plugins/slice/ObjectSizeCache.h

/** @file cache.h

  Metadata cache to store object sizes.

  @section license License

  Licensed to the Apache Software Foundation (ASF) under one
  or more contributor license agreements.  See the NOTICE file
  distributed with this work for additional information
  regarding copyright ownership.  The ASF licenses this file
  to you 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.
 */

#include <optional>
#include <vector>
#include <string>
#include <string_view>
#include <unordered_map>
#include <mutex>

class ObjectSizeCache
{
public:
  using cache_size_type  = size_t;
  using object_size_type = uint64_t;

  ObjectSizeCache(cache_size_type capacity);

  /**
   * @brief Get an object size from cache.
   *
   * @param url The URL of the object
   * @return std::optional<uint64_t> If the object size was found, return the size of the object.  If not, return std::nullopt.
   */
  std::optional<object_size_type> get(const std::string_view url);

  /**
   * @brief Add an object size to cache.
   *
   * @param url The URL of the object
   * @param object_size The size of the object
   */
  void set(const std::string_view url, object_size_type object_size);

  /**
   * @brief Remove an object from cache
   *
   * @param url The URL of the object
   */
  void remove(const std::string_view url);

  // Max capacity of the cache
  cache_size_type cache_capacity();

  // Current number of used entries in the cache
  cache_size_type cache_count();

private:
  void find_eviction_slot();

  cache_size_type                                       _cache_capacity;
  cache_size_type                                       _hand{0};
  std::vector<std::string>                              _urls;
  std::vector<object_size_type>                         _object_sizes;
  std::vector<bool>                                     _visits;
  std::unordered_map<std::string_view, cache_size_type> _index;
  std::mutex                                            _mutex;
};