[docs] add doxygen docs to coin selection code

Co-authored-by: Xekyo <murch@murch.one>

src/wallet/coinselection.h

@@ -15,6 +15,7 @@ static constexpr CAmount MIN_CHANGE{COIN / 100};
 //! final minimum change amount after paying for fees
 static const CAmount MIN_FINAL_CHANGE = MIN_CHANGE/2;
 
+/** A UTXO under consideration for use in funding a new transaction. */
 class CInputCoin {
 public:
     CInputCoin(const CTransactionRef& tx, unsigned int i)
@@ -56,31 +57,58 @@ class CInputCoin {
     }
 };
 
+/** Parameters for filtering which OutputGroups we may use in coin selection.
+ * We start by being very selective and requiring multiple confirmations and
+ * then get more permissive if we cannot fund the transaction. */
 struct CoinEligibilityFilter
 {
+    /** Minimum number of confirmations for outputs that we sent to ourselves.
+     * We may use unconfirmed UTXOs sent from ourselves, e.g. change outputs. */
     const int conf_mine;
+    /** Minimum number of confirmations for outputs received from a different
+     * wallet. We never spend unconfirmed foreign outputs as we cannot rely on these funds yet. */
     const int conf_theirs;
+    /** Maximum number of total unconfirmed ancestors for all UTXOs in the OutputGroup. */
     const uint64_t max_ancestors;
+    /** Maximum number of descendants that a single UTXO in the OutputGroup may have. */
     const uint64_t max_descendants;
-    const bool m_include_partial_groups{false}; //! Include partial destination groups when avoid_reuse and there are full groups
+    /** When avoid_reuse=true and there are full groups (OUTPUT_GROUP_MAX_ENTRIES), whether or not to use any partial groups.*/
+    const bool m_include_partial_groups{false};
 
     CoinEligibilityFilter(int conf_mine, int conf_theirs, uint64_t max_ancestors) : conf_mine(conf_mine), conf_theirs(conf_theirs), max_ancestors(max_ancestors), max_descendants(max_ancestors) {}
     CoinEligibilityFilter(int conf_mine, int conf_theirs, uint64_t max_ancestors, uint64_t max_descendants) : conf_mine(conf_mine), conf_theirs(conf_theirs), max_ancestors(max_ancestors), max_descendants(max_descendants) {}
     CoinEligibilityFilter(int conf_mine, int conf_theirs, uint64_t max_ancestors, uint64_t max_descendants, bool include_partial) : conf_mine(conf_mine), conf_theirs(conf_theirs), max_ancestors(max_ancestors), max_descendants(max_descendants), m_include_partial_groups(include_partial) {}
 };
 
+/** A group of UTXOs paid to the same output script. */
 struct OutputGroup
 {
+    /** The list of UTXOs contained in this output group. */
     std::vector<CInputCoin> m_outputs;
+    /** Whether the UTXOs were sent by the wallet to itself. This is relevant because we may want at
+     * least certain number of confirmations on UTXOs received from outside wallets while trusting
+     * our own UTXOs more. */
     bool m_from_me{true};
+    /** The total value of the UTXOs in sum. */
     CAmount m_value{0};
+    /** The minimum number of confirmations the UTXOs in the group have. Unconfirmed is 0. */
     int m_depth{999};
+    /** The aggregated count of unconfirmed ancestors of all UTXOs in this
+     * group. Not deduplicated and may overestimate when ancestors are shared. */
     size_t m_ancestors{0};
+    /** The maximum count of descendants of a single UTXO in this output group. */
     size_t m_descendants{0};
+    /** The value of the UTXOs after deducting the cost of spending them at the effective feerate. */
     CAmount effective_value{0};
+    /** The fee to spend these UTXOs at the effective feerate. */
     CAmount fee{0};
+    /** The target feerate of the transaction we're trying to build. */
     CFeeRate m_effective_feerate{0};
+    /** The fee to spend these UTXOs at the long term feerate. */
     CAmount long_term_fee{0};
+    /** The feerate for spending a created change output eventually (i.e. not urgently, and thus at
+     * a lower feerate). Calculated using long term fee estimate. This is used to decide whether
+     * it could be economical to create a change output. */
     CFeeRate m_long_term_feerate{0};
 
     OutputGroup() {}

src/wallet/wallet.h

@@ -612,17 +612,28 @@ class COutput
     }
 };
 
+/** Parameters for one iteration of Coin Selection. */
 struct CoinSelectionParams
 {
+    /** When true, try to use the Branch and Bound algorithm before falling back to knapsack. */
     bool use_bnb = true;
+    /** Size of a change output in bytes, determined by the output type. */
     size_t change_output_size = 0;
+    /** Size of the input to spend a change output in virtual bytes. */
     size_t change_spend_size = 0;
+    /** The targeted feerate of the transaction being built. */
     CFeeRate m_effective_feerate;
+    /** The feerate estimate used to calculate the upper bound of the cost to spend a change output. */
     CFeeRate m_long_term_feerate;
+    /** If the cost to spend a change output at the discard feerate exceeds its value, drop it to fees. */
     CFeeRate m_discard_feerate;
+    /** Size of the transaction before inputs are selected, the header and recipient output(s). */
     size_t tx_noinputs_size = 0;
     /** Indicate that we are subtracting the fee from outputs */
     bool m_subtract_fee_outputs = false;
+    /** When true, always spend all (up to OUTPUT_GROUP_MAX_ENTRIES) or none of the outputs
+     * associated with the same address. This helps reduce privacy leaks resulting from address
+     * reuse. Dust outputs are not eligible to be added to output groups and thus not considered. */
     bool m_avoid_partial_spends = false;
 
     CoinSelectionParams(bool use_bnb, size_t change_output_size, size_t change_spend_size, CFeeRate effective_feerate,
@@ -765,8 +776,11 @@ class CWallet final : public WalletStorage, public interfaces::Chain::Notificati
 
     /**
      * Select a set of coins such that nValueRet >= nTargetValue and at least
-     * all coins from coinControl are selected; Never select unconfirmed coins
-     * if they are not ours
+     * all coins from coin_ontrol are selected; Never select unconfirmed coins if they are not ours
+     * param@[out]  setCoinsRet         Populated with inputs including pre-selected inputs from
+     *                                  coin_control and Coin Selection if successful.
+     * param@[out]  nValueRet           Total value of selected coins including pre-selected ones
+     *                                  from coin_control and Coin Selection if successful.
      */
     bool SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet,
                     const CCoinControl& coin_control, CoinSelectionParams& coin_selection_params, bool& bnb_used) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
@@ -851,6 +865,11 @@ class CWallet final : public WalletStorage, public interfaces::Chain::Notificati
      * small change; This method is stochastic for some inputs and upon
      * completion the coin set and corresponding actual target value is
      * assembled
+     * param@[in]   coins           Set of UTXOs to consider. These will be categorized into
+     *                              OutputGroups and filtered using eligibility_filter before
+     *                              selecting coins.
+     * param@[out]  setCoinsRet     Populated with the coins selected if successful.
+     * param@[out]  nValueRet       Used to return the total value of selected coins.
      */
     bool SelectCoinsMinConf(const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutput> coins,
         std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CoinSelectionParams& coin_selection_params, bool& bnb_used) const;