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// Copyright 2011 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// This file contains a template for a Least Recently Used cache that allows
// constant-time access to items, but easy identification of the
// least-recently-used items for removal. Variations exist to support use as a
// Map (`base::LRUCache`), HashMap (`base::HashingLRUCache`), Set
// (`base::LRUCacheSet`), or HashSet (`base::HashingLRUCacheSet`). These are
// implemented as aliases of `base::internal::LRUCacheBase`, defined at the
// bottom of this file.
//
// The key object (which is identical to the value, in the Set variations) will
// be stored twice, so it should support efficient copying.
#ifndef BASE_CONTAINERS_LRU_CACHE_H_
#define BASE_CONTAINERS_LRU_CACHE_H_
#include <stddef.h>
#include <algorithm>
#include <functional>
#include <list>
#include <map>
#include <type_traits>
#include <unordered_map>
#include <utility>
#include "base/check.h"
#include "base/functional/identity.h"
namespace base {
namespace trace_event::internal {
template <class LruCacheType>
size_t DoEstimateMemoryUsageForLruCache(const LruCacheType&);
} // namespace trace_event::internal
namespace internal {
struct GetKeyFromKVPair {
template <typename T1, typename T2>
constexpr const T1& operator()(const std::pair<T1, T2>& pair) {
return pair.first;
}
};
// Base class for the LRU cache specializations defined below.
template <class ValueType, class GetKeyFromValue, class KeyIndexTemplate>
class LRUCacheBase {
public:
// The contents of the list. This must contain a copy of the key (that may be
// extracted via `GetKeyFromValue()(value)` so we can efficiently delete
// things given an element of the list.
using value_type = ValueType;
private:
using ValueList = std::list<value_type>;
using KeyIndex =
typename KeyIndexTemplate::template Type<typename ValueList::iterator>;
public:
using size_type = typename ValueList::size_type;
using key_type = typename KeyIndex::key_type;
using iterator = typename ValueList::iterator;
using const_iterator = typename ValueList::const_iterator;
using reverse_iterator = typename ValueList::reverse_iterator;
using const_reverse_iterator = typename ValueList::const_reverse_iterator;
enum { NO_AUTO_EVICT = 0 };
// The max_size is the size at which the cache will prune its members to when
// a new item is inserted. If the caller wants to manage this itself (for
// example, maybe it has special work to do when something is evicted), it
// can pass NO_AUTO_EVICT to not restrict the cache size.
explicit LRUCacheBase(size_type max_size) : max_size_(max_size) {}
LRUCacheBase(const LRUCacheBase&) = delete;
LRUCacheBase& operator=(const LRUCacheBase&) = delete;
~LRUCacheBase() = default;
size_type max_size() const { return max_size_; }
// Inserts an item into the list. If an existing item has the same key, it is
// removed prior to insertion. An iterator indicating the inserted item will
// be returned (this will always be the front of the list).
// In the map variations of this container, `value_type` is a `std::pair` and
// it's preferred to use the `Put(k, v)` overload of this method.
iterator Put(value_type&& value) {
// Remove any existing item with that key.
key_type key = GetKeyFromValue{}(value);
typename KeyIndex::iterator index_iter = index_.find(key);
if (index_iter != index_.end()) {
// Erase the reference to it. The index reference will be replaced in the
// code below.
Erase(index_iter->second);
} else if (max_size_ != NO_AUTO_EVICT) {
// New item is being inserted which might make it larger than the maximum
// size: kick the oldest thing out if necessary.
ShrinkToSize(max_size_ - 1);
}
ordering_.push_front(std::move(value));
index_.emplace(std::move(key), ordering_.begin());
return ordering_.begin();
}
// Inserts an item into the list. If an existing item has the same key, it is
// removed prior to insertion. An iterator indicating the inserted item will
// be returned (this will always be the front of the list).
template <
class K,
class V,
class MapKeyGetter = GetKeyFromKVPair,
class = std::enable_if_t<std::is_same_v<MapKeyGetter, GetKeyFromValue>>>
iterator Put(K&& key, V&& value) {
return Put(value_type{std::forward<K>(key), std::forward<V>(value)});
}
// Retrieves the contents of the given key, or end() if not found. This method
// has the side effect of moving the requested item to the front of the
// recency list.
iterator Get(const key_type& key) {
typename KeyIndex::iterator index_iter = index_.find(key);
if (index_iter == index_.end())
return end();
typename ValueList::iterator iter = index_iter->second;
// Move the touched item to the front of the recency ordering.
ordering_.splice(ordering_.begin(), ordering_, iter);
return ordering_.begin();
}
// Retrieves the item associated with a given key and returns it via
// result without affecting the ordering (unlike Get()).
iterator Peek(const key_type& key) {
typename KeyIndex::const_iterator index_iter = index_.find(key);
if (index_iter == index_.end())
return end();
return index_iter->second;
}
const_iterator Peek(const key_type& key) const {
typename KeyIndex::const_iterator index_iter = index_.find(key);
if (index_iter == index_.end())
return end();
return index_iter->second;
}
// Exchanges the contents of |this| by the contents of the |other|.
void Swap(LRUCacheBase& other) {
ordering_.swap(other.ordering_);
index_.swap(other.index_);
std::swap(max_size_, other.max_size_);
}
// Erases the item referenced by the given iterator. An iterator to the item
// following it will be returned. The iterator must be valid.
iterator Erase(iterator pos) {
index_.erase(GetKeyFromValue()(*pos));
return ordering_.erase(pos);
}
// LRUCache entries are often processed in reverse order, so we add this
// convenience function (not typically defined by STL containers).
reverse_iterator Erase(reverse_iterator pos) {
// We have to actually give it the incremented iterator to delete, since
// the forward iterator that base() returns is actually one past the item
// being iterated over.
return reverse_iterator(Erase((++pos).base()));
}
// Shrinks the cache so it only holds |new_size| items. If |new_size| is
// bigger or equal to the current number of items, this will do nothing.
void ShrinkToSize(size_type new_size) {
for (size_type i = size(); i > new_size; i--)
Erase(rbegin());
}
// Deletes everything from the cache.
void Clear() {
index_.clear();
ordering_.clear();
}
// Returns the number of elements in the cache.
size_type size() const {
// We don't use ordering_.size() for the return value because
// (as a linked list) it can be O(n).
DCHECK(index_.size() == ordering_.size());
return index_.size();
}
// Allows iteration over the list. Forward iteration starts with the most
// recent item and works backwards.
//
// Note that since these iterators are actually iterators over a list, you
// can keep them as you insert or delete things (as long as you don't delete
// the one you are pointing to) and they will still be valid.
iterator begin() { return ordering_.begin(); }
const_iterator begin() const { return ordering_.begin(); }
iterator end() { return ordering_.end(); }
const_iterator end() const { return ordering_.end(); }
reverse_iterator rbegin() { return ordering_.rbegin(); }
const_reverse_iterator rbegin() const { return ordering_.rbegin(); }
reverse_iterator rend() { return ordering_.rend(); }
const_reverse_iterator rend() const { return ordering_.rend(); }
bool empty() const { return ordering_.empty(); }
private:
template <class LruCacheType>
friend size_t trace_event::internal::DoEstimateMemoryUsageForLruCache(
const LruCacheType&);
ValueList ordering_;
KeyIndex index_;
size_type max_size_;
};
template <class KeyType, class KeyCompare>
struct LRUCacheKeyIndex {
template <class ValueType>
using Type = std::map<KeyType, ValueType, KeyCompare>;
};
template <class KeyType, class KeyHash, class KeyEqual>
struct HashingLRUCacheKeyIndex {
template <class ValueType>
using Type = std::unordered_map<KeyType, ValueType, KeyHash, KeyEqual>;
};
} // namespace internal
// Implements an LRU cache of `ValueType`, where each value can be uniquely
// referenced by `KeyType`. Entries can be iterated in order of
// least-recently-used to most-recently-used by iterating from `rbegin()` to
// `rend()`, where a "use" is defined as a call to `Put(k, v)` or `Get(k)`.
template <class KeyType, class ValueType, class KeyCompare = std::less<KeyType>>
using LRUCache =
internal::LRUCacheBase<std::pair<KeyType, ValueType>,
internal::GetKeyFromKVPair,
internal::LRUCacheKeyIndex<KeyType, KeyCompare>>;
// Implements an LRU cache of `ValueType`, where each value can be uniquely
// referenced by `KeyType`, and `KeyType` may be hashed for O(1) insertion,
// removal, and lookup. Entries can be iterated in order of least-recently-used
// to most-recently-used by iterating from `rbegin()` to `rend()`, where a "use"
// is defined as a call to `Put(k, v)` or `Get(k)`.
template <class KeyType,
class ValueType,
class KeyHash = std::hash<KeyType>,
class KeyEqual = std::equal_to<KeyType>>
using HashingLRUCache = internal::LRUCacheBase<
std::pair<KeyType, ValueType>,
internal::GetKeyFromKVPair,
internal::HashingLRUCacheKeyIndex<KeyType, KeyHash, KeyEqual>>;
// Implements an LRU cache of `ValueType`, where each value is unique. Entries
// can be iterated in order of least-recently-used to most-recently-used by
// iterating from `rbegin()` to `rend()`, where a "use" is defined as a call to
// `Put(v)` or `Get(v)`.
template <class ValueType, class Compare = std::less<ValueType>>
using LRUCacheSet =
internal::LRUCacheBase<ValueType,
identity,
internal::LRUCacheKeyIndex<ValueType, Compare>>;
// Implements an LRU cache of `ValueType`, where is value is unique, and may be
// hashed for O(1) insertion, removal, and lookup. Entries can be iterated in
// order of least-recently-used to most-recently-used by iterating from
// `rbegin()` to `rend()`, where a "use" is defined as a call to `Put(v)` or
// `Get(v)`.
template <class ValueType,
class Hash = std::hash<ValueType>,
class Equal = std::equal_to<ValueType>>
using HashingLRUCacheSet = internal::LRUCacheBase<
ValueType,
identity,
internal::HashingLRUCacheKeyIndex<ValueType, Hash, Equal>>;
} // namespace base
#endif // BASE_CONTAINERS_LRU_CACHE_H_