quic/core/quic_lru_cache.h

// Copyright (c) 2018 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 QUICHE_QUIC_CORE_QUIC_LRU_CACHE_H_
#define QUICHE_QUIC_CORE_QUIC_LRU_CACHE_H_

#include <memory>

#include "quic/platform/api/quic_export.h"
#include "quic/platform/api/quic_flag_utils.h"
#include "quic/platform/api/quic_flags.h"
#include "quic/platform/api/quic_logging.h"
#include "common/quiche_linked_hash_map.h"

namespace quic {

// A LRU cache that maps from type Key to Value* in QUIC.
// This cache CANNOT be shared by multiple threads (even with locks) because
// Value* returned by Lookup() can be invalid if the entry is evicted by other
// threads.
template <class K, class V, class Hash = std::hash<K>,
          class Eq = std::equal_to<K>>
class QUIC_NO_EXPORT QuicLRUCache {
 private:
  using HashMapType =
      typename quiche::QuicheLinkedHashMap<K, std::unique_ptr<V>, Hash, Eq>;

 public:
  // The iterator, if valid, points to std::pair<K, std::unique_ptr<V>>.
  using iterator = typename HashMapType::iterator;
  using const_iterator = typename HashMapType::const_iterator;
  using reverse_iterator = typename HashMapType::reverse_iterator;
  using const_reverse_iterator = typename HashMapType::const_reverse_iterator;

  explicit QuicLRUCache(size_t capacity) : capacity_(capacity) {}
  QuicLRUCache(const QuicLRUCache&) = delete;
  QuicLRUCache& operator=(const QuicLRUCache&) = delete;

  iterator begin() { return cache_.begin(); }
  const_iterator begin() const { return cache_.begin(); }

  iterator end() { return cache_.end(); }
  const_iterator end() const { return cache_.end(); }

  reverse_iterator rbegin() { return cache_.rbegin(); }
  const_reverse_iterator rbegin() const { return cache_.rbegin(); }

  reverse_iterator rend() { return cache_.rend(); }
  const_reverse_iterator rend() const { return cache_.rend(); }

  // Inserts one unit of |key|, |value| pair to the cache. Cache takes ownership
  // of inserted |value|.
  void Insert(const K& key, std::unique_ptr<V> value) {
    auto it = cache_.find(key);
    if (it != cache_.end()) {
      cache_.erase(it);
    }
    cache_.emplace(key, std::move(value));

    if (cache_.size() > capacity_) {
      cache_.pop_front();
    }
    QUICHE_DCHECK_LE(cache_.size(), capacity_);
  }

  iterator Lookup(const K& key) {
    auto iter = cache_.find(key);
    if (iter == cache_.end()) {
      return iter;
    }

    std::unique_ptr<V> value = std::move(iter->second);
    cache_.erase(iter);
    auto result = cache_.emplace(key, std::move(value));
    QUICHE_DCHECK(result.second);
    return result.first;
  }

  iterator Erase(iterator iter) { return cache_.erase(iter); }

  // Removes all entries from the cache.
  void Clear() { cache_.clear(); }

  // Returns maximum size of the cache.
  size_t MaxSize() const { return capacity_; }

  // Returns current size of the cache.
  size_t Size() const { return cache_.size(); }

 private:
  quiche::QuicheLinkedHashMap<K, std::unique_ptr<V>, Hash, Eq> cache_;
  const size_t capacity_;
};

}  // namespace quic

#endif  // QUICHE_QUIC_CORE_QUIC_LRU_CACHE_H_