Simple LRU class

media/blink/lru.h

Index: media/blink/lru.h
diff --git a/media/blink/lru.h b/media/blink/lru.h
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+// Copyright 2015 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 MEDIA_BLINK_LRU_H_
+#define MEDIA_BLINK_LRU_H_
+
+#include <list>
+
+#include "base/containers/hash_tables.h"
+#include "base/macros.h"
+
+namespace media {
+
+// Simple LRU (least recently used) class.
+// Keeps track of a set of data and lets you get the least recently used
+// (oldest) element at any time. All operations are O(1). Elements are expected
+// to be hashable and unique.
+// Example:
+//  LRU<int> lru;
+//  lru.Insert(1);
+//  lru.Insert(2);
+//  lru.Insert(3);
+//  lru.Use(1);
+//  cout << lru.Pop();  // this will print "2"
+template <typename T>
+class LRU {
+ public:
+  LRU() {}
+
+  // Adds |x| to LRU.
+  // |x| must not already be in the LRU.
+  // Faster than Use(), and will DCHECK that |x| is not in the LRU.
+  void Insert(const T& x) {
+    DCHECK(!Contains(x));
+    lru_.push_front(x);
+    pos_[x] = lru_.begin();
+  }
+
+  // Removes |x| from LRU.
+  // |x| must be in the LRU.
+  void Remove(const T& x) {
+    DCHECK(Contains(x));
+    lru_.erase(pos_[x]);
+    pos_.erase(x);
+  }
+
+  // Moves |x| to front of LRU. (most recently used)
+  // If |x| is not in LRU, it is added.
+  // Please call Insert() if you know that |x| is not in the LRU.
+  void Use(const T& x) {
+    if (Contains(x))
+      Remove(x);
+    Insert(x);
+  }
+
+  bool Empty() const { return lru_.empty(); }
+
+  // Returns the Least Recently Used T and removes it.
+  T Pop() {
+    DCHECK(!Empty());
+    T ret = lru_.back();
+    lru_.pop_back();
+    pos_.erase(ret);
+    return ret;
+  }
+
+  // Returns the Least Recently Used T _without_ removing it.
+  T Peek() const {
+    DCHECK(!Empty());
+    return lru_.back();
+  }
+
+  bool Contains(const T& x) const { return pos_.find(x) != pos_.end(); }
+
+  size_t Size() const { return pos_.size(); }
+
+ private:
+  friend class LRUTest;
+
+  // Linear list of elements, most recently used first.
+  std::list<T> lru_;
+
+  // Maps element values to positions in the list so that we
+  // can quickly remove elements.
+  base::hash_map<T, typename std::list<T>::iterator> pos_;
+
+  DISALLOW_COPY_AND_ASSIGN(LRU);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BLINK_LRU_H