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// Copyright (c) 2011 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.
// This file contains a template for a Most Recently Used cache that allows
// constant-time access to items using a key, but easy identification of the
// least-recently-used items for removal. Each key can only be associated with
// one payload item at a time.
// The key object will be stored twice, so it should support efficient copying.
// NOTE: While all operations are O(1), this code is written for
// legibility rather than optimality. If future profiling identifies this as
// a bottleneck, there is room for smaller values of 1 in the O(1). :]
#include <algorithm>
#include <functional>
#include <list>
#include <map>
#include <unordered_map>
#include <utility>
#include "base/logging.h"
#include "base/macros.h"
#include "starboard/types.h"
namespace base {
namespace trace_event {
namespace internal {
template <class MruCacheType>
size_t DoEstimateMemoryUsageForMruCache(const MruCacheType&);
} // namespace internal
} // namespace trace_event
// MRUCacheBase ----------------------------------------------------------------
// This template is used to standardize map type containers that can be used
// by MRUCacheBase. This level of indirection is necessary because of the way
// that template template params and default template params interact.
template <class KeyType, class ValueType, class CompareType>
struct MRUCacheStandardMap {
typedef std::map<KeyType, ValueType, CompareType> Type;
// Base class for the MRU cache specializations defined below.
template <class KeyType,
class PayloadType,
class HashOrCompareType,
template <typename, typename, typename> class MapType =
class MRUCacheBase {
// The payload of the list. This maintains a copy of the key so we can
// efficiently delete things given an element of the list.
typedef std::pair<KeyType, PayloadType> value_type;
typedef std::list<value_type> PayloadList;
typedef typename MapType<KeyType,
typename PayloadList::iterator,
HashOrCompareType>::Type KeyIndex;
typedef typename PayloadList::size_type size_type;
typedef typename PayloadList::iterator iterator;
typedef typename PayloadList::const_iterator const_iterator;
typedef typename PayloadList::reverse_iterator reverse_iterator;
typedef typename PayloadList::const_reverse_iterator 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 manager 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 MRUCacheBase(size_type max_size) : max_size_(max_size) {}
virtual ~MRUCacheBase() = default;
size_type max_size() const { return max_size_; }
// Inserts a payload item with the given key. 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).
// The payload will be forwarded.
template <typename Payload>
iterator Put(const KeyType& key, Payload&& payload) {
// Remove any existing payload with that key.
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.
} 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_.emplace_front(key, std::forward<Payload>(payload));
index_.emplace(key, ordering_.begin());
return ordering_.begin();
// 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 KeyType& key) {
typename KeyIndex::iterator index_iter = index_.find(key);
if (index_iter == index_.end())
return end();
typename PayloadList::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 payload associated with a given key and returns it via
// result without affecting the ordering (unlike Get).
iterator Peek(const KeyType& 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 KeyType& 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(MRUCacheBase& other) {
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) {
return ordering_.erase(pos);
// MRUCache 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--)
// Deletes everything from the cache.
void 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(); }
template <class MruCacheType>
friend size_t trace_event::internal::DoEstimateMemoryUsageForMruCache(
const MruCacheType&);
PayloadList ordering_;
KeyIndex index_;
size_type max_size_;
// MRUCache --------------------------------------------------------------------
// A container that does not do anything to free its data. Use this when storing
// value types (as opposed to pointers) in the list.
template <class KeyType,
class PayloadType,
class CompareType = std::less<KeyType>>
class MRUCache : public MRUCacheBase<KeyType, PayloadType, CompareType> {
using ParentType = MRUCacheBase<KeyType, PayloadType, CompareType>;
// See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
explicit MRUCache(typename ParentType::size_type max_size)
: ParentType(max_size) {}
virtual ~MRUCache() = default;
// HashingMRUCache ------------------------------------------------------------
template <class KeyType, class ValueType, class HashType>
struct MRUCacheHashMap {
typedef std::unordered_map<KeyType, ValueType, HashType> Type;
// This class is similar to MRUCache, except that it uses std::unordered_map as
// the map type instead of std::map. Note that your KeyType must be hashable to
// use this cache or you need to provide a hashing class.
template <class KeyType, class PayloadType, class HashType = std::hash<KeyType>>
class HashingMRUCache
: public MRUCacheBase<KeyType, PayloadType, HashType, MRUCacheHashMap> {
using ParentType =
MRUCacheBase<KeyType, PayloadType, HashType, MRUCacheHashMap>;
// See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
explicit HashingMRUCache(typename ParentType::size_type max_size)
: ParentType(max_size) {}
virtual ~HashingMRUCache() = default;
} // namespace base