Source/WebCore/html/parser/HTMLAtomStringCache.h

/*
 * Copyright (C) 2021 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 *
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 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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#pragma once

#include <wtf/NeverDestroyed.h>
#include <wtf/Vector.h>
#include <wtf/text/AtomString.h>

namespace WebCore {

class HTMLAtomStringCache {
public:
    template<size_t inlineCapacity>
    ALWAYS_INLINE static AtomString makeTagOrAttributeName(const Vector<UChar, inlineCapacity>& string)
    {
        return make<Type::TagOrAttributeName>(string);
    }

    template<size_t inlineCapacity>
    ALWAYS_INLINE static AtomString makeAttributeValue(const Vector<UChar, inlineCapacity>& string)
    {
        return make<Type::AttributeValue>(string);
    }

    ALWAYS_INLINE static void clear()
    {
        // FIXME (webkit.org/b/230019): We should try to find more opportunities to clear this cache without hindering this performance optimization.
        cache(Type::TagOrAttributeName).fill({ });
        cache(Type::AttributeValue).fill({ });
    }

private:
    enum class Type : bool { TagOrAttributeName, AttributeValue };

    template<HTMLAtomStringCache::Type type, size_t inlineCapacity>
    ALWAYS_INLINE static AtomString make(const Vector<UChar, inlineCapacity>& string)
    {
        if (string.isEmpty())
            return emptyAtom();

        auto length = string.size();
        if (length > maxStringLengthForCache)
            return AtomString(string);

        auto firstCharacter = string[0];
        auto lastCharacter = string[length - 1];
        auto& slot = cacheSlot(type, firstCharacter, lastCharacter, length);
        if (!equal(slot.impl(), string.data(), length)) {
            AtomString result(string);
            slot = result;
            return result;
        }

        return slot;
    }

    ALWAYS_INLINE static AtomString& cacheSlot(Type type, UChar firstCharacter, UChar lastCharacter, UChar length)
    {
        unsigned hash = (firstCharacter << 6) ^ ((lastCharacter << 14) ^ firstCharacter);
        hash += (hash >> 14) + (length << 14);
        hash ^= hash << 14;
        return cache(type)[(hash + (hash >> 6)) % capacity];
    }

    static constexpr auto maxStringLengthForCache = 36;
    static constexpr auto capacity = 512;
    using Cache = std::array<AtomString, capacity>;
    static Cache& cache(Type);
};

} // namespace WebCore