -> DispatchResult {
			// Anyone can call this function.
			let caller = ensure_signed(origin)?;
			let ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
			let stash = ledger.stash;

			// In order for one user to chill another user, the following conditions must be met:
			// * A `ChillThreshold` is set which defines how close to the max nominators or
			//   validators we must reach before users can start chilling one-another.
			// * A `MaxNominatorCount` and `MaxValidatorCount` which is used to determine how close
			//   we are to the threshold.
			// * A `MinNominatorBond` and `MinValidatorBond` which is the final condition checked to
			//   determine this is a person that should be chilled because they have not met the
			//   threshold bond required.
			//
			// Otherwise, if caller is the same as the controller, this is just like `chill`.
			if caller != controller {
				let threshold = ChillThreshold::<T>::get().ok_or(Error::<T>::CannotChillOther)?;
				let min_active_bond = if Nominators::<T>::contains_key(&stash) {
					let max_nominator_count = MaxNominatorsCount::<T>::get().ok_or(Error::<T>::CannotChillOther)?;
					let current_nominator_count = CounterForNominators::<T>::get();
					ensure!(threshold * max_nominator_count < current_nominator_count, Error::<T>::CannotChillOther);
					MinNominatorBond::<T>::get()
				} else if Validators::<T>::contains_key(&stash) {
					let max_validator_count = MaxValidatorsCount::<T>::get().ok_or(Error::<T>::CannotChillOther)?;
					let current_validator_count = CounterForValidators::<T>::get();
					ensure!(threshold * max_validator_count < current_validator_count, Error::<T>::CannotChillOther);
					MinValidatorBond::<T>::get()
				} else {
					Zero::zero()
				};

				ensure!(ledger.active < min_active_bond, Error::<T>::CannotChillOther);
			}

			Self::chill_stash(&stash);
			Ok(())
		}

		/// Declare that some `stash` has, through rewards or penalties, sufficiently changed its
		/// stake that it should properly fall into a different bag than its current position.
		///
		/// This will adjust its position into the appropriate bag. This will affect its position
		/// among the nominator/validator set once the snapshot is prepared for the election.
		///
		/// Anyone can call this function about any stash.
		#[pallet::weight(T::WeightInfo::rebag())]
		pub fn rebag(
			origin: OriginFor<T>,
			stash: AccountIdOf<T>,
		) -> DispatchResult {
			ensure_signed(origin)?;
			Pallet::<T>::do_rebag(&stash);
			Ok(())
		}
	}
}

impl<T: Config> Pallet<T> {
	/// The total balance that can be slashed from a stash account as of right now.
	pub fn slashable_balance_of(stash: &T::AccountId) -> BalanceOf<T> {
		// Weight note: consider making the stake accessible through stash.
		Self::bonded(stash).and_then(Self::ledger).map(|l| l.active).unwrap_or_default()
	}

	/// Internal impl of [`Self::slashable_balance_of`] that returns [`VoteWeight`].
	pub fn slashable_balance_of_vote_weight(
		stash: &T::AccountId,
		issuance: BalanceOf<T>,
	) -> VoteWeight {
		T::CurrencyToVote::to_vote(Self::slashable_balance_of(stash), issuance)
	}

	/// Returns a closure around `slashable_balance_of_vote_weight` that can be passed around.
	///
	/// This prevents call sites from repeatedly requesting `total_issuance` from backend. But it is
	/// important to be only used while the total issuance is not changing.
	pub fn weight_of_fn() -> Box<dyn Fn(&T::AccountId) -> VoteWeight> {
		// NOTE: changing this to unboxed `impl Fn(..)` return type and the pallet will still
		// compile, while some types in mock fail to resolve.
		let issuance = T::Currency::total_issuance();
		Box::new(move |who: &T::AccountId| -> VoteWeight {
			Self::slashable_balance_of_vote_weight(who, issuance)
		})
	}

	fn do_payout_stakers(validator_stash: T::AccountId, era: EraIndex) -> DispatchResultWithPostInfo {
		// Validate input data
		let current_era = CurrentEra::<T>::get().ok_or(
			Error::<T>::InvalidEraToReward.with_weight(T::WeightInfo::payout_stakers_alive_staked(0))
		)?;
		let history_depth = Self::history_depth();
		ensure!(
			era <= current_era && era >= current_era.saturating_sub(history_depth),
			Error::<T>::InvalidEraToReward.with_weight(T::WeightInfo::payout_stakers_alive_staked(0))
		);

		// Note: if era has no reward to be claimed, era may be future. better not to update
		// `ledger.claimed_rewards` in this case.
		let era_payout = <ErasValidatorReward<T>>::get(&era)
			.ok_or_else(||
				Error::<T>::InvalidEraToReward
					.with_weight(T::WeightInfo::payout_stakers_alive_staked(0))
			)?;

		let controller = Self::bonded(&validator_stash).ok_or(
			Error::<T>::NotStash.with_weight(T::WeightInfo::payout_stakers_alive_staked(0))
		)?;
		let mut ledger = <Ledger<T>>::get(&controller).ok_or_else(|| Error::<T>::NotController)?;

		ledger.claimed_rewards.retain(|&x| x >= current_era.saturating_sub(history_depth));
		match ledger.claimed_rewards.binary_search(&era) {
			Ok(_) => Err(
				Error::<T>::AlreadyClaimed.with_weight(T::WeightInfo::payout_stakers_alive_staked(0))
			)?,
			Err(pos) => ledger.claimed_rewards.insert(pos, era),
		}

		let exposure = <ErasStakersClipped<T>>::get(&era, &ledger.stash);

		/* Input data seems good, no errors allowed after this point */

		<Ledger<T>>::insert(&controller, &ledger);

		// Get Era reward points. It has TOTAL and INDIVIDUAL
		// Find the fraction of the era reward that belongs to the validator
		// Take that fraction of the eras rewards to split to nominator and validator
		//
		// Then look at the validator, figure out the proportion of their reward
		// which goes to them and each of their nominators.

		let era_reward_points = <ErasRewardPoints<T>>::get(&era);
		let total_reward_points = era_reward_points.total;
		let validator_reward_points = era_reward_points.individual.get(&ledger.stash)
			.map(|points| *points)
			.unwrap_or_else(|| Zero::zero());

		// Nothing to do if they have no reward points.
		if validator_reward_points.is_zero() {
			return Ok(Some(T::WeightInfo::payout_stakers_alive_staked(0)).into())
		}

		// This is the fraction of the total reward that the validator and the
		// nominators will get.
		let validator_total_reward_part = Perbill::from_rational(
			validator_reward_points,
			total_reward_points,
		);

		// This is how much validator + nominators are entitled to.
		let validator_total_payout = validator_total_reward_part * era_payout;

		let validator_prefs = Self::eras_validator_prefs(&era, &validator_stash);
		// Validator first gets a cut off the top.
		let validator_commission = validator_prefs.commission;
		let validator_commission_payout = validator_commission * validator_total_payout;

		let validator_leftover_payout = validator_total_payout - validator_commission_payout;
		// Now let's calculate how this is split to the validator.
		let validator_exposure_part = Perbill::from_rational(
			exposure.own,
			exposure.total,
		);
		let validator_staking_payout = validator_exposure_part * validator_leftover_payout;

		// We can now make total validator payout:
		if let Some(imbalance) = Self::make_payout(
			&ledger.stash,
			validator_staking_payout + validator_commission_payout
		) {
			Self::deposit_event(Event::<T>::Reward(ledger.stash, imbalance.peek()));
		}

		// Track the number of payout ops to nominators. Note: `WeightInfo::payout_stakers_alive_staked`
		// always assumes at least a validator is paid out, so we do not need to count their payout op.
		let mut nominator_payout_count: u32 = 0;

		// Lets now calculate how this is split to the nominators.
		// Reward only the clipped exposures. Note this is not necessarily sorted.
		for nominator in exposure.others.iter() {
			let nominator_exposure_part = Perbill::from_rational(
				nominator.value,
				exposure.total,
			);

			let nominator_reward: BalanceOf<T> = nominator_exposure_part * validator_leftover_payout;
			// We can now make nominator payout:
			if let Some(imbalance) = Self::make_payout(&nominator.who, nominator_reward) {
				// Note: this logic does not count payouts for `RewardDestination::None`.
				nominator_payout_count += 1;
				Self::deposit_event(Event::<T>::Reward(nominator.who.clone(), imbalance.peek()));
			}
		}

		debug_assert!(nominator_payout_count <= T::MaxNominatorRewardedPerValidator::get());
		Ok(Some(T::WeightInfo::payout_stakers_alive_staked(nominator_payout_count)).into())
	}

	/// Update the ledger for a controller.
	///
	/// This will also update the stash lock.
	fn update_ledger(
		controller: &T::AccountId,
		ledger: &StakingLedger<T::AccountId, BalanceOf<T>>
	) {
		T::Currency::set_lock(
			STAKING_ID,
			&ledger.stash,
			ledger.total,
			WithdrawReasons::all(),
		);
		<Ledger<T>>::insert(controller, ledger);
	}

	/// Chill a stash account.
	fn chill_stash(stash: &T::AccountId) {
		Self::do_remove_validator(stash);
		Self::do_remove_nominator(stash);
	}

	/// Actually make a payment to a staker. This uses the currency's reward function
	/// to pay the right payee for the given staker account.
	fn make_payout(stash: &T::AccountId, amount: BalanceOf<T>) -> Option<PositiveImbalanceOf<T>> {
		let dest = Self::payee(stash);
		match dest {
			RewardDestination::Controller => Self::bonded(stash)
				.and_then(|controller|
					Some(T::Currency::deposit_creating(&controller, amount))
				),
			RewardDestination::Stash =>
				T::Currency::deposit_into_existing(stash, amount).ok(),
			RewardDestination::Staked => Self::bonded(stash)
				.and_then(|c| Self::ledger(&c).map(|l| (c, l)))
				.and_then(|(controller, mut l)| {
					l.active += amount;
					l.total += amount;
					let r = T::Currency::deposit_into_existing(stash, amount).ok();
					Self::update_ledger(&controller, &l);
					r
				}),
			RewardDestination::Account(dest_account) => {
				Some(T::Currency::deposit_creating(&dest_account, amount))
			},
			RewardDestination::None => None,
		}
	}

	/// Plan a new session potentially trigger a new era.
	fn new_session(session_index: SessionIndex, is_genesis: bool) -> Option<Vec<T::AccountId>> {
		if let Some(current_era) = Self::current_era() {
			// Initial era has been set.
			let current_era_start_session_index = Self::eras_start_session_index(current_era)
				.unwrap_or_else(|| {
					frame_support::print("Error: start_session_index must be set for current_era");
					0
				});

			let era_length = session_index.checked_sub(current_era_start_session_index)
				.unwrap_or(0); // Must never happen.

			match ForceEra::<T>::get() {
				// Will be set to `NotForcing` again if a new era has been triggered.
				Forcing::ForceNew => (),
				// Short circuit to `try_trigger_new_era`.
				Forcing::ForceAlways => (),
				// Only go to `try_trigger_new_era` if deadline reached.
				Forcing::NotForcing if era_length >= T::SessionsPerEra::get() => (),
				_ => {
					// Either `Forcing::ForceNone`,
					// or `Forcing::NotForcing if era_length >= T::SessionsPerEra::get()`.
					return None
				},
			}

			// New era.
			let maybe_new_era_validators = Self::try_trigger_new_era(session_index, is_genesis);
			if maybe_new_era_validators.is_some()
				&& matches!(ForceEra::<T>::get(), Forcing::ForceNew)
			{
				ForceEra::<T>::put(Forcing::NotForcing);
			}

			maybe_new_era_validators
		} else {
			// Set initial era.
			log!(debug, "Starting the first era.");
			Self::try_trigger_new_era(session_index, is_genesis)
		}
	}

	/// Start a session potentially starting an era.
	fn start_session(start_session: SessionIndex) {
		let next_active_era = Self::active_era().map(|e| e.index + 1).unwrap_or(0);
		// This is only `Some` when current era has already progressed to the next era, while the
		// active era is one behind (i.e. in the *last session of the active era*, or *first session
		// of the new current era*, depending on how you look at it).
		if let Some(next_active_era_start_session_index) =
			Self::eras_start_session_index(next_active_era)
		{
			if next_active_era_start_session_index == start_session {
				Self::start_era(start_session);
			} else if next_active_era_start_session_index < start_session {
				// This arm should never happen, but better handle it than to stall the staking
				// pallet.
				frame_support::print("Warning: A session appears to have been skipped.");
				Self::start_era(start_session);
			}
		}
	}

	/// End a session potentially ending an era.
	fn end_session(session_index: SessionIndex) {
		if let Some(active_era) = Self::active_era() {
			if let Some(next_active_era_start_session_index) =
				Self::eras_start_session_index(active_era.index + 1)
			{
				if next_active_era_start_session_index == session_index + 1 {
					Self::end_era(active_era, session_index);
				}
			}
		}
	}

	/// * Increment `active_era.index`,
	/// * reset `active_era.start`,
	/// * update `BondedEras` and apply slashes.
	fn start_era(start_session: SessionIndex) {
		let active_era = ActiveEra::<T>::mutate(|active_era| {
			let new_index = active_era.as_ref().map(|info| info.index + 1).unwrap_or(0);
			*active_era = Some(ActiveEraInfo {
				index: new_index,
				// Set new active era start in next `on_finalize`. To guarantee usage of `Time`
				start: None,
			});
			new_index
		});

		let bonding_duration = T::BondingDuration::get();

		BondedEras::<T>::mutate(|bonded| {
			bonded.push((active_era, start_session));

			if active_era > bonding_duration {
				let first_kept = active_era - bonding_duration;

				// Prune out everything that's from before the first-kept index.
				let n_to_prune = bonded.iter()
					.take_while(|&&(era_idx, _)| era_idx < first_kept)
					.count();

				// Kill slashing metadata.
				for (pruned_era, _) in bonded.drain(..n_to_prune) {
					slashing::clear_era_metadata::<T>(pruned_era);
				}

				if let Some(&(_, first_session)) = bonded.first() {
					T::SessionInterface::prune_historical_up_to(first_session);
				}
			}
		});

		Self::apply_unapplied_slashes(active_era);
	}

	/// Compute payout for era.
	fn end_era(active_era: ActiveEraInfo, _session_index: SessionIndex) {
		// Note: active_era_start can be None if end era is called during genesis config.
		if let Some(active_era_start) = active_era.start {
			let now_as_millis_u64 = T::UnixTime::now().as_millis().saturated_into::<u64>();

			let era_duration = (now_as_millis_u64 - active_era_start).saturated_into::<u64>();
			let staked = Self::eras_total_stake(&active_era.index);
			let issuance = T::Currency::total_issuance();
			let (validator_payout, rest) = T::EraPayout::era_payout(staked, issuance, era_duration);

			Self::deposit_event(Event::<T>::EraPayout(active_era.index, validator_payout, rest));

			// Set ending era reward.
			<ErasValidatorReward<T>>::insert(&active_era.index, validator_payout);
			T::RewardRemainder::on_unbalanced(T::Currency::issue(rest));
		}
	}

	/// Plan a new era.
	///
	/// * Bump the current era storage (which holds the latest planned era).
	/// * Store start session index for the new planned era.
	/// * Clean old era information.
	/// * Store staking information for the new planned era
	///
	/// Returns the new validator set.
	pub fn trigger_new_era(
		start_session_index: SessionIndex,
		exposures: Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)>,
	) -> Vec<T::AccountId> {
		// Increment or set current era.
		let new_planned_era = CurrentEra::<T>::mutate(|s| {
			*s = Some(s.map(|s| s + 1).unwrap_or(0));
			s.unwrap()
		});
		ErasStartSessionIndex::<T>::insert(&new_planned_era, &start_session_index);

		// Clean old era information.
		if let Some(old_era) = new_planned_era.checked_sub(Self::history_depth() + 1) {
			Self::clear_era_information(old_era);
		}

		// Set staking information for the new era.
		Self::store_stakers_info(exposures, new_planned_era)
	}

	/// Potentially plan a new era.
	///
	/// Get election result from `T::ElectionProvider`.
	/// In case election result has more than [`MinimumValidatorCount`] validator trigger a new era.
	///
	/// In case a new era is planned, the new validator set is returned.
	fn try_trigger_new_era(start_session_index: SessionIndex, is_genesis: bool) -> Option<Vec<T::AccountId>> {
		let (election_result, weight) = if is_genesis {
			T::GenesisElectionProvider::elect().map_err(|e| {
				log!(warn, "genesis election provider failed due to {:?}", e);
				Self::deposit_event(Event::StakingElectionFailed);
			})
		} else {
			T::ElectionProvider::elect().map_err(|e| {
				log!(warn, "election provider failed due to {:?}", e);
				Self::deposit_event(Event::StakingElectionFailed);
			})
		}
		.ok()?;

		<frame_system::Pallet<T>>::register_extra_weight_unchecked(
			weight,
			frame_support::weights::DispatchClass::Mandatory,
		);

		let exposures = Self::collect_exposures(election_result);

		if (exposures.len() as u32) < Self::minimum_validator_count().max(1) {
			// Session will panic if we ever return an empty validator set, thus max(1) ^^.
			match CurrentEra::<T>::get() {
				Some(current_era) if current_era > 0 => log!(
					warn,
					"chain does not have enough staking candidates to operate for era {:?} ({} \
					elected, minimum is {})",
					CurrentEra::<T>::get().unwrap_or(0),
					exposures.len(),
					Self::minimum_validator_count(),
				),
				None => {
					// The initial era is allowed to have no exposures.
					// In this case the SessionManager is expected to choose a sensible validator
					// set.
					// TODO: this should be simplified #8911
					CurrentEra::<T>::put(0);
					ErasStartSessionIndex::<T>::insert(&0, &start_session_index);
				},
				_ => ()
			}

			Self::deposit_event(Event::StakingElectionFailed);
			return None
		}

		Self::deposit_event(Event::StakingElection);
		Some(Self::trigger_new_era(start_session_index, exposures))
	}

	/// Process the output of the election.
	///
	/// Store staking information for the new planned era
	pub fn store_stakers_info(
		exposures: Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)>,
		new_planned_era: EraIndex,
	) -> Vec<T::AccountId> {
		let elected_stashes = exposures.iter().cloned().map(|(x, _)| x).collect::<Vec<_>>();

		// Populate stakers, exposures, and the snapshot of validator prefs.
		let mut total_stake: BalanceOf<T> = Zero::zero();
		exposures.into_iter().for_each(|(stash, exposure)| {
			total_stake = total_stake.saturating_add(exposure.total);
			<ErasStakers<T>>::insert(new_planned_era, &stash, &exposure);

			let mut exposure_clipped = exposure;
			let clipped_max_len = T::MaxNominatorRewardedPerValidator::get() as usize;
			if exposure_clipped.others.len() > clipped_max_len {
				exposure_clipped.others.sort_by(|a, b| a.value.cmp(&b.value).reverse());
				exposure_clipped.others.truncate(clipped_max_len);
			}
			<ErasStakersClipped<T>>::insert(&new_planned_era, &stash, exposure_clipped);
		});

		// Insert current era staking information
		<ErasTotalStake<T>>::insert(&new_planned_era, total_stake);

		// Collect the pref of all winners.
		for stash in &elected_stashes {
			let pref = Self::validators(stash);
			<ErasValidatorPrefs<T>>::insert(&new_planned_era, stash, pref);
		}

		if new_planned_era > 0 {
			log!(
				info,
				"new validator set of size {:?} has been processed for era {:?}",
				elected_stashes.len(),
				new_planned_era,
			);
		}

		elected_stashes
	}

	/// Consume a set of [`Supports`] from [`sp_npos_elections`] and collect them into a
	/// [`Exposure`].
	fn collect_exposures(
		supports: Supports<T::AccountId>,
	) -> Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)> {
		let total_issuance = T::Currency::total_issuance();
		let to_currency = |e: frame_election_provider_support::ExtendedBalance| {
			T::CurrencyToVote::to_currency(e, total_issuance)
		};

		supports
			.into_iter()
			.map(|(validator, support)| {
				// Build `struct exposure` from `support`.
				let mut others = Vec::with_capacity(support.voters.len());
				let mut own: BalanceOf<T> = Zero::zero();
				let mut total: BalanceOf<T> = Zero::zero();
				support
					.voters
					.into_iter()
					.map(|(nominator, weight)| (nominator, to_currency(weight)))
					.for_each(|(nominator, stake)| {
						if nominator == validator {
						own = own.saturating_add(stake);
						} else {
							others.push(IndividualExposure { who: nominator, value: stake });
						}
						total = total.saturating_add(stake);
					});

				let exposure = Exposure { own, others, total };
				(validator, exposure)
			})
			.collect::<Vec<(T::AccountId, Exposure<_, _>)>>()
	}

	/// Remove all associated data of a stash account from the staking system.
	///
	/// Assumes storage is upgraded before calling.
	///
	/// This is called:
	/// - after a `withdraw_unbonded()` call that frees all of a stash's bonded balance.
	/// - through `reap_stash()` if the balance has fallen to zero (through slashing).
	fn kill_stash(stash: &T::AccountId, num_slashing_spans: u32) -> DispatchResult {
		let controller = <Bonded<T>>::get(stash).ok_or(Error::<T>::NotStash)?;

		slashing::clear_stash_metadata::<T>(stash, num_slashing_spans)?;

		<Bonded<T>>::remove(stash);
		<Ledger<T>>::remove(&controller);

		<Payee<T>>::remove(stash);
		Self::do_remove_validator(stash);
		Self::do_remove_nominator(stash);

		VoterList::<T>::remove(stash);

		frame_system::Pallet::<T>::dec_consumers(stash);

		Ok(())
	}

	/// Clear all era information for given era.
	fn clear_era_information(era_index: EraIndex) {
		<ErasStakers<T>>::remove_prefix(era_index, None);
		<ErasStakersClipped<T>>::remove_prefix(era_index, None);
		<ErasValidatorPrefs<T>>::remove_prefix(era_index, None);
		<ErasValidatorReward<T>>::remove(era_index);
		<ErasRewardPoints<T>>::remove(era_index);
		<ErasTotalStake<T>>::remove(era_index);
		ErasStartSessionIndex::<T>::remove(era_index);
	}

	/// Apply previously-unapplied slashes on the beginning of a new era, after a delay.
	fn apply_unapplied_slashes(active_era: EraIndex) {
		let slash_defer_duration = T::SlashDeferDuration::get();
		<Self as Store>::EarliestUnappliedSlash::mutate(|earliest| if let Some(ref mut earliest) = earliest {
			let keep_from = active_era.saturating_sub(slash_defer_duration);
			for era in (*earliest)..keep_from {
				let era_slashes = <Self as Store>::UnappliedSlashes::take(&era);
				for slash in era_slashes {
					slashing::apply_slash::<T>(slash);
				}
			}

			*earliest = (*earliest).max(keep_from)
		})
	}

	/// Add reward points to validators using their stash account ID.
	///
	/// Validators are keyed by stash account ID and must be in the current elected set.
	///
	/// For each element in the iterator the given number of points in u32 is added to the
	/// validator, thus duplicates are handled.
	///
	/// At the end of the era each the total payout will be distributed among validator
	/// relatively to their points.
	///
	/// COMPLEXITY: Complexity is `number_of_validator_to_reward x current_elected_len`.
	pub fn reward_by_ids(
		validators_points: impl IntoIterator<Item = (T::AccountId, u32)>
	) {
		if let Some(active_era) = Self::active_era() {
			<ErasRewardPoints<T>>::mutate(active_era.index, |era_rewards| {
				for (validator, points) in validators_points.into_iter() {
					*era_rewards.individual.entry(validator).or_default() += points;
					era_rewards.total += points;
				}
			});
		}
	}

	/// Ensures that at the end of the current session there will be a new era.
	fn ensure_new_era() {
		match ForceEra::<T>::get() {
			Forcing::ForceAlways | Forcing::ForceNew => (),
			_ => ForceEra::<T>::put(Forcing::ForceNew),
		}
	}

	#[cfg(feature = "runtime-benchmarks")]
	pub fn add_era_stakers(
		current_era: EraIndex,
		controller: T::AccountId,
		exposure: Exposure<T::AccountId, BalanceOf<T>>,
	) {
		<ErasStakers<T>>::insert(&current_era, &controller, &exposure);
	}

	#[cfg(feature = "runtime-benchmarks")]
	pub fn set_slash_reward_fraction(fraction: Perbill) {
		SlashRewardFraction::<T>::put(fraction);
	}

	/// Get all of the voters that are eligible for the npos election.
	///
	/// `voter_count` imposes an implicit cap on the number of voters returned; care should be taken
	/// to ensure that it is accurate.
	///
	/// This will use all on-chain nominators, and all the validators will inject a self vote.
	///
	/// ### Slashing
	///
	/// All nominations that have been submitted before the last non-zero slash of the validator are
	/// auto-chilled.
	///
	/// Note that this is fairly expensive: it must iterate over the min of `maybe_max_len` and
	/// `voter_count` voters. Use with care.
	pub fn get_npos_voters(
		maybe_max_len: Option<usize>,
		voter_count: usize,
	) -> Vec<VotingDataOf<T>> {
		debug_assert_eq!(
			voter_count,
			VoterList::<T>::decode_len().unwrap_or_default(),
			"voter_count must be accurate",
		);

		let wanted_voters = maybe_max_len.unwrap_or(voter_count).min(voter_count);

		let weight_of = Self::weight_of_fn();
		// collect all slashing spans into a BTreeMap for further queries.
		let slashing_spans = <SlashingSpans<T>>::iter().collect::<BTreeMap<_, _>>();

		VoterList::<T>::iter()
			.filter_map(|node| node.voting_data(&weight_of, &slashing_spans))
			.take(wanted_voters)
			.collect()
	}

	/// This is a very expensive function and result should be cached versus being called multiple times.
	pub fn get_npos_targets() -> Vec<T::AccountId> {
		Validators::<T>::iter().map(|(v, _)| v).collect::<Vec<_>>()
	}

	/// This function will add a nominator to the `Nominators` storage map,
	/// and keep track of the `CounterForNominators`.
	///
	/// If the nominator already exists, their nominations will be updated.
	pub fn do_add_nominator(who: &T::AccountId, nominations: Nominations<T::AccountId>) {
		if !Nominators::<T>::contains_key(who) {
			CounterForNominators::<T>::mutate(|x| x.saturating_inc())
		}
		Nominators::<T>::insert(who, nominations);
		VoterList::<T>::insert_as(who, VoterType::Nominator);
		debug_assert!(VoterCount::<T>::get() == CounterForNominators::<T>::get() + CounterForValidators::<T>::get());
	}

	/// This function will remove a nominator from the `Nominators` storage map,
	/// and keep track of the `CounterForNominators`.
	pub fn do_remove_nominator(who: &T::AccountId) {
		if Nominators::<T>::contains_key(who) {
			Nominators::<T>::remove(who);
			CounterForNominators::<T>::mutate(|x| x.saturating_dec());
			VoterList::<T>::remove(who);
			debug_assert!(VoterCount::<T>::get() == CounterForNominators::<T>::get() + CounterForValidators::<T>::get());
		}
	}

	/// This function will add a validator to the `Validators` storage map,
	/// and keep track of the `CounterForValidators`.
	///
	/// If the validator already exists, their preferences will be updated.
	pub fn do_add_validator(who: &T::AccountId, prefs: ValidatorPrefs) {
		if !Validators::<T>::contains_key(who) {
			CounterForValidators::<T>::mutate(|x| x.saturating_inc())
		}
		Validators::<T>::insert(who, prefs);
		VoterList::<T>::insert_as(who, VoterType::Validator);
		debug_assert!(VoterCount::<T>::get() == CounterForNominators::<T>::get() + CounterForValidators::<T>::get());
	}

	/// This function will remove a validator from the `Validators` storage map,
	/// and keep track of the `CounterForValidators`.
	pub fn do_remove_validator(who: &T::AccountId) {
		if Validators::<T>::contains_key(who) {
			Validators::<T>::remove(who);
			CounterForValidators::<T>::mutate(|x| x.saturating_dec());
			VoterList::<T>::remove(who);
			debug_assert!(VoterCount::<T>::get() == CounterForNominators::<T>::get() + CounterForValidators::<T>::get());
		}
	}

	/// Move a stash account from one bag to another, depositing an event on success.
	///
	/// If the stash changed bags, returns `Some((from, to))`.
	pub fn do_rebag(stash: &T::AccountId) -> Option<(VoteWeight, VoteWeight)> {
		// if no voter at that node, don't do anything.
		// the caller just wasted the fee to call this.
		let maybe_movement = voter_bags::Node::<T>::from_id(&stash).and_then(|node| {
			let weight_of = Self::weight_of_fn();
			VoterList::update_position_for(node, weight_of)
		});
		if let Some((from, to)) = maybe_movement {
			Self::deposit_event(Event::<T>::Rebagged(stash.clone(), from, to));
		};
		maybe_movement
	}
}

impl<T: Config>
	frame_election_provider_support::ElectionDataProvider<T::AccountId, BlockNumberFor<T>>
	for Pallet<T>
{
	const MAXIMUM_VOTES_PER_VOTER: u32 = T::MAX_NOMINATIONS;
	fn desired_targets() -> data_provider::Result<(u32, Weight)> {
		Ok((Self::validator_count(), <T as frame_system::Config>::DbWeight::get().reads(1)))
	}

	fn voters(
		maybe_max_len: Option<usize>,
	) -> data_provider::Result<(Vec<(T::AccountId, VoteWeight, Vec<T::AccountId>)>, Weight)> {
		let nominator_count = CounterForNominators::<T>::get();
		let validator_count = CounterForValidators::<T>::get();
		let voter_count = nominator_count.saturating_add(validator_count) as usize;
		debug_assert!(<Nominators<T>>::iter().count() as u32 == CounterForNominators::<T>::get());
		debug_assert!(<Validators<T>>::iter().count() as u32 == CounterForValidators::<T>::get());

		let slashing_span_count = <SlashingSpans<T>>::iter().count();
		let weight = T::WeightInfo::get_npos_voters(
			nominator_count,
			validator_count,
			slashing_span_count as u32,
		);
		Ok((Self::get_npos_voters(maybe_max_len, voter_count), weight))
	}

	fn targets(maybe_max_len: Option<usize>) -> data_provider::Result<(Vec<T::AccountId>, Weight)> {
		let target_count = CounterForValidators::<T>::get() as usize;

		if maybe_max_len.map_or(false, |max_len| target_count > max_len) {
			return Err("Target snapshot too big");
		}

		let weight = <T as frame_system::Config>::DbWeight::get().reads(target_count as u64);
		Ok((Self::get_npos_targets(), weight))
	}

	fn next_election_prediction(now: T::BlockNumber) -> T::BlockNumber {
		let current_era = Self::current_era().unwrap_or(0);
		let current_session = Self::current_planned_session();
		let current_era_start_session_index =
			Self::eras_start_session_index(current_era).unwrap_or(0);
		// Number of session in the current era or the maximum session per era if reached.
		let era_progress = current_session
			.saturating_sub(current_era_start_session_index)
			.min(T::SessionsPerEra::get());

		let until_this_session_end = T::NextNewSession::estimate_next_new_session(now)
			.0
			.unwrap_or_default()
			.saturating_sub(now);

		let session_length = T::NextNewSession::average_session_length();

		let sessions_left: T::BlockNumber = match ForceEra::<T>::get() {
			Forcing::ForceNone => Bounded::max_value(),
			Forcing::ForceNew | Forcing::ForceAlways => Zero::zero(),
			Forcing::NotForcing if era_progress >= T::SessionsPerEra::get() => Zero::zero(),
			Forcing::NotForcing => T::SessionsPerEra::get()
				.saturating_sub(era_progress)
				// One session is computed in this_session_end.
				.saturating_sub(1)
				.into(),
		};

		now.saturating_add(
			until_this_session_end.saturating_add(sessions_left.saturating_mul(session_length)),
		)
	}

	#[cfg(any(feature = "runtime-benchmarks", test))]
	fn put_snapshot(
		voters: Vec<(T::AccountId, VoteWeight, Vec<T::AccountId>)>,
		targets: Vec<T::AccountId>,
		target_stake: Option<VoteWeight>,
	) {
		use sp_std::convert::TryFrom;
		targets.into_iter().for_each(|v| {
			let stake: BalanceOf<T> = target_stake
				.and_then(|w| <BalanceOf<T>>::try_from(w).ok())
				.unwrap_or(MinNominatorBond::<T>::get() * 100u32.into());
			<Bonded<T>>::insert(v.clone(), v.clone());
			<Ledger<T>>::insert(
				v.clone(),
				StakingLedger {
					stash: v.clone(),
					active: stake,
					total: stake,
					unlocking: vec![],
					claimed_rewards: vec![],
				},
			);
			Self::do_add_validator(
				&v,
				ValidatorPrefs { commission: Perbill::zero(), blocked: false },
			);
		});

		voters.into_iter().for_each(|(v, s, t)| {
			let stake = <BalanceOf<T>>::try_from(s).unwrap_or_else(|_| {
				panic!("cannot convert a VoteWeight into BalanceOf, benchmark needs reconfiguring.")
			});
			<Bonded<T>>::insert(v.clone(), v.clone());
			<Ledger<T>>::insert(
				v.clone(),
				StakingLedger {
					stash: v.clone(),
					active: stake,
					total: stake,
					unlocking: vec![],
					claimed_rewards: vec![],
				},
			);
			Self::do_add_nominator(
				&v,
				Nominations { targets: t, submitted_in: 0, suppressed: false },
			);
		});
	}
}

/// In this implementation `new_session(session)` must be called before `end_session(session-1)`
/// i.e. the new session must be planned before the ending of the previous session.
///
/// Once the first new_session is planned, all session must start and then end in order, though
/// some session can lag in between the newest session planned and the latest session started.
impl<T: Config> pallet_session::SessionManager<T::AccountId> for Pallet<T> {
	fn new_session(new_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		log!(trace, "planning new session {}", new_index);
		CurrentPlannedSession::<T>::put(new_index);
		Self::new_session(new_index, false)
	}
	fn new_session_genesis(new_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		log!(trace, "planning new session {} at genesis", new_index);
		CurrentPlannedSession::<T>::put(new_index);
		Self::new_session(new_index, true)
	}
	fn start_session(start_index: SessionIndex) {
		log!(trace, "starting session {}", start_index);
		Self::start_session(start_index)
	}
	fn end_session(end_index: SessionIndex) {
		log!(trace, "ending session {}", end_index);
		Self::end_session(end_index)
	}
}

impl<T: Config> historical::SessionManager<T::AccountId, Exposure<T::AccountId, BalanceOf<T>>>
	for Pallet<T>
{
	fn new_session(
		new_index: SessionIndex,
	) -> Option<Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)>> {
		<Self as pallet_session::SessionManager<_>>::new_session(new_index).map(|validators| {
			let current_era = Self::current_era()
				// Must be some as a new era has been created.
				.unwrap_or(0);

			validators.into_iter().map(|v| {
				let exposure = Self::eras_stakers(current_era, &v);
				(v, exposure)
			}).collect()
		})
	}
	fn new_session_genesis(
		new_index: SessionIndex,
	) -> Option<Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)>> {
		<Self as pallet_session::SessionManager<_>>::new_session_genesis(new_index).map(|validators| {
			let current_era = Self::current_era()
				// Must be some as a new era has been created.
				.unwrap_or(0);

			validators.into_iter().map(|v| {
				let exposure = Self::eras_stakers(current_era, &v);
				(v, exposure)
			}).collect()
		})
	}
	fn start_session(start_index: SessionIndex) {
		<Self as pallet_session::SessionManager<_>>::start_session(start_index)
	}
	fn end_session(end_index: SessionIndex) {
		<Self as pallet_session::SessionManager<_>>::end_session(end_index)
	}
}

/// Add reward points to block authors:
/// * 20 points to the block producer for producing a (non-uncle) block in the relay chain,
/// * 2 points to the block producer for each reference to a previously unreferenced uncle, and
/// * 1 point to the producer of each referenced uncle block.
impl<T> pallet_authorship::EventHandler<T::AccountId, T::BlockNumber> for Pallet<T>
where
	T: Config + pallet_authorship::Config + pallet_session::Config,
{
	fn note_author(author: T::AccountId) {
		Self::reward_by_ids(vec![(author, 20)])
	}
	fn note_uncle(author: T::AccountId, _age: T::BlockNumber) {
		Self::reward_by_ids(vec![
			(<pallet_authorship::Pallet<T>>::author(), 2),
			(author, 1)
		])
	}
}

/// A `Convert` implementation that finds the stash of the given controller account,
/// if any.
pub struct StashOf<T>(sp_std::marker::PhantomData<T>);

impl<T: Config> Convert<T::AccountId, Option<T::AccountId>> for StashOf<T> {
	fn convert(controller: T::AccountId) -> Option<T::AccountId> {
		<Pallet<T>>::ledger(&controller).map(|l| l.stash)
	}
}

/// A typed conversion from stash account ID to the active exposure of nominators
/// on that account.
///
/// Active exposure is the exposure of the validator set currently validating, i.e. in
/// `active_era`. It can differ from the latest planned exposure in `current_era`.
pub struct ExposureOf<T>(sp_std::marker::PhantomData<T>);

impl<T: Config> Convert<T::AccountId, Option<Exposure<T::AccountId, BalanceOf<T>>>>
	for ExposureOf<T>
{
	fn convert(validator: T::AccountId) -> Option<Exposure<T::AccountId, BalanceOf<T>>> {
		<Pallet<T>>::active_era()
			.map(|active_era| <Pallet<T>>::eras_stakers(active_era.index, &validator))
	}
}

/// This is intended to be used with `FilterHistoricalOffences`.
impl<T: Config>
	OnOffenceHandler<T::AccountId, pallet_session::historical::IdentificationTuple<T>, Weight>
	for Pallet<T>
where
	T: pallet_session::Config<ValidatorId = <T as frame_system::Config>::AccountId>,
	T: pallet_session::historical::Config<
		FullIdentification = Exposure<<T as frame_system::Config>::AccountId, BalanceOf<T>>,
		FullIdentificationOf = ExposureOf<T>,
	>,
	T::SessionHandler: pallet_session::SessionHandler<<T as frame_system::Config>::AccountId>,
	T::SessionManager: pallet_session::SessionManager<<T as frame_system::Config>::AccountId>,
	T::ValidatorIdOf: Convert<
		<T as frame_system::Config>::AccountId,
		Option<<T as frame_system::Config>::AccountId>,
	>,
{
	fn on_offence(
		offenders: &[OffenceDetails<
			T::AccountId,
			pallet_session::historical::IdentificationTuple<T>,
		>],
		slash_fraction: &[Perbill],
		slash_session: SessionIndex,
	) -> Weight {
		let reward_proportion = SlashRewardFraction::<T>::get();
		let mut consumed_weight: Weight = 0;
		let mut add_db_reads_writes = |reads, writes| {
			consumed_weight += T::DbWeight::get().reads_writes(reads, writes);
		};

		let active_era = {
			let active_era = Self::active_era();
			add_db_reads_writes(1, 0);
			if active_era.is_none() {
				// This offence need not be re-submitted.
				return consumed_weight
			}
			active_era.expect("value checked not to be `None`; qed").index
		};
		let active_era_start_session_index = Self::eras_start_session_index(active_era)
			.unwrap_or_else(|| {
				frame_support::print("Error: start_session_index must be set for current_era");
				0
			});
		add_db_reads_writes(1, 0);

		let window_start = active_era.saturating_sub(T::BondingDuration::get());

		// Fast path for active-era report - most likely.
		// `slash_session` cannot be in a future active era. It must be in `active_era` or before.
		let slash_era = if slash_session >= active_era_start_session_index {
			active_era
		} else {
			let eras = BondedEras::<T>::get();
			add_db_reads_writes(1, 0);

			// Reverse because it's more likely to find reports from recent eras.
			match eras.iter().rev().filter(|&&(_, ref sesh)| sesh <= &slash_session).next() {
				Some(&(ref slash_era, _)) => *slash_era,
				// Before bonding period. defensive - should be filtered out.
				None => return consumed_weight,
			}
		};

		<Self as Store>::EarliestUnappliedSlash::mutate(|earliest| {
			if earliest.is_none() {
				*earliest = Some(active_era)
			}
		});
		add_db_reads_writes(1, 1);

		let slash_defer_duration = T::SlashDeferDuration::get();

		let invulnerables = Self::invulnerables();
		add_db_reads_writes(1, 0);

		for (details, slash_fraction) in offenders.iter().zip(slash_fraction) {
			let (stash, exposure) = &details.offender;

			// Skip if the validator is invulnerable.
			if invulnerables.contains(stash) {
				continue
			}

			let unapplied = slashing::compute_slash::<T>(slashing::SlashParams {
				stash,
				slash: *slash_fraction,
				exposure,
				slash_era,
				window_start,
				now: active_era,
				reward_proportion,
			});

			if let Some(mut unapplied) = unapplied {
				let nominators_len = unapplied.others.len() as u64;
				let reporters_len = details.reporters.len() as u64;

				{
					let upper_bound = 1 /* Validator/NominatorSlashInEra */ + 2 /* fetch_spans */;
					let rw = upper_bound + nominators_len * upper_bound;
					add_db_reads_writes(rw, rw);
				}
				unapplied.reporters = details.reporters.clone();
				if slash_defer_duration == 0 {
					// Apply right away.
					slashing::apply_slash::<T>(unapplied);
					{
						let slash_cost = (6, 5);
						let reward_cost = (2, 2);
						add_db_reads_writes(
							(1 + nominators_len) * slash_cost.0 + reward_cost.0 * reporters_len,
							(1 + nominators_len) * slash_cost.1 + reward_cost.1 * reporters_len
						);
					}
				} else {
					// Defer to end of some `slash_defer_duration` from now.
					<Self as Store>::UnappliedSlashes::mutate(
						active_era,
						move |for_later| for_later.push(unapplied),
					);
					add_db_reads_writes(1, 1);
				}
			} else {
				add_db_reads_writes(4 /* fetch_spans */, 5 /* kick_out_if_recent */)
			}
		}

		consumed_weight
	}
}

/// Filter historical offences out and only allow those from the bonding period.
pub struct FilterHistoricalOffences<T, R> {
	_inner: sp_std::marker::PhantomData<(T, R)>,
}

impl<T, Reporter, Offender, R, O> ReportOffence<Reporter, Offender, O>
	for FilterHistoricalOffences<Pallet<T>, R>
where
	T: Config,
	R: ReportOffence<Reporter, Offender, O>,
	O: Offence<Offender>,
{
	fn report_offence(reporters: Vec<Reporter>, offence: O) -> Result<(), OffenceError> {
		// Disallow any slashing from before the current bonding period.
		let offence_session = offence.session_index();
		let bonded_eras = BondedEras::<T>::get();

		if bonded_eras.first().filter(|(_, start)| offence_session >= *start).is_some() {
			R::report_offence(reporters, offence)
		} else {
			<Pallet<T>>::deposit_event(
				Event::<T>::OldSlashingReportDiscarded(offence_session)
			);
			Ok(())
		}
	}

	fn is_known_offence(offenders: &[Offender], time_slot: &O::TimeSlot) -> bool {
		R::is_known_offence(offenders, time_slot)
	}
}

/// Check that list is sorted and has no duplicates.
fn is_sorted_and_unique(list: &[u32]) -> bool {
	list.windows(2).all(|w| w[0] < w[1])
}