ccls/src/lru_cache.h

/* Copyright 2017-2018 ccls Authors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/

#pragma once

#include <algorithm>
#include <cassert>
#include <limits>
#include <memory>
#include <vector>

// Cache that evicts old entries which have not been used recently. Implemented
// using array/linear search so this works well for small array sizes.
template <typename TKey, typename TValue> struct LruCache {
  explicit LruCache(int max_entries);

  // Fetches an entry for |key|. If it does not exist, |allocator| will be
  // invoked to create one.
  template <typename TAllocator>
  std::shared_ptr<TValue> Get(const TKey &key, TAllocator allocator);
  // Fetches the entry for |filename| and updates it's usage so it is less
  // likely to be evicted.
  std::shared_ptr<TValue> TryGet(const TKey &key);
  // TryGetEntry, except the entry is removed from the cache.
  std::shared_ptr<TValue> TryTake(const TKey &key);
  // Inserts an entry. Evicts the oldest unused entry if there is no space.
  void Insert(const TKey &key, const std::shared_ptr<TValue> &value);

  // Call |func| on existing entries. If |func| returns false iteration
  // temrinates early.
  template <typename TFunc> void IterateValues(TFunc func);

  // Empties the cache
  void Clear(void);

private:
  // There is a global score counter, when we access an element we increase
  // its score to the current global value, so it has the highest overall
  // score. This means that the oldest/least recently accessed value has the
  // lowest score.
  //
  // There is a bit of special logic to handle score overlow.
  struct Entry {
    uint32_t score = 0;
    TKey key;
    std::shared_ptr<TValue> value;
    bool operator<(const Entry &other) const { return score < other.score; }
  };

  void IncrementScore();

  std::vector<Entry> entries_;
  int max_entries_ = 1;
  uint32_t next_score_ = 0;
};

template <typename TKey, typename TValue>
LruCache<TKey, TValue>::LruCache(int max_entries) : max_entries_(max_entries) {
  assert(max_entries > 0);
}

template <typename TKey, typename TValue>
template <typename TAllocator>
std::shared_ptr<TValue> LruCache<TKey, TValue>::Get(const TKey &key,
                                                    TAllocator allocator) {
  std::shared_ptr<TValue> result = TryGet(key);
  if (!result)
    Insert(key, result = allocator());
  return result;
}

template <typename TKey, typename TValue>
std::shared_ptr<TValue> LruCache<TKey, TValue>::TryGet(const TKey &key) {
  // Assign new score.
  for (Entry &entry : entries_) {
    if (entry.key == key) {
      entry.score = next_score_;
      IncrementScore();
      return entry.value;
    }
  }

  return nullptr;
}

template <typename TKey, typename TValue>
std::shared_ptr<TValue> LruCache<TKey, TValue>::TryTake(const TKey &key) {
  for (size_t i = 0; i < entries_.size(); ++i) {
    if (entries_[i].key == key) {
      std::shared_ptr<TValue> copy = entries_[i].value;
      entries_.erase(entries_.begin() + i);
      return copy;
    }
  }

  return nullptr;
}

template <typename TKey, typename TValue>
void LruCache<TKey, TValue>::Insert(const TKey &key,
                                    const std::shared_ptr<TValue> &value) {
  if ((int)entries_.size() >= max_entries_)
    entries_.erase(std::min_element(entries_.begin(), entries_.end()));

  Entry entry;
  entry.score = next_score_;
  IncrementScore();
  entry.key = key;
  entry.value = value;
  entries_.push_back(entry);
}

template <typename TKey, typename TValue>
template <typename TFunc>
void LruCache<TKey, TValue>::IterateValues(TFunc func) {
  for (Entry &entry : entries_) {
    if (!func(entry.value))
      break;
  }
}

template <typename TKey, typename TValue>
void LruCache<TKey, TValue>::IncrementScore() {
  // Overflow.
  if (++next_score_ == 0) {
    std::sort(entries_.begin(), entries_.end());
    for (Entry &entry : entries_)
      entry.score = next_score_++;
  }
}

template <typename TKey, typename TValue>
void LruCache<TKey, TValue>::Clear(void) {
  entries_.clear();
  next_score_ = 0;
}