ClickHouse/base/poco/Foundation/include/Poco/LRUStrategy.h

//
// LRUStrategy.h
//
// Library: Foundation
// Package: Cache
// Module:  LRUStrategy
//
// Definition of the LRUStrategy class.
//
// Copyright (c) 2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// SPDX-License-Identifier:	BSL-1.0
//


#ifndef Foundation_LRUStrategy_INCLUDED
#define Foundation_LRUStrategy_INCLUDED


#include <cstddef>
#include <list>
#include <map>
#include "Poco/AbstractStrategy.h"
#include "Poco/EventArgs.h"
#include "Poco/Exception.h"
#include "Poco/KeyValueArgs.h"
#include "Poco/ValidArgs.h"


namespace Poco
{


template <class TKey, class TValue>
class LRUStrategy : public AbstractStrategy<TKey, TValue>
/// An LRUStrategy implements least recently used cache replacement.
{
public:
    typedef std::list<TKey> Keys;
    typedef typename Keys::iterator Iterator;
    typedef typename Keys::const_iterator ConstIterator;
    typedef std::map<TKey, Iterator> KeyIndex;
    typedef typename KeyIndex::iterator IndexIterator;
    typedef typename KeyIndex::const_iterator ConstIndexIterator;

public:
    LRUStrategy(std::size_t size) : _size(size)
    {
        if (_size < 1)
            throw InvalidArgumentException("size must be > 0");
    }

    ~LRUStrategy() { }

    void onAdd(const void *, const KeyValueArgs<TKey, TValue> & args)
    {
        _keys.push_front(args.key());
        std::pair<IndexIterator, bool> stat = _keyIndex.insert(std::make_pair(args.key(), _keys.begin()));
        if (!stat.second)
        {
            stat.first->second = _keys.begin();
        }
    }

    void onRemove(const void *, const TKey & key)
    {
        IndexIterator it = _keyIndex.find(key);

        if (it != _keyIndex.end())
        {
            _keys.erase(it->second);
            _keyIndex.erase(it);
        }
    }

    void onGet(const void *, const TKey & key)
    {
        // LRU: in case of an hit, move to begin
        IndexIterator it = _keyIndex.find(key);

        if (it != _keyIndex.end())
        {
            _keys.splice(_keys.begin(), _keys, it->second); //_keys.erase(it->second)+_keys.push_front(key);
            it->second = _keys.begin();
        }
    }

    void onClear(const void *, const EventArgs & args)
    {
        _keys.clear();
        _keyIndex.clear();
    }

    void onIsValid(const void *, ValidArgs<TKey> & args)
    {
        if (_keyIndex.find(args.key()) == _keyIndex.end())
        {
            args.invalidate();
        }
    }

    void onReplace(const void *, std::set<TKey> & elemsToRemove)
    {
        // Note: replace only informs the cache which elements
        // it would like to remove!
        // it does not remove them on its own!
        std::size_t curSize = _keyIndex.size();

        if (curSize < _size)
        {
            return;
        }

        std::size_t diff = curSize - _size;
        Iterator it = --_keys.end(); /// --keys can never be invoked on an empty list due to the minSize==1 requirement of LRU
        std::size_t i = 0;

        while (i++ < diff)
        {
            elemsToRemove.insert(*it);
            if (it != _keys.begin())
            {
                --it;
            }
        }
    }

protected:
    std::size_t _size; /// Number of keys the cache can store.
    Keys _keys;
    KeyIndex _keyIndex; /// For faster access to _keys
};


} // namespace Poco


#endif // Foundation_LRUStrategy_INCLUDED