306 lines
11 KiB
C++
306 lines
11 KiB
C++
// Copyright (c) 2011 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// This file contains a template for a Most Recently Used cache that allows
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// constant-time access to items using a key, but easy identification of the
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// least-recently-used items for removal. Each key can only be associated with
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// one payload item at a time.
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//
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// The key object will be stored twice, so it should support efficient copying.
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//
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// NOTE: While all operations are O(1), this code is written for
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// legibility rather than optimality. If future profiling identifies this as
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// a bottleneck, there is room for smaller values of 1 in the O(1). :]
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#ifndef BASE_CONTAINERS_MRU_CACHE_H_
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#define BASE_CONTAINERS_MRU_CACHE_H_
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#include <list>
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#include <map>
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#include <utility>
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#include "base/basictypes.h"
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#include "base/containers/hash_tables.h"
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#include "base/logging.h"
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namespace base {
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// MRUCacheBase ----------------------------------------------------------------
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// This template is used to standardize map type containers that can be used
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// by MRUCacheBase. This level of indirection is necessary because of the way
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// that template template params and default template params interact.
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template <class KeyType, class ValueType>
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struct MRUCacheStandardMap {
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typedef std::map<KeyType, ValueType> Type;
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};
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// Base class for the MRU cache specializations defined below.
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// The deletor will get called on all payloads that are being removed or
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// replaced.
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template <class KeyType, class PayloadType, class DeletorType,
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template <typename, typename> class MapType = MRUCacheStandardMap>
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class MRUCacheBase {
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public:
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// The payload of the list. This maintains a copy of the key so we can
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// efficiently delete things given an element of the list.
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typedef std::pair<KeyType, PayloadType> value_type;
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private:
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typedef std::list<value_type> PayloadList;
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typedef typename MapType<KeyType,
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typename PayloadList::iterator>::Type KeyIndex;
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public:
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typedef typename PayloadList::size_type size_type;
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typedef typename PayloadList::iterator iterator;
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typedef typename PayloadList::const_iterator const_iterator;
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typedef typename PayloadList::reverse_iterator reverse_iterator;
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typedef typename PayloadList::const_reverse_iterator const_reverse_iterator;
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enum { NO_AUTO_EVICT = 0 };
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// The max_size is the size at which the cache will prune its members to when
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// a new item is inserted. If the caller wants to manager this itself (for
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// example, maybe it has special work to do when something is evicted), it
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// can pass NO_AUTO_EVICT to not restrict the cache size.
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explicit MRUCacheBase(size_type max_size) : max_size_(max_size) {
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}
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MRUCacheBase(size_type max_size, const DeletorType& deletor)
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: max_size_(max_size), deletor_(deletor) {
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}
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virtual ~MRUCacheBase() {
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iterator i = begin();
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while (i != end())
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i = Erase(i);
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}
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size_type max_size() const { return max_size_; }
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// Inserts a payload item with the given key. If an existing item has
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// the same key, it is removed prior to insertion. An iterator indicating the
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// inserted item will be returned (this will always be the front of the list).
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//
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// The payload will be copied. In the case of an OwningMRUCache, this function
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// will take ownership of the pointer.
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iterator Put(const KeyType& key, const PayloadType& payload) {
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// Remove any existing payload with that key.
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typename KeyIndex::iterator index_iter = index_.find(key);
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if (index_iter != index_.end()) {
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// Erase the reference to it. This will call the deletor on the removed
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// element. The index reference will be replaced in the code below.
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Erase(index_iter->second);
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} else if (max_size_ != NO_AUTO_EVICT) {
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// New item is being inserted which might make it larger than the maximum
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// size: kick the oldest thing out if necessary.
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ShrinkToSize(max_size_ - 1);
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}
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ordering_.push_front(value_type(key, payload));
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index_.insert(std::make_pair(key, ordering_.begin()));
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return ordering_.begin();
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}
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// Retrieves the contents of the given key, or end() if not found. This method
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// has the side effect of moving the requested item to the front of the
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// recency list.
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//
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// TODO(brettw) We may want a const version of this function in the future.
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iterator Get(const KeyType& key) {
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typename KeyIndex::iterator index_iter = index_.find(key);
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if (index_iter == index_.end())
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return end();
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typename PayloadList::iterator iter = index_iter->second;
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// Move the touched item to the front of the recency ordering.
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ordering_.splice(ordering_.begin(), ordering_, iter);
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return ordering_.begin();
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}
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// Retrieves the payload associated with a given key and returns it via
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// result without affecting the ordering (unlike Get).
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//
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// TODO(brettw) We may want a const version of this function in the future.
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iterator Peek(const KeyType& key) {
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typename KeyIndex::const_iterator index_iter = index_.find(key);
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if (index_iter == index_.end())
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return end();
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return index_iter->second;
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}
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// Erases the item referenced by the given iterator. An iterator to the item
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// following it will be returned. The iterator must be valid.
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iterator Erase(iterator pos) {
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deletor_(pos->second);
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index_.erase(pos->first);
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return ordering_.erase(pos);
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}
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// MRUCache entries are often processed in reverse order, so we add this
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// convenience function (not typically defined by STL containers).
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reverse_iterator Erase(reverse_iterator pos) {
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// We have to actually give it the incremented iterator to delete, since
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// the forward iterator that base() returns is actually one past the item
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// being iterated over.
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return reverse_iterator(Erase((++pos).base()));
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}
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// Shrinks the cache so it only holds |new_size| items. If |new_size| is
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// bigger or equal to the current number of items, this will do nothing.
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void ShrinkToSize(size_type new_size) {
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for (size_type i = size(); i > new_size; i--)
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Erase(rbegin());
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}
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// Deletes everything from the cache.
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void Clear() {
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for (typename PayloadList::iterator i(ordering_.begin());
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i != ordering_.end(); ++i)
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deletor_(i->second);
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index_.clear();
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ordering_.clear();
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}
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// Returns the number of elements in the cache.
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size_type size() const {
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// We don't use ordering_.size() for the return value because
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// (as a linked list) it can be O(n).
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DCHECK(index_.size() == ordering_.size());
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return index_.size();
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}
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// Allows iteration over the list. Forward iteration starts with the most
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// recent item and works backwards.
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//
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// Note that since these iterators are actually iterators over a list, you
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// can keep them as you insert or delete things (as long as you don't delete
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// the one you are pointing to) and they will still be valid.
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iterator begin() { return ordering_.begin(); }
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const_iterator begin() const { return ordering_.begin(); }
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iterator end() { return ordering_.end(); }
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const_iterator end() const { return ordering_.end(); }
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reverse_iterator rbegin() { return ordering_.rbegin(); }
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const_reverse_iterator rbegin() const { return ordering_.rbegin(); }
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reverse_iterator rend() { return ordering_.rend(); }
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const_reverse_iterator rend() const { return ordering_.rend(); }
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bool empty() const { return ordering_.empty(); }
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private:
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PayloadList ordering_;
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KeyIndex index_;
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size_type max_size_;
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DeletorType deletor_;
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DISALLOW_COPY_AND_ASSIGN(MRUCacheBase);
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};
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// MRUCache --------------------------------------------------------------------
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// A functor that does nothing. Used by the MRUCache.
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template<class PayloadType>
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class MRUCacheNullDeletor {
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public:
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void operator()(PayloadType& payload) {
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}
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};
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// A container that does not do anything to free its data. Use this when storing
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// value types (as opposed to pointers) in the list.
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template <class KeyType, class PayloadType>
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class MRUCache : public MRUCacheBase<KeyType,
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PayloadType,
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MRUCacheNullDeletor<PayloadType> > {
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private:
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typedef MRUCacheBase<KeyType, PayloadType,
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MRUCacheNullDeletor<PayloadType> > ParentType;
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public:
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// See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
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explicit MRUCache(typename ParentType::size_type max_size)
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: ParentType(max_size) {
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}
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virtual ~MRUCache() {
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}
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private:
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DISALLOW_COPY_AND_ASSIGN(MRUCache);
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};
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// OwningMRUCache --------------------------------------------------------------
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template<class PayloadType>
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class MRUCachePointerDeletor {
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public:
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void operator()(PayloadType& payload) {
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delete payload;
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}
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};
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// A cache that owns the payload type, which must be a non-const pointer type.
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// The pointers will be deleted when they are removed, replaced, or when the
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// cache is destroyed.
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template <class KeyType, class PayloadType>
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class OwningMRUCache
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: public MRUCacheBase<KeyType,
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PayloadType,
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MRUCachePointerDeletor<PayloadType> > {
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private:
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typedef MRUCacheBase<KeyType, PayloadType,
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MRUCachePointerDeletor<PayloadType> > ParentType;
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public:
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// See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
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explicit OwningMRUCache(typename ParentType::size_type max_size)
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: ParentType(max_size) {
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}
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virtual ~OwningMRUCache() {
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}
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private:
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DISALLOW_COPY_AND_ASSIGN(OwningMRUCache);
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};
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// HashingMRUCache ------------------------------------------------------------
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template <class KeyType, class ValueType>
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struct MRUCacheHashMap {
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typedef base::hash_map<KeyType, ValueType> Type;
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};
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// This class is similar to MRUCache, except that it uses base::hash_map as
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// the map type instead of std::map. Note that your KeyType must be hashable
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// to use this cache.
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template <class KeyType, class PayloadType>
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class HashingMRUCache : public MRUCacheBase<KeyType,
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PayloadType,
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MRUCacheNullDeletor<PayloadType>,
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MRUCacheHashMap> {
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private:
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typedef MRUCacheBase<KeyType, PayloadType,
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MRUCacheNullDeletor<PayloadType>,
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MRUCacheHashMap> ParentType;
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public:
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// See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
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explicit HashingMRUCache(typename ParentType::size_type max_size)
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: ParentType(max_size) {
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}
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virtual ~HashingMRUCache() {
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}
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private:
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DISALLOW_COPY_AND_ASSIGN(HashingMRUCache);
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};
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} // namespace base
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#endif // BASE_CONTAINERS_MRU_CACHE_H_
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