Source/WTF/wtf/SmallMap.h

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 * Copyright (C) 2024 Apple Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 * 1. Redistributions of source code must retain the above copyright
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 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
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#pragma once

#include <utility>
#include <variant>

#include <wtf/HashMap.h>
#include <wtf/ScopedLambda.h>
#include <wtf/StdLibExtras.h>
#include <wtf/Variant.h>

namespace WTF {

// This is a map optimized for holding 0 or 1 items with no hashing or allocations in those cases.
template<typename Key, typename Value>
class SmallMap {
public:
    using Pair = KeyValuePair<Key, Value>;
    using Map = HashMap<Key, Value>;
    using Storage = Variant<std::monostate, Pair, Map>;

    static_assert(sizeof(Pair) <= 4 * sizeof(uint64_t), "Don't use SmallMap with large types. It probably wastes memory.");

    Value& ensure(const Key& key, NOESCAPE const auto& functor) LIFETIME_BOUND
    {
        ASSERT(Map::isValidKey(key));
        if (std::holds_alternative<std::monostate>(m_storage)) {
            m_storage = Pair { key, functor() };
            return std::get<Pair>(m_storage).value;
        }
        if (auto* pair = std::get_if<Pair>(&m_storage)) {
            if (pair->key == key)
                return pair->value;
            Map map;
            map.add(WTF::move(pair->key), WTF::move(pair->value));
            m_storage = WTF::move(map);
            return std::get<Map>(m_storage).add(key, functor()).iterator->value;
        }
        return std::get<Map>(m_storage).ensure(key, functor).iterator->value;
    }

    void remove(const Key& key)
    {
        ASSERT(Map::isValidKey(key));
        if (auto* pair = std::get_if<Pair>(&m_storage)) {
            if (pair->key == key)
                m_storage = std::monostate { };
        } else if (auto* map = std::get_if<Map>(&m_storage))
            map->remove(key);
    }

    const Value* get(const Key& key) const LIFETIME_BOUND
    {
        ASSERT(Map::isValidKey(key));
        if (auto* pair = std::get_if<Pair>(&m_storage)) {
            if (pair->key == key)
                return std::addressof(pair->value);
        } else if (auto* map = std::get_if<Map>(&m_storage)) {
            if (auto it = map->find(key); it != map->end())
                return std::addressof(it->value);
        }
        return nullptr;
    }

    void forEach(NOESCAPE const auto& callback) const
    {
        switchOn(m_storage, [&] (const std::monostate&) {
        }, [&] (const Pair& pair) {
            callback(pair.key, pair.value);
        }, [&] (const Map& map) {
            for (auto& [key, value] : map)
                callback(key, value);
        });
    }

    size_t size() const
    {
        return switchOn(m_storage, [&] (const std::monostate&) {
            return 0u;
        }, [&] (const Pair&) {
            return 1u;
        }, [&] (const Map& map) {
            return map.size();
        });
    }

    const Storage& rawStorage() const LIFETIME_BOUND { return m_storage; }

private:
    Storage m_storage;
};

} // namespace WTF

using WTF::SmallMap;