system/hwc3/LruCache.h

// Copyright 2022 The Android Open Source Project
//
// 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 <list>
#include <unordered_map>

template <typename Key, typename Value>
class LruCache {
  public:
    LruCache(std::size_t maxSize) : m_maxSize(maxSize) {
        m_table.reserve(maxSize);
    }

    Value* get(const Key& key) {
        auto tableIt = m_table.find(key);
        if (tableIt == m_table.end()) {
            return nullptr;
        }

        // Move to front.
        auto elementsIt = tableIt->second;
        m_elements.splice(elementsIt, m_elements, m_elements.begin());
        return &elementsIt->value;
    }

    void set(const Key& key, Value&& value) {
        auto tableIt = m_table.find(key);
        if (tableIt == m_table.end()) {
            if (m_table.size() >= m_maxSize) {
                auto& kv = m_elements.back();
                m_table.erase(kv.key);
                m_elements.pop_back();
            }
        } else {
            auto elementsIt = tableIt->second;
            m_elements.erase(elementsIt);
        }
        m_elements.emplace_front(KeyValue{
            key,
            std::forward<Value>(value),
        });
        m_table[key] = m_elements.begin();
    }

    void remove(const Key& key) {
        auto tableIt = m_table.find(key);
        if (tableIt == m_table.end()) {
            return;
        }
        auto elementsIt = tableIt->second;
        m_elements.erase(elementsIt);
        m_table.erase(tableIt);
    }

    void clear() {
        m_elements.clear();
        m_table.clear();
    }

  private:
    struct KeyValue {
        Key key;
        Value value;
    };

    const std::size_t m_maxSize;
    // Front is the most recently used and back is the least recently used.
    std::list<KeyValue> m_elements;
    std::unordered_map<Key, typename std::list<KeyValue>::iterator> m_table;
};