qtbase/tests/benchmarks/corelib/tools/qhash/outofline.cpp

// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only

#include "tst_bench_qhash.h"

QT_BEGIN_NAMESPACE

size_t qHash(const Qt4String &str, size_t /* never used */)
{
    qsizetype n = str.size();
    const QChar *p = str.unicode();
    uint h = 0;

    while (n--) {
        h = (h << 4) + (*p++).unicode();
        h ^= (h & 0xf0000000) >> 23;
        h &= 0x0fffffff;
    }
    return h;
}

size_t qHash(const Qt50String &key, size_t seed)
{
    const QChar *p = key.unicode();
    qsizetype len = key.size();
    size_t h = seed;
    for (int i = 0; i < len; ++i)
        h = 31 * h + p[i].unicode();
    return h;
}

// The Java's hashing algorithm for strings is a variation of D. J. Bernstein
// hashing algorithm appeared here http://cr.yp.to/cdb/cdb.txt
// and informally known as DJB33XX - DJB's 33 Times Xor.
// Java uses DJB31XA, that is, 31 Times Add.
// The original algorithm was a loop around  "(h << 5) + h ^ c",
// which is indeed "h * 33 ^ c"; it was then changed to
// "(h << 5) - h ^ c", so "h * 31 ^ c", and the XOR changed to a sum:
// "(h << 5) - h + c", which can save some assembly instructions.
// Still, we can avoid writing the multiplication as "(h << 5) - h"
// -- the compiler will turn it into a shift and an addition anyway
// (for instance, gcc 4.4 does that even at -O0).
size_t qHash(const JavaString &str, size_t /* never used */)
{
    const auto *p = reinterpret_cast<const char16_t *>(str.constData());
    const qsizetype len = str.size();

    uint h = 0;

    for (int i = 0; i < len; ++i)
        h = 31 * h + p[i];

    return h;
}

QT_END_NAMESPACE