path: root/third_party/WebKit/Source/wtf/StringHasher.h

/*
 * Copyright (C) 2005, 2006, 2008, 2010, 2013 Apple Inc. All rights reserved.
 * Copyright (C) 2010 Patrick Gansterer <paroga@paroga.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public License
 * along with this library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
 * Boston, MA 02110-1301, USA.
 *
 */

#ifndef WTF_StringHasher_h
#define WTF_StringHasher_h

#include "wtf/Allocator.h"
#include "wtf/text/Unicode.h"

namespace WTF {

// Paul Hsieh's SuperFastHash
// http://www.azillionmonkeys.com/qed/hash.html

// LChar data is interpreted as Latin-1-encoded (zero extended to 16 bits).

// NOTE: The hash computation here must stay in sync with the create_hash_table script in
// JavaScriptCore and the CodeGeneratorJS.pm script in WebCore.

// Golden ratio. Arbitrary start value to avoid mapping all zeros to a hash value of zero.
static const unsigned stringHashingStartValue = 0x9E3779B9U;

class StringHasher {
    DISALLOW_NEW();
public:
    static const unsigned flagCount = 8; // Save 8 bits for StringImpl to use as flags.

    StringHasher()
        : m_hash(stringHashingStartValue)
        , m_hasPendingCharacter(false)
        , m_pendingCharacter(0)
    {
    }

    // The hasher hashes two characters at a time, and thus an "aligned" hasher is one
    // where an even number of characters have been added. Callers that always add
    // characters two at a time can use the "assuming aligned" functions.
    void addCharactersAssumingAligned(UChar a, UChar b)
    {
        ASSERT(!m_hasPendingCharacter);
        m_hash += a;
        m_hash = (m_hash << 16) ^ ((b << 11) ^ m_hash);
        m_hash += m_hash >> 11;
    }

    void addCharacter(UChar character)
    {
        if (m_hasPendingCharacter) {
            m_hasPendingCharacter = false;
            addCharactersAssumingAligned(m_pendingCharacter, character);
            return;
        }

        m_pendingCharacter = character;
        m_hasPendingCharacter = true;
    }

    void addCharacters(UChar a, UChar b)
    {
        if (m_hasPendingCharacter) {
#if ENABLE(ASSERT)
            m_hasPendingCharacter = false;
#endif
            addCharactersAssumingAligned(m_pendingCharacter, a);
            m_pendingCharacter = b;
#if ENABLE(ASSERT)
            m_hasPendingCharacter = true;
#endif
            return;
        }

        addCharactersAssumingAligned(a, b);
    }

    template<typename T, UChar Converter(T)> void addCharactersAssumingAligned(const T* data, unsigned length)
    {
        ASSERT(!m_hasPendingCharacter);

        bool remainder = length & 1;
        length >>= 1;

        while (length--) {
            addCharactersAssumingAligned(Converter(data[0]), Converter(data[1]));
            data += 2;
        }

        if (remainder)
            addCharacter(Converter(*data));
    }

    template<typename T> void addCharactersAssumingAligned(const T* data, unsigned length)
    {
        addCharactersAssumingAligned<T, defaultConverter>(data, length);
    }

    template<typename T, UChar Converter(T)> void addCharacters(const T* data, unsigned length)
    {
        if (m_hasPendingCharacter && length) {
            m_hasPendingCharacter = false;
            addCharactersAssumingAligned(m_pendingCharacter, Converter(*data++));
            --length;
        }
        addCharactersAssumingAligned<T, Converter>(data, length);
    }

    template<typename T> void addCharacters(const T* data, unsigned length)
    {
        addCharacters<T, defaultConverter>(data, length);
    }

    unsigned hashWithTop8BitsMasked() const
    {
        unsigned result = avalancheBits();

        // Reserving space from the high bits for flags preserves most of the hash's
        // value, since hash lookup typically masks out the high bits anyway.
        result &= (1U << (sizeof(result) * 8 - flagCount)) - 1;

        // This avoids ever returning a hash code of 0, since that is used to
        // signal "hash not computed yet". Setting the high bit maintains
        // reasonable fidelity to a hash code of 0 because it is likely to yield
        // exactly 0 when hash lookup masks out the high bits.
        if (!result)
            result = 0x80000000 >> flagCount;

        return result;
    }

    unsigned hash() const
    {
        unsigned result = avalancheBits();

        // This avoids ever returning a hash code of 0, since that is used to
        // signal "hash not computed yet". Setting the high bit maintains
        // reasonable fidelity to a hash code of 0 because it is likely to yield
        // exactly 0 when hash lookup masks out the high bits.
        if (!result)
            result = 0x80000000;

        return result;
    }

    template<typename T, UChar Converter(T)> static unsigned computeHashAndMaskTop8Bits(const T* data, unsigned length)
    {
        StringHasher hasher;
        hasher.addCharactersAssumingAligned<T, Converter>(data, length);
        return hasher.hashWithTop8BitsMasked();
    }

    template<typename T> static unsigned computeHashAndMaskTop8Bits(const T* data, unsigned length)
    {
        return computeHashAndMaskTop8Bits<T, defaultConverter>(data, length);
    }

    template<typename T, UChar Converter(T)> static unsigned computeHash(const T* data, unsigned length)
    {
        StringHasher hasher;
        hasher.addCharactersAssumingAligned<T, Converter>(data, length);
        return hasher.hash();
    }

    template<typename T> static unsigned computeHash(const T* data, unsigned length)
    {
        return computeHash<T, defaultConverter>(data, length);
    }

    static unsigned hashMemory(const void* data, unsigned length)
    {
        // FIXME: Why does this function use the version of the hash that drops the top 8 bits?
        // We want that for all string hashing so we can use those bits in StringImpl and hash
        // strings consistently, but I don't see why we'd want that for general memory hashing.
        ASSERT(!(length % 2));
        return computeHashAndMaskTop8Bits<UChar>(static_cast<const UChar*>(data), length / sizeof(UChar));
    }

    template<size_t length> static unsigned hashMemory(const void* data)
    {
        static_assert(!(length % 2), "length must be a multiple of two");
        return hashMemory(data, length);
    }

private:
    static UChar defaultConverter(UChar character)
    {
        return character;
    }

    static UChar defaultConverter(LChar character)
    {
        return character;
    }

    unsigned avalancheBits() const
    {
        unsigned result = m_hash;

        // Handle end case.
        if (m_hasPendingCharacter) {
            result += m_pendingCharacter;
            result ^= result << 11;
            result += result >> 17;
        }

        // Force "avalanching" of final 31 bits.
        result ^= result << 3;
        result += result >> 5;
        result ^= result << 2;
        result += result >> 15;
        result ^= result << 10;

        return result;
    }

    unsigned m_hash;
    bool m_hasPendingCharacter;
    UChar m_pendingCharacter;
};

} // namespace WTF

using WTF::StringHasher;

#endif // WTF_StringHasher_h