source: trunk/src/corelib/tools/qhash.cpp@ 595

/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation ([email protected])
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Nokia.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain additional
** rights.  These rights are described in the Nokia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you have questions regarding the use of this file, please contact
** Nokia at [email protected].
** $QT_END_LICENSE$
**
****************************************************************************/

#include "qhash.h"

#ifdef truncate
#undef truncate
#endif

#include <qbitarray.h>
#include <qstring.h>
#include <stdlib.h>
#ifdef QT_QHASH_DEBUG
#include <qstring.h>
#endif

QT_BEGIN_NAMESPACE

/*
    These functions are based on Peter J. Weinberger's hash function
    (from the Dragon Book). The constant 24 in the original function
    was replaced with 23 to produce fewer collisions on input such as
    "a", "aa", "aaa", "aaaa", ...
*/

static uint hash(const uchar *p, int n)
{
    uint h = 0;
    uint g;

    while (n--) {
        h = (h << 4) + *p++;
        if ((g = (h & 0xf0000000)) != 0)
            h ^= g >> 23;
        h &= ~g;
    }
    return h;
}

static uint hash(const QChar *p, int n)
{
    uint h = 0;
    uint g;

    while (n--) {
        h = (h << 4) + (*p++).unicode();
        if ((g = (h & 0xf0000000)) != 0)
            h ^= g >> 23;
        h &= ~g;
    }
    return h;
}

uint qHash(const QByteArray &key)
{
    return hash(reinterpret_cast<const uchar *>(key.constData()), key.size());
}

uint qHash(const QString &key)
{
    return hash(key.unicode(), key.size());
}

uint qHash(const QStringRef &key)
{
    return hash(key.unicode(), key.size());
}

uint qHash(const QBitArray &bitArray)
{
    int m = bitArray.d.size() - 1;
    uint result = hash(reinterpret_cast<const uchar *>(bitArray.d.constData()), qMax(0, m));

    // deal with the last 0 to 7 bits manually, because we can't trust that
    // the padding is initialized to 0 in bitArray.d
    int n = bitArray.size();
    if (n & 0x7)
        result = ((result << 4) + bitArray.d.at(m)) & ((1 << n) - 1);
    return result;
}

/*
    The prime_deltas array is a table of selected prime values, even
    though it doesn't look like one. The primes we are using are 1,
    2, 5, 11, 17, 37, 67, 131, 257, ..., i.e. primes in the immediate
    surrounding of a power of two.

    The primeForNumBits() function returns the prime associated to a
    power of two. For example, primeForNumBits(8) returns 257.
*/

static const uchar prime_deltas[] = {
    0,  0,  1,  3,  1,  5,  3,  3,  1,  9,  7,  5,  3,  9, 25,  3,
    1, 21,  3, 21,  7, 15,  9,  5,  3, 29, 15,  0,  0,  0,  0,  0
};

static inline int primeForNumBits(int numBits)
{
    return (1 << numBits) + prime_deltas[numBits];
}

/*