safe_btree.h

// Copyright 2013 Google Inc. All Rights Reserved.
//
// 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.
//
// A safe_btree<> wraps around a btree<> and removes the caveat that insertion
// and deletion invalidate iterators. A safe_btree<> maintains a generation
// number that is incremented on every mutation. A safe_btree<>::iterator keeps
// a pointer to the safe_btree<> it came from, the generation of the tree when
// it was last validated and the key the underlying btree<>::iterator points
// to. If an iterator is accessed and its generation differs from the tree
// generation it is revalidated.
//
// References and pointers returned by safe_btree iterators are not safe.
//
// See the incorrect usage examples mentioned in safe_btree_set.h and
// safe_btree_map.h.

#ifndef UTIL_BTREE_SAFE_BTREE_H__
#define UTIL_BTREE_SAFE_BTREE_H__

#include <stddef.h>
#include <iosfwd>
#include <utility>

#include "btree.h"

namespace btree {

template <typename Tree, typename Iterator>
class safe_btree_iterator {
 public:
  typedef typename Iterator::key_type key_type;
  typedef typename Iterator::value_type value_type;
  typedef typename Iterator::size_type size_type;
  typedef typename Iterator::difference_type difference_type;
  typedef typename Iterator::pointer pointer;
  typedef typename Iterator::reference reference;
  typedef typename Iterator::const_pointer const_pointer;
  typedef typename Iterator::const_reference const_reference;
  typedef typename Iterator::iterator_category iterator_category;
  typedef typename Tree::iterator iterator;
  typedef typename Tree::const_iterator const_iterator;
  typedef safe_btree_iterator<Tree, Iterator> self_type;

  void update() const {
    if (iter_ != tree_->internal_btree()->end()) {
      // A positive generation indicates a valid key.
      generation_ = tree_->generation();
      key_ = iter_.key();
    } else {
      // Use a negative generation to indicate iter_ points to end().
      generation_ = -tree_->generation();
    }
  }

 public:
  safe_btree_iterator()
      : generation_(0),
        key_(),
        iter_(),
        tree_(NULL) {
  }
  safe_btree_iterator(const iterator &x)
      : generation_(x.generation()),
        key_(x.key()),
        iter_(x.iter()),
        tree_(x.tree()) {
  }
  safe_btree_iterator(Tree *tree, const Iterator &iter)
      : generation_(),
        key_(),
        iter_(iter),
        tree_(tree) {
    update();
  }

  safe_btree_iterator &operator=(const iterator &x) {
    this->generation_ = x.generation();
    this->key_ = x.key();
    this->iter_ = x.iter();
    this->tree_ = x.tree();
    return *this;
  }

  Tree* tree() const { return tree_; }
  int64_t generation() const { return generation_; }

  Iterator* mutable_iter() const {
    if (generation_ != tree_->generation()) {
      if (generation_ > 0) {
        // This does the wrong thing for a multi{set,map}. If my iter was
        // pointing to the 2nd of 2 values with the same key, then this will
        // reset it to point to the first. This is why we don't provide a
        // safe_btree_multi{set,map}.
        iter_ = tree_->internal_btree()->lower_bound(key_);
        update();
      } else if (-generation_ != tree_->generation()) {
        iter_ = tree_->internal_btree()->end();
        generation_ = -tree_->generation();
      }
    }
    return &iter_;
  }
  const Iterator& iter() const {
    return *mutable_iter();
  }

  // Equality/inequality operators.
#if defined(__cplusplus) && __cplusplus >= 202002L
  friend bool operator==(const safe_btree_iterator &a, const safe_btree_iterator &b) noexcept {
    return a.iter() == b.iter();
  }
#else
  bool operator==(const const_iterator &x) const {
    return iter() == x.iter();
  }
  bool operator!=(const const_iterator &x) const {
    return iter() != x.iter();
  }
#endif

  // Accessors for the key/value the iterator is pointing at.
  const key_type& key() const {
    return key_;
  }
  // This reference value is potentially invalidated by any non-const
  // method on the tree; it is NOT safe.
  reference operator*() const {
    assert(generation_ > 0);
    return iter().operator*();
  }
  // This pointer value is potentially invalidated by any non-const
  // method on the tree; it is NOT safe.
  pointer operator->() const {
    assert(generation_ > 0);
    return iter().operator->();
  }

  // Increment/decrement operators.
  self_type& operator++() {
    ++(*mutable_iter());
    update();
    return *this;
  }
  self_type& operator--() {
    --(*mutable_iter());
    update();
    return *this;
  }
  self_type operator++(int) {
    self_type tmp = *this;
    ++*this;
    return tmp;
  }
  self_type operator--(int) {
    self_type tmp = *this;
    --*this;
    return tmp;
  }

 private:
  // The generation of the tree when "iter" was updated.
  mutable int64_t generation_;
  // The key the iterator points to.
  mutable key_type key_;
  // The underlying iterator.
  mutable Iterator iter_;
  // The tree the iterator is associated with.
  Tree *tree_;
};

template <typename Params>
class safe_btree {
  typedef safe_btree<Params> self_type;

  typedef btree<Params> btree_type;
  typedef typename btree_type::iterator tree_iterator;
  typedef typename btree_type::const_iterator tree_const_iterator;

 public:
  typedef typename btree_type::params_type params_type;
  typedef typename btree_type::key_type key_type;
  typedef typename btree_type::data_type data_type;
  typedef typename btree_type::mapped_type mapped_type;
  typedef typename btree_type::value_type value_type;
  typedef typename btree_type::key_compare key_compare;
  typedef typename btree_type::allocator_type allocator_type;
  typedef typename btree_type::pointer pointer;
  typedef typename btree_type::const_pointer const_pointer;
  typedef typename btree_type::reference reference;
  typedef typename btree_type::const_reference const_reference;
  typedef typename btree_type::size_type size_type;
  typedef typename btree_type::difference_type difference_type;
  typedef safe_btree_iterator<self_type, tree_iterator> iterator;
  typedef safe_btree_iterator<
    const self_type, tree_const_iterator> const_iterator;
  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
  typedef std::reverse_iterator<iterator> reverse_iterator;

 public:
  // Default constructor.
  safe_btree(const key_compare &comp, const allocator_type &alloc)
      : tree_(comp, alloc),
        generation_(1) {
  }

  // Copy constructor.
  safe_btree(const self_type &x)
      : tree_(x.tree_),
        generation_(1) {
  }

  iterator begin() {
    return iterator(this, tree_.begin());
  }
  const_iterator begin() const {
    return const_iterator(this, tree_.begin());
  }
  iterator end() {
    return iterator(this, tree_.end());
  }
  const_iterator end() const {
    return const_iterator(this, tree_.end());
  }
  reverse_iterator rbegin() {
    return reverse_iterator(end());
  }
  const_reverse_iterator rbegin() const {
    return const_reverse_iterator(end());
  }
  reverse_iterator rend() {
    return reverse_iterator(begin());
  }
  const_reverse_iterator rend() const {
    return const_reverse_iterator(begin());
  }

  // Lookup routines.
  iterator lower_bound(const key_type &key) {
    return iterator(this, tree_.lower_bound(key));
  }
  const_iterator lower_bound(const key_type &key) const {
    return const_iterator(this, tree_.lower_bound(key));
  }
  iterator upper_bound(const key_type &key) {
    return iterator(this, tree_.upper_bound(key));
  }
  const_iterator upper_bound(const key_type &key) const {
    return const_iterator(this, tree_.upper_bound(key));
  }
  std::pair<iterator, iterator> equal_range(const key_type &key) {
    std::pair<tree_iterator, tree_iterator> p = tree_.equal_range(key);
    return std::make_pair(iterator(this, p.first),
                     iterator(this, p.second));
  }
  std::pair<const_iterator, const_iterator> equal_range(const key_type &key) const {
    std::pair<tree_const_iterator, tree_const_iterator> p = tree_.equal_range(key);
    return std::make_pair(const_iterator(this, p.first),
                     const_iterator(this, p.second));
  }
  iterator find_unique(const key_type &key) {
    return iterator(this, tree_.find_unique(key));
  }
  const_iterator find_unique(const key_type &key) const {
    return const_iterator(this, tree_.find_unique(key));
  }
  iterator find_multi(const key_type &key) {
    return iterator(this, tree_.find_multi(key));
  }
  const_iterator find_multi(const key_type &key) const {
    return const_iterator(this, tree_.find_multi(key));
  }
  size_type count_unique(const key_type &key) const {
    return tree_.count_unique(key);
  }
  size_type count_multi(const key_type &key) const {
    return tree_.count_multi(key);
  }

  // Insertion routines.
  template <typename... Args>
  std::pair<iterator,bool> insert_unique_args(const key_type &key, Args&&... args) {
    std::pair<tree_iterator, bool> p = tree_.insert_unique_args(key, std::forward<Args>(args)...);
    generation_ += p.second;
    return std::make_pair(iterator(this, p.first), p.second);
  }
  std::pair<iterator,bool> insert_unique(const value_type &v) {
    return insert_unique_args(params_type::key(v), v);
  }
  std::pair<iterator,bool> insert_unique(value_type &&v) {
    return insert_unique_args(params_type::key(v), std::move(v));
  }

  template <typename... Args>
  iterator insert_unique_hint_args(iterator position, const key_type &key, Args&&... args) {
    tree_iterator tree_pos = position.iter();
    ++generation_;
    return iterator(this, tree_.insert_unique_hint_args(tree_pos, key, std::forward<Args>(args)...));
  }
  iterator insert_unique(iterator position, const value_type &v) {
    return insert_unique_hint_args(position, params_type::key(v), v);
  }
  iterator insert_unique(iterator position, value_type &&v) {
    return insert_unique_hint_args(position, params_type::key(v), std::move(v));
  }

  template <typename InputIterator>
  void insert_unique(InputIterator b, InputIterator e) {
    for (; b != e; ++b) {
      insert_unique(*b);
    }
  }

  template <typename... Args>
  iterator insert_multi_args(const key_type &key,  Args&&... args) {
    ++generation_;
    return iterator(this, tree_.insert_multi_args(key, std::forward<Args>(args)...));
  }

  iterator insert_multi(const value_type &v) {
    return insert_multi_args(params_type::key(v), v);
  }
  iterator insert_multi(value_type &&v) {
    return insert_multi_args(params_type::key(v), std::move(v));
  }

  template <typename... Args>
  iterator insert_multi_hint_args(iterator position, const key_type &key, Args&&... args) {
    tree_iterator tree_pos = position.iter();
    ++generation_;
    return iterator(this, tree_.insert_multi_hint_args(tree_pos, key, std::forward<Args>(args)...));
  }

  iterator insert_multi(iterator position, const value_type &v) {
    return insert_multi_hint_args(position, params_type::key(v), v);
  }
  iterator insert_multi(iterator position, value_type &&v) {
    return insert_multi_hint_args(position, params_type::key(v), std::move(v));
  }
  template <typename InputIterator>
  void insert_multi(InputIterator b, InputIterator e) {
    for (; b != e; ++b) {
      insert_multi(*b);
    }
  }
  self_type& operator=(const self_type &x) {
    if (&x == this) {
      // Don't copy onto ourselves.
      return *this;
    }
    ++generation_;
    tree_ = x.tree_;
    return *this;
  }

  // Deletion routines.
  void erase(const iterator &begin, const iterator &end) {
    tree_.erase(begin.iter(), end.iter());
    ++generation_;
  }
  // Erase the specified iterator from the btree. The iterator must be valid
  // (i.e. not equal to end()).  Return an iterator pointing to the node after
  // the one that was erased (or end() if none exists).
  iterator erase(iterator iter) {
    tree_iterator res = tree_.erase(iter.iter());
    ++generation_;
    return iterator(this, res);
  }
  int erase_unique(const key_type &key) {
    int res = tree_.erase_unique(key);
    generation_ += res;
    return res;
  }
  int erase_multi(const key_type &key) {
    int res = tree_.erase_multi(key);
    generation_ += res;
    return res;
  }

  // Access to the underlying btree.
  btree_type* internal_btree() { return &tree_; }
  const btree_type* internal_btree() const { return &tree_; }

  // Utility routines.
  void clear() {
    ++generation_;
    tree_.clear();
  }
  void swap(self_type &x) {
    ++generation_;
    ++x.generation_;
    tree_.swap(x.tree_);
  }
#ifndef BTREE_NO_IOSTREAM
  void dump(std::ostream &os) const {
    tree_.dump(os);
  }
#endif
  void verify() const {
    tree_.verify();
  }
  int64_t generation() const {
    return generation_;
  }
  key_compare key_comp() const { return tree_.key_comp(); }

  // Size routines.
  size_type size() const { return tree_.size(); }
  size_type max_size() const { return tree_.max_size(); }
  bool empty() const { return tree_.empty(); }
  size_type height() const { return tree_.height(); }
  size_type internal_nodes() const { return tree_.internal_nodes(); }
  size_type leaf_nodes() const { return tree_.leaf_nodes(); }
  size_type nodes() const { return tree_.nodes(); }
  size_type bytes_used() const { return tree_.bytes_used(); }
  static double average_bytes_per_value() {
    return btree_type::average_bytes_per_value();
  }
  double fullness() const { return tree_.fullness(); }
  double overhead() const { return tree_.overhead(); }

 private:
  btree_type tree_;
  int64_t generation_;
};

}  // namespace btree

#endif  // UTIL_BTREE_SAFE_BTREE_H__