First implementation of flat sets

base/containers/flat_set.h

Index: base/containers/flat_set.h
diff --git a/base/containers/flat_set.h b/base/containers/flat_set.h
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_CONTAINERS_FLAT_SET_H_
+#define BASE_CONTAINERS_FLAT_SET_H_
+
+#include <algorithm>
+#include <functional>
+#include <utility>
+#include <vector>
+
+namespace base {
+
+// Overview:
+// This file implements flat_set container. It is an alternative to standard
+// sorted containers that stores it's elements in contiguous memory (current
+// version uses sorted std::vector).
+// Discussion that preceded introduction of this container can be found here:
+// https://groups.google.com/a/chromium.org/forum/#!searchin/chromium-dev/vector$20based/chromium-dev/4uQMma9vj9w/HaQ-WvMOAwAJ
+//
+// Motivation:
+// Contiguous memory is very beneficial to iteration and copy speed at the cost
+// of worse algorithmic complexity of insertion/erasure operations. They can
+// be very fast for set operations and for small number of elements.
+//
+// Usage guidance:
+// Prefer base::flat_set for:
+//  * Very small sets, something that is an easy fit for cache. Consider
+//    "very small" to be under a 100 32bit integers.
+//  * Sets that are built once (using flat_set::flat_set(first, last)). Consider
+//    collecting all data in a vector and then building flat_set out of it.
+//    TODO(dyaroshev): improve the interface to better support this pattern
+//    (crbug.com/682254).
+//  * Sets where mutating happens in big bulks: use erase(std::remove()) idiom
+//    for erasing many elements. Insertion is harder - consider set operations
+//    or building a new vector. Set operations can be slow if one of the sets
+//    is considerably bigger. Also be aware that beating performance of
+//    sort + unique (implementation of flat_set's constructor) is hard, clever
+//    merge of many sets might not win. Generally avoid inserting into flat set
+//    without benchmarks.
+//  * Copying and iterating.
+//  * Set operations (union/intersect etc).
+//
+// TODO(dyaroshev): develop standalone benchmarks to find performance boundaries
+// for different types of sets crbug.com/682215.
+//
+// If you do write a benchmark that significantly depends on using sets please
+// share your results at:
+// https://groups.google.com/a/chromium.org/forum/#!searchin/chromium-dev/vector$20based/chromium-dev/4uQMma9vj9w/HaQ-WvMOAwAJ
+//
+// Important usability aspects:
+//   * flat_set implements std::set interface from C++11 where possible. It
+//     also has reserve(), capacity() and shrink_to_fit() from std::vector.
+//   * iteration invalidation rules differ:
+//     - all cases of std::vector::iterator invalidation also apply.
+//     - we ask (for now) to assume that move operations invalidate iterators.
+//       TODO(dyaroshev): Research the possibility of using a small buffer
+//       optimization crbug.com/682240.
+//   * Constructor sorts elements in a non-stable manner (unlike std::set). So
+//     among equivalent (with respect to provided compare) elements passed to
+//     the constructor it is unspecified with one will end up in the set.
+//     However insert()/emplace() methods are stable with respect to already
+//     inserted elements - an element that is already in the set will not be
+//     replaced.
+//   * allocator support is not implemented.
+//   * insert(first, last) and insert(std::initializer_list) are not
+//     implemented (see Notes section).
+//
+// Notes:
+// Current implementation is based on boost::containers::flat_set,
+// eastl::vector_set and folly::sorted_vector. All of these implementations do
+// insert(first, last) as insertion one by one (some implementations with hints
+// and/or reserve). Boost documentation claims this algorithm to be O(n*log(n))
+// but it seems to be a quadratic algorithm. For now we do not implement this
+// method.
+// TODO(dyaroshev): research an algorithm for range insertion crbug.com/682249.
+
+template <class Key, class Compare = std::less<Key>>
+// Meets the requirements of Container, AssociativeContainer,
+// ReversibleContainer.
+// Requires: Key is Movable, Compare is a StrictWeakOrdering on Key.
+class flat_set {
+ private:
+  using underlying_type = std::vector<Key>;
+
+ public:
+  // --------------------------------------------------------------------------
+  // Types.
+  //
+  using key_type = Key;
+  using key_compare = Compare;
+  using value_type = Key;
+  using value_compare = Compare;
+
+  using pointer = typename underlying_type::pointer;
+  using const_pointer = typename underlying_type::const_pointer;
+  using reference = typename underlying_type::reference;
+  using const_reference = typename underlying_type::const_reference;
+  using size_type = typename underlying_type::size_type;
+  using difference_type = typename underlying_type::difference_type;
+  using iterator = typename underlying_type::iterator;
+  using const_iterator = typename underlying_type::const_iterator;
+  using reverse_iterator = typename underlying_type::reverse_iterator;
+  using const_reverse_iterator =
+      typename underlying_type::const_reverse_iterator;
+
+  // --------------------------------------------------------------------------
+  // Lifetime.
+  //
+  // Constructors that take range guarantee O(N * log^2(N)) + O(N) complexity
+  // and take O(N * log(N)) + O(N) if extra memory is available (N is a range
+  // length).
+  //
+  // Assume that move constructors invalidate iterators and references.
+
+  flat_set();
+  explicit flat_set(const Compare& comp);
+
+  template <class InputIterator>
+  flat_set(InputIterator first,
+           InputIterator last,
+           const Compare& comp = Compare());
+
+  flat_set(const flat_set&);
+  flat_set(flat_set&&);
+
+  flat_set(std::initializer_list<value_type> ilist,
+           const Compare& comp = Compare());
+
+  ~flat_set();
+
+  // --------------------------------------------------------------------------
+  // Assignments.
+  //
+  // Assume that move assignment invalidates iterators and references.
+
+  flat_set& operator=(const flat_set&);
+  flat_set& operator=(flat_set&&);
+  flat_set& operator=(std::initializer_list<value_type> ilist);
+
+  // --------------------------------------------------------------------------
+  // Memory management.
+  //
+  // Beware that shrink_to_fit() simply forwards the request to the
+  // underlying_type and its implementation is free to optimize otherwise and
+  // leave capacity() to be greater that its size.
+  //
+  // reserve() and shrink_to_fit() invalidate iterators and references.
+
+  void reserve(size_type new_capacity);
+  size_type capacity() const;
+  void shrink_to_fit();
+
+  // --------------------------------------------------------------------------
+  // Size management.
+  //
+  // clear() leaves the capacity() of the flat_set unchanged.
+
+  void clear();
+
+  size_type size() const;
+  size_type max_size() const;
+  bool empty() const;
+
+  // --------------------------------------------------------------------------
+  // Iterators.
+
+  iterator begin();
+  const_iterator begin() const;
+  const_iterator cbegin() const;
+
+  iterator end();
+  const_iterator end() const;
+  const_iterator cend() const;
+
+  reverse_iterator rbegin();
+  const_reverse_iterator rbegin() const;
+  const_reverse_iterator crbegin() const;
+
+  reverse_iterator rend();
+  const_reverse_iterator rend() const;
+  const_reverse_iterator crend() const;
+
+  // --------------------------------------------------------------------------
+  // Insert operations.
+  //
+  // Assume that every operation invalidates iterators and references.
+  // Insertion of one element can take O(size). See the Notes section in the
+  // class comments on why we do not currently implement range insertion.
+  // Capacity of flat_set grows in an implementation-defined manner.
+
+  std::pair<iterator, bool> insert(const value_type& val);
+  std::pair<iterator, bool> insert(value_type&& val);
+
+  iterator insert(const_iterator position_hint, const value_type& x);
+  iterator insert(const_iterator position_hint, value_type&& x);
+
+  template <class... Args>
+  std::pair<iterator, bool> emplace(Args&&... args);
+
+  template <class... Args>
+  iterator emplace_hint(const_iterator position_hint, Args&&... args);
+
+  // --------------------------------------------------------------------------
+  // Erase operations.
+  //
+  // Assume that every operation invalidates iterators and references.
+  //
+  // erase(position), erase(first, last) can take O(size).
+  // erase(key) may take O(size) + O(log(size)).
+  //
+  // Prefer the erase(std::remove(), end()) idiom for deleting multiple
+  // elements.
+
+  iterator erase(const_iterator position);
+  iterator erase(const_iterator first, const_iterator last);
+  size_type erase(const key_type& key);
+
+  // --------------------------------------------------------------------------
+  // Comparators.
+
+  key_compare key_comp() const;
+  value_compare value_comp() const;
+
+  // --------------------------------------------------------------------------
+  // Search operations.
+  //
+  // Search operations have O(log(size)) complexity.
+
+  size_type count(const key_type& key) const;
+
+  iterator find(const key_type& key);
+  const_iterator find(const key_type& key) const;
+
+  std::pair<iterator, iterator> equal_range(const key_type& ket);
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_type& key) const;
+
+  iterator lower_bound(const key_type& key);
+  const_iterator lower_bound(const key_type& key) const;
+
+  iterator upper_bound(const key_type& key);
+  const_iterator upper_bound(const key_type& key) const;
+
+  // --------------------------------------------------------------------------
+  // General operations.
+  //
+  // Assume that swap invalidates iterators and references.
+  //
+  // As with std::set, equality and ordering operations for the whole flat_set
+  // are equivalent to using equal() and lexicographical_compare() on the key
+  // types, rather than using element-wise key_comp() as e.g. lower_bound()
+  // does. Implementation note: currently we use operator==() and operator<() on
+  // std::vector, because they have the same contract we need, so we use them
+  // directly for brevity and in case it is more optimal than calling equal()
+  // and lexicograhpical_compare(). If the underlying container type is changed,
+  // this code may need to be modified.
+
+  void swap(flat_set& other);
+
+  friend bool operator==(const flat_set& lhs, const flat_set& rhs) {
+    return lhs.impl_.body_ == rhs.impl_.body_;
+  }
+
+  friend bool operator!=(const flat_set& lhs, const flat_set& rhs) {
+    return !(lhs == rhs);
+  }
+
+  friend bool operator<(const flat_set& lhs, const flat_set& rhs) {
+    return lhs.impl_.body_ < rhs.impl_.body_;
+  }
+
+  friend bool operator>(const flat_set& lhs, const flat_set& rhs) {
+    return rhs < lhs;
+  }
+
+  friend bool operator>=(const flat_set& lhs, const flat_set& rhs) {
+    return !(lhs < rhs);
+  }
+
+  friend bool operator<=(const flat_set& lhs, const flat_set& rhs) {
+    return !(lhs > rhs);
+  }
+
+  friend void swap(flat_set& lhs, flat_set& rhs) { lhs.swap(rhs); }
+
+ private:
+  const flat_set& as_const() { return *this; }
+
+  iterator const_cast_it(const_iterator c_it) {
+    auto distance = std::distance(cbegin(), c_it);
+    return std::next(begin(), distance);
+  }
+
+  void sort_and_unique() {
+    // std::set sorts elements preserving stability because it doesn't have any
+    // performance wins in not doing that. We do, so we use an unstable sort.
+    std::sort(begin(), end(), value_comp());
+    erase(std::unique(begin(), end(),
+                      [this](const value_type& lhs, const value_type& rhs) {
+                        // lhs is already <= rhs due to sort, therefore
+                        // !(lhs < rhs) <=> lhs == rhs.
+                        return !value_comp()(lhs, rhs);
+                      }),
+          end());
+  }
+
+  // To support comparators that may not be possible to default-construct, we
+  // have to store an instance of Compare. Using this to store all internal
+  // state of flat_set and using private inheritance to store compare lets us
+  // take advantage of an empty base class optimization to avoid extra space in
+  // the common case when Compare has no state.
+  struct Impl : private Compare {
+    Impl() = default;
+
+    template <class Cmp, class... Body>
+    explicit Impl(Cmp&& compare_arg, Body&&... underlying_type_args)
+        : Compare(std::forward<Cmp>(compare_arg)),
+          body_(std::forward<Body>(underlying_type_args)...) {}
+
+    Compare compare() const { return *this; }
+
+    underlying_type body_;
+  } impl_;
+};
+
+// ----------------------------------------------------------------------------
+// Lifetime.
+
+template <class Key, class Compare>
+flat_set<Key, Compare>::flat_set() = default;
+
+template <class Key, class Compare>
+flat_set<Key, Compare>::flat_set(const Compare& comp) : impl_(comp) {}
+
+template <class Key, class Compare>
+template <class InputIterator>
+flat_set<Key, Compare>::flat_set(InputIterator first,
+                                 InputIterator last,
+                                 const Compare& comp)
+    : impl_(comp, first, last) {
+  sort_and_unique();
+}
+
+template <class Key, class Compare>
+flat_set<Key, Compare>::flat_set(const flat_set&) = default;
+
+template <class Key, class Compare>
+flat_set<Key, Compare>::flat_set(flat_set&&) = default;
+
+template <class Key, class Compare>
+flat_set<Key, Compare>::flat_set(std::initializer_list<value_type> ilist,
+                                 const Compare& comp)
+    : flat_set(std::begin(ilist), std::end(ilist), comp) {}
+
+template <class Key, class Compare>
+flat_set<Key, Compare>::~flat_set() = default;
+
+// ----------------------------------------------------------------------------
+// Assignments.
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::operator=(const flat_set&) -> flat_set& = default;
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::operator=(flat_set &&) -> flat_set& = default;
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::operator=(std::initializer_list<value_type> ilist)
+    -> flat_set& {
+  impl_.body_ = ilist;
+  sort_and_unique();
+  return *this;
+}
+
+// ----------------------------------------------------------------------------
+// Memory management.
+
+template <class Key, class Compare>
+void flat_set<Key, Compare>::reserve(size_type new_capacity) {
+  impl_.body_.reserve(new_capacity);
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::capacity() const -> size_type {
+  return impl_.body_.capacity();
+}
+
+template <class Key, class Compare>
+void flat_set<Key, Compare>::shrink_to_fit() {
+  impl_.body_.shrink_to_fit();
+}
+
+// ----------------------------------------------------------------------------
+// Size management.
+
+template <class Key, class Compare>
+void flat_set<Key, Compare>::clear() {
+  impl_.body_.clear();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::size() const -> size_type {
+  return impl_.body_.size();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::max_size() const -> size_type {
+  return impl_.body_.max_size();
+}
+
+template <class Key, class Compare>
+bool flat_set<Key, Compare>::empty() const {
+  return impl_.body_.empty();
+}
+
+// ----------------------------------------------------------------------------
+// Iterators.
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::begin() -> iterator {
+  return impl_.body_.begin();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::begin() const -> const_iterator {
+  return impl_.body_.begin();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::cbegin() const -> const_iterator {
+  return impl_.body_.cbegin();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::end() -> iterator {
+  return impl_.body_.end();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::end() const -> const_iterator {
+  return impl_.body_.end();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::cend() const -> const_iterator {
+  return impl_.body_.cend();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::rbegin() -> reverse_iterator {
+  return impl_.body_.rbegin();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::rbegin() const -> const_reverse_iterator {
+  return impl_.body_.rbegin();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::crbegin() const -> const_reverse_iterator {
+  return impl_.body_.crbegin();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::rend() -> reverse_iterator {
+  return impl_.body_.rend();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::rend() const -> const_reverse_iterator {
+  return impl_.body_.rend();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::crend() const -> const_reverse_iterator {
+  return impl_.body_.crend();
+}
+
+// ----------------------------------------------------------------------------
+// Insert operations.
+//
+// Currently we use position_hint the same way as eastl or boost:
+// https://github.com/electronicarts/EASTL/blob/master/include/EASTL/vector_set.h#L493
+//
+// We duplicate code between copy and move version so that we can avoid
+// creating a temporary value.
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::insert(const value_type& val)
+    -> std::pair<iterator, bool> {
+  auto position = lower_bound(val);
+
+  if (position == end() || value_comp()(val, *position))
+    return {impl_.body_.insert(position, val), true};
+
+  return {position, false};
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::insert(value_type&& val)
+    -> std::pair<iterator, bool> {
+  auto position = lower_bound(val);
+
+  if (position == end() || value_comp()(val, *position))
+    return {impl_.body_.insert(position, std::move(val)), true};
+
+  return {position, false};
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::insert(const_iterator position_hint,
+                                    const value_type& val) -> iterator {
+  if (position_hint == end() || value_comp()(val, *position_hint)) {
+    if (position_hint == begin() || value_comp()(*(position_hint - 1), val))
+      // We have to cast away const because of crbug.com/677044.
+      return impl_.body_.insert(const_cast_it(position_hint), val);
+  }
+  return insert(val).first;
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::insert(const_iterator position_hint,
+                                    value_type&& val) -> iterator {
+  if (position_hint == end() || value_comp()(val, *position_hint)) {
+    if (position_hint == begin() || value_comp()(*(position_hint - 1), val))
+      // We have to cast away const because of crbug.com/677044.
+      return impl_.body_.insert(const_cast_it(position_hint), std::move(val));
+  }
+  return insert(std::move(val)).first;
+}
+
+template <class Key, class Compare>
+template <class... Args>
+auto flat_set<Key, Compare>::emplace(Args&&... args)
+    -> std::pair<iterator, bool> {
+  return insert(value_type(std::forward<Args>(args)...));
+}
+
+template <class Key, class Compare>
+template <class... Args>
+auto flat_set<Key, Compare>::emplace_hint(const_iterator position_hint,
+                                          Args&&... args) -> iterator {
+  return insert(position_hint, value_type(std::forward<Args>(args)...));
+}
+
+// ----------------------------------------------------------------------------
+// Erase operations.
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::erase(const_iterator position) -> iterator {
+  // We have to cast away const because of crbug.com/677044.
+  return impl_.body_.erase(const_cast_it(position));
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::erase(const key_type& val) -> size_type {
+  auto eq_range = equal_range(val);
+  auto res = std::distance(eq_range.first, eq_range.second);
+  // We have to cast away const because of crbug.com/677044.
+  erase(const_cast_it(eq_range.first), const_cast_it(eq_range.second));
+  return res;
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::erase(const_iterator first, const_iterator last)
+    -> iterator {
+  // We have to cast away const because of crbug.com/677044.
+  return impl_.body_.erase(const_cast_it(first), const_cast_it(last));
+}
+
+// ----------------------------------------------------------------------------
+// Comparators.
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::key_comp() const -> key_compare {
+  return impl_.compare();
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::value_comp() const -> value_compare {
+  return impl_.compare();
+}
+
+// ----------------------------------------------------------------------------
+// Search operations.
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::count(const key_type& key) const -> size_type {
+  auto eq_range = equal_range(key);
+  return std::distance(eq_range.first, eq_range.second);
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::find(const key_type& key) -> iterator {
+  return const_cast_it(as_const().find(key));
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::find(const key_type& key) const -> const_iterator {
+  auto eq_range = equal_range(key);
+  return (eq_range.first == eq_range.second) ? end() : eq_range.first;
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::equal_range(const key_type& key)
+    -> std::pair<iterator, iterator> {
+  auto res = as_const().equal_range(key);
+  return {const_cast_it(res.first), const_cast_it(res.second)};
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::equal_range(const key_type& key) const
+    -> std::pair<const_iterator, const_iterator> {
+  auto lower = lower_bound(key);
+
+  if (lower == end() || key_comp()(key, *lower))
+    return {lower, lower};
+
+  return {lower, std::next(lower)};
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::lower_bound(const key_type& key) -> iterator {
+  return const_cast_it(as_const().lower_bound(key));
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::lower_bound(const key_type& key) const
+    -> const_iterator {
+  return std::lower_bound(begin(), end(), key, key_comp());
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::upper_bound(const key_type& key) -> iterator {
+  return const_cast_it(as_const().upper_bound(key));
+}
+
+template <class Key, class Compare>
+auto flat_set<Key, Compare>::upper_bound(const key_type& key) const
+    -> const_iterator {
+  return std::upper_bound(begin(), end(), key, key_comp());
+}
+
+// ----------------------------------------------------------------------------
+// General operations.
+
+template <class Key, class Compare>
+void flat_set<Key, Compare>::swap(flat_set& other) {
+  std::swap(impl_, other.impl_);
+}
+
+}  // namespace  base
+
+#endif  // BASE_CONTAINERS_FLAT_SET_H_