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/*
* Copyright (C) 2005, 2006, 2008, 2010 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/unicode/Unicode.h>
namespace WTF {
// Golden ratio - arbitrary start value to avoid mapping all 0's to all 0's
static const unsigned stringHashingStartValue = 0x9e3779b9U;
// Paul Hsieh's SuperFastHash
// http://www.azillionmonkeys.com/qed/hash.html
// char* data is interpreted as latin-encoded (zero extended to 16 bits).
// NOTE: This class must stay in sync with the create_hash_table script in
// JavaScriptCore and the CodeGeneratorJS.pm script in WebCore.
class StringHasher {
public:
static const unsigned flagCount = 8; // Save 8 bits for StringImpl to use as flags.
inline StringHasher()
: m_hash(stringHashingStartValue)
, m_hasPendingCharacter(false)
, m_pendingCharacter(0)
{
}
inline void addCharacters(UChar a, UChar b)
{
ASSERT(!m_hasPendingCharacter);
addCharactersToHash(a, b);
}
inline void addCharacter(UChar ch)
{
if (m_hasPendingCharacter) {
addCharactersToHash(m_pendingCharacter, ch);
m_hasPendingCharacter = false;
return;
}
m_pendingCharacter = ch;
m_hasPendingCharacter = true;
}
inline unsigned hash() 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;
// 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;
}
template<typename T, UChar Converter(T)> static inline unsigned computeHash(const T* data, unsigned length)
{
StringHasher hasher;
bool rem = length & 1;
length >>= 1;
while (length--) {
hasher.addCharacters(Converter(data[0]), Converter(data[1]));
data += 2;
}
if (rem)
hasher.addCharacter(Converter(*data));
return hasher.hash();
}
template<typename T, UChar Converter(T)> static inline unsigned computeHash(const T* data)
{
StringHasher hasher;
while (true) {
UChar b0 = Converter(*data++);
if (!b0)
break;
UChar b1 = Converter(*data++);
if (!b1) {
hasher.addCharacter(b0);
break;
}
hasher.addCharacters(b0, b1);
}
return hasher.hash();
}
template<typename T> static inline unsigned computeHash(const T* data, unsigned length)
{
return computeHash<T, defaultConverter>(data, length);
}
template<typename T> static inline unsigned computeHash(const T* data)
{
return computeHash<T, defaultConverter>(data);
}
template<size_t length> static inline unsigned hashMemory(const void* data)
{
COMPILE_ASSERT(!(length % 4), length_must_be_a_multible_of_four);
return computeHash<UChar>(static_cast<const UChar*>(data), length / sizeof(UChar));
}
static inline unsigned hashMemory(const void* data, unsigned size)
{
ASSERT(!(size % 2));
return computeHash<UChar>(static_cast<const UChar*>(data), size / sizeof(UChar));
}
private:
static inline UChar defaultConverter(UChar ch)
{
return ch;
}
static inline UChar defaultConverter(LChar ch)
{
return ch;
}
inline void addCharactersToHash(UChar a, UChar b)
{
m_hash += a;
unsigned tmp = (b << 11) ^ m_hash;
m_hash = (m_hash << 16) ^ tmp;
m_hash += m_hash >> 11;
}
unsigned m_hash;
bool m_hasPendingCharacter;
UChar m_pendingCharacter;
};
} // namespace WTF
using WTF::StringHasher;
#endif // WTF_StringHasher_h
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