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TMessagesProj/jni/voip/webrtc/base/containers/mru_cache.h

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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). :]
+
+#ifndef BASE_CONTAINERS_MRU_CACHE_H_
+#define BASE_CONTAINERS_MRU_CACHE_H_
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <functional>
+#include <list>
+#include <map>
+#include <unordered_map>
+#include <utility>
+
+#include "base/logging.h"
+#include "base/macros.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 =
+              MRUCacheStandardMap>
+class MRUCacheBase {
+ public:
+  // 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;
+
+ private:
+  typedef std::list<value_type> PayloadList;
+  typedef typename MapType<KeyType,
+                           typename PayloadList::iterator,
+                           HashOrCompareType>::Type KeyIndex;
+
+ public:
+  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.
+      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_.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) {
+    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(pos->first);
+    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--)
+      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 MruCacheType>
+  friend size_t trace_event::internal::DoEstimateMemoryUsageForMruCache(
+      const MruCacheType&);
+
+  PayloadList ordering_;
+  KeyIndex index_;
+
+  size_type max_size_;
+
+  DISALLOW_COPY_AND_ASSIGN(MRUCacheBase);
+};
+
+// 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> {
+ private:
+  using ParentType = MRUCacheBase<KeyType, PayloadType, CompareType>;
+
+ public:
+  // See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
+  explicit MRUCache(typename ParentType::size_type max_size)
+      : ParentType(max_size) {}
+  virtual ~MRUCache() = default;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MRUCache);
+};
+
+// 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> {
+ private:
+  using ParentType =
+      MRUCacheBase<KeyType, PayloadType, HashType, MRUCacheHashMap>;
+
+ public:
+  // See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
+  explicit HashingMRUCache(typename ParentType::size_type max_size)
+      : ParentType(max_size) {}
+  virtual ~HashingMRUCache() = default;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(HashingMRUCache);
+};
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_MRU_CACHE_H_