pcsx2/common/LRUCache.h

/*  PCSX2 - PS2 Emulator for PCs
 *  Copyright (C) 2002-2022  PCSX2 Dev Team
 *
 *  PCSX2 is free software: you can redistribute it and/or modify it under the terms
 *  of the GNU Lesser General Public License as published by the Free Software Found-
 *  ation, either version 3 of the License, or (at your option) any later version.
 *
 *  PCSX2 is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
 *  without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
 *  PURPOSE.  See the GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along with PCSX2.
 *  If not, see <http://www.gnu.org/licenses/>.
 */

#pragma once
#include <cstdint>
#include <map>

template <class K, class V>
class LRUCache
{
	using CounterType = std::uint64_t;

	struct Item
	{
		V value;
		CounterType last_access;
	};

	using MapType = std::map<K, Item>;

public:
	LRUCache(std::size_t max_capacity = 16, bool manual_evict = false)
		: m_max_capacity(max_capacity)
		, m_manual_evict(manual_evict)
	{
	}
	~LRUCache() = default;

	std::size_t GetSize() const { return m_items.size(); }
	std::size_t GetMaxCapacity() const { return m_max_capacity; }

	void Clear() { m_items.clear(); }

	void SetMaxCapacity(std::size_t capacity)
	{
		m_max_capacity = capacity;
		if (m_items.size() > m_max_capacity)
			Evict(m_items.size() - m_max_capacity);
	}

	template<typename KeyT>
	V* Lookup(const KeyT& key)
	{
		auto iter = m_items.find(key);
		if (iter == m_items.end())
			return nullptr;

		iter->second.last_access = ++m_last_counter;
		return &iter->second.value;
	}

	V* Insert(K key, V value)
	{
		ShrinkForNewItem();

		auto iter = m_items.find(key);
		if (iter != m_items.end())
		{
			iter->second.value = std::move(value);
			iter->second.last_access = ++m_last_counter;
			return &iter->second.value;
		}
		else
		{
			Item it;
			it.last_access = ++m_last_counter;
			it.value = std::move(value);
			auto ip = m_items.emplace(std::move(key), std::move(it));
			return &ip.first->second.value;
		}
	}

	void Evict(std::size_t count = 1)
	{
		while (m_items.size() >= count)
		{
			typename MapType::iterator lowest = m_items.end();
			for (auto iter = m_items.begin(); iter != m_items.end(); ++iter)
			{
				if (lowest == m_items.end() || iter->second.last_access < lowest->second.last_access)
					lowest = iter;
			}
			m_items.erase(lowest);
		}
	}

	template<typename KeyT>
	bool Remove(const KeyT& key)
	{
		auto iter = m_items.find(key);
		if (iter == m_items.end())
			return false;

		m_items.erase(iter);
		return true;
	}

	void SetManualEvict(bool block)
	{
		m_manual_evict = block;
		if (!m_manual_evict)
			ManualEvict();
	}

	void ManualEvict()
	{
		// evict if we went over
		while (m_items.size() > m_max_capacity)
			Evict(m_items.size() - (m_max_capacity - 1));
	}

private:
	void ShrinkForNewItem()
	{
		if (m_items.size() < m_max_capacity)
			return;

		Evict(m_items.size() - (m_max_capacity - 1));
	}

	MapType m_items;
	CounterType m_last_counter = 0;
	std::size_t m_max_capacity = 0;
	bool m_manual_evict = false;
};