transmission/libtransmission/lru-cache.h

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// This file Copyright © 2022-2023 Mnemosyne LLC.
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// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.
#pragma once
#include <algorithm>
#include <array>
#include <cstddef>
#include <cstdint>
#include <functional>
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#include <utility>
// A fixed-size cache that erases least-recently-used items to make room for new ones.
template<typename Key, typename Val, std::size_t N>
class tr_lru_cache
{
public:
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[[nodiscard]] constexpr Val* get(Key const& key) noexcept
{
if (auto const found = find(key); found != nullptr)
{
found->sequence_ = next_sequence_++;
return &found->val_;
}
return nullptr;
}
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[[nodiscard]] constexpr bool contains(Key const& key) const noexcept
{
return !!find(key);
}
Val& add(Key&& key)
{
auto& entry = getFreeSlot();
entry.key_ = std::move(key);
entry.sequence_ = next_sequence_++;
key = {};
return entry.val_;
}
void erase(Key const& key)
{
if (auto* const found = find(key); found != nullptr)
{
this->erase(*found);
}
}
void erase_if(std::function<bool(Key const&, Val const&)> test)
{
for (auto& entry : entries_)
{
if (entry.sequence_ != InvalidSeq && test(entry.key_, entry.val_))
{
erase(entry);
}
}
}
void clear()
{
for (auto& entry : entries_)
{
erase(entry);
}
}
using PreEraseCallback = std::function<void(Key const&, Val&)>;
void setPreErase(PreEraseCallback&& func)
{
pre_erase_cb_ = std::move(func);
}
private:
PreEraseCallback pre_erase_cb_ = [](Key const&, Val&) {
};
struct Entry
{
Key key_ = {};
Val val_ = {};
uint64_t sequence_ = InvalidSeq;
};
void erase(Entry& entry)
{
if (entry.sequence_ != InvalidSeq)
{
pre_erase_cb_(entry.key_, entry.val_);
}
entry.key_ = {};
entry.val_ = {};
entry.sequence_ = InvalidSeq;
}
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[[nodiscard]] constexpr Entry* find(Key const& key) noexcept
{
for (auto& entry : entries_)
{
if (entry.sequence_ != InvalidSeq && entry.key_ == key)
{
return &entry;
}
}
return nullptr;
}
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[[nodiscard]] constexpr Entry const* find(Key const& key) const noexcept
{
for (auto const& entry : entries_)
{
if (entry.sequence_ != InvalidSeq && entry.key_ == key)
{
return &entry;
}
}
return nullptr;
}
Entry& getFreeSlot()
{
auto const iter = std::min_element(
std::begin(entries_),
std::end(entries_),
[](auto const& a, auto const& b) { return a.sequence_ < b.sequence_; });
this->erase(*iter);
return *iter;
}
std::array<Entry, N> entries_;
uint64_t next_sequence_ = 1;
static uint64_t constexpr InvalidSeq = 0;
};