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/*
* Copyright (C) 2010-2024 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
* THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include <algorithm>
#include <cstddef>
#include <span>
#include <wtf/NeverDestroyed.h>
WTF_ALLOW_UNSAFE_BUFFER_USAGE_BEGIN
namespace WTF {
template<typename KeyType, typename ValueType>
struct TinyLRUCachePolicy {
static bool isKeyNull(const KeyType&) { return false; }
static ValueType createValueForNullKey() { return { }; }
static ValueType createValueForKey(const KeyType&) { return { }; }
static KeyType createKeyForStorage(const KeyType& key) { return key; }
};
template<typename KeyType, typename ValueType, size_t capacity = 4, typename Policy = TinyLRUCachePolicy<KeyType, ValueType>>
class TinyLRUCache {
WTF_MAKE_FAST_ALLOCATED;
public:
const ValueType& get(const KeyType& key)
{
if (Policy::isKeyNull(key)) {
static NeverDestroyed<ValueType> valueForNull = Policy::createValueForNullKey();
return valueForNull;
}
auto cacheBuffer = this->cacheBuffer();
for (size_t i = m_size; i-- > 0;) {
if (cacheBuffer[i].first == key) {
if (i < m_size - 1) {
// Move entry to the end of the cache if necessary.
auto entry = WTFMove(cacheBuffer[i]);
do {
cacheBuffer[i] = WTFMove(cacheBuffer[i + 1]);
} while (++i < m_size - 1);
cacheBuffer[m_size - 1] = WTFMove(entry);
}
return cacheBuffer[m_size - 1].second;
}
}
// cacheBuffer[0] is the LRU entry, so remove it.
if (m_size == capacity) {
for (size_t i = 0; i < m_size - 1; ++i)
cacheBuffer[i] = WTFMove(cacheBuffer[i + 1]);
} else
++m_size;
cacheBuffer[m_size - 1] = std::pair { Policy::createKeyForStorage(key), Policy::createValueForKey(key) };
return cacheBuffer[m_size - 1].second;
}
private:
using Entry = std::pair<KeyType, ValueType>;
std::span<Entry, capacity> cacheBuffer() { return m_cacheBuffer; }
alignas(Entry) std::array<Entry, capacity> m_cacheBuffer;
size_t m_size { 0 };
};
} // namespace WTF
using WTF::TinyLRUCache;
using WTF::TinyLRUCachePolicy;
WTF_ALLOW_UNSAFE_BUFFER_USAGE_END
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