LRU.h

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// 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
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// specific language governing permissions and limitations
// under the License.


#pragma once

#include <unordered_map>
#include <list>
#include <cstddef>
#include <stdexcept>
#include <utility>

namespace zilliz {
namespace milvus {
namespace cache {

template<typename key_t, typename value_t>
class LRU {
 public:
    typedef typename std::pair<key_t, value_t> key_value_pair_t;
    typedef typename std::list<key_value_pair_t>::iterator list_iterator_t;
    typedef typename std::list<key_value_pair_t>::reverse_iterator reverse_list_iterator_t;

    explicit LRU(size_t max_size) : max_size_(max_size) {
    }

    void put(const key_t &key, const value_t &value) {
        auto it = cache_items_map_.find(key);
        cache_items_list_.push_front(key_value_pair_t(key, value));
        if (it != cache_items_map_.end()) {
            cache_items_list_.erase(it->second);
            cache_items_map_.erase(it);
        }
        cache_items_map_[key] = cache_items_list_.begin();

        if (cache_items_map_.size() > max_size_) {
            auto last = cache_items_list_.end();
            last--;
            cache_items_map_.erase(last->first);
            cache_items_list_.pop_back();
        }
    }

    const value_t &get(const key_t &key) {
        auto it = cache_items_map_.find(key);
        if (it == cache_items_map_.end()) {
            throw std::range_error("There is no such key in cache");
        } else {
            cache_items_list_.splice(cache_items_list_.begin(), cache_items_list_, it->second);
            return it->second->second;
        }
    }

    void erase(const key_t &key) {
        auto it = cache_items_map_.find(key);
        if (it != cache_items_map_.end()) {
            cache_items_list_.erase(it->second);
            cache_items_map_.erase(it);
        }
    }

    bool exists(const key_t &key) const {
        return cache_items_map_.find(key) != cache_items_map_.end();
    }

    size_t size() const {
        return cache_items_map_.size();
    }

    list_iterator_t begin() {
        iter_ = cache_items_list_.begin();
        return iter_;
    }

    list_iterator_t end() {
        return cache_items_list_.end();
    }

    reverse_list_iterator_t rbegin() {
        return cache_items_list_.rbegin();
    }

    reverse_list_iterator_t rend() {
        return cache_items_list_.rend();
    }

    void clear() {
        cache_items_list_.clear();
        cache_items_map_.clear();
    }

 private:
    std::list<key_value_pair_t> cache_items_list_;
    std::unordered_map<key_t, list_iterator_t> cache_items_map_;
    size_t max_size_;
    list_iterator_t iter_;
};

} // namespace cache
} // namespace milvus
} // namespace zilliz