Quelle hashfunctions.h Sprache: C

//
// Datastructures: GAP package providing common datastructures.
//
// Copyright (C) 2015-2017  The datastructures team.
// For list of the team members, please refer to the COPYRIGHT file.
//
// This package is licensed under the GPL 2 or later, please refer
// to the COPYRIGHT.md and LICENSE files for details.
//
//
// hashfunctions: various hash functions
//

#ifndef HASHFUNCTIONS_H
#define HASHFUNCTIONS_H

#include "datastructures.h"

extern struct DatastructuresModule HashFunctionsModule;

// Perform a shuffle of a UInt. Ideally, changing any
// bit would have a 50/50 chance of changing every other bit.
// The main purpose of this is to allow adding values when
// we want to hash an unordered list -- while 1+3 = 2+2,
// HashUInt(1) + HashUInt(3) should not be equal to
// HashUInt(2) + HashUInt(2)
// From: http://www.concentric.net/~Ttwang/tech/inthash.htm
static UInt ShuffleUInt(UInt key)
{
#ifdef SYS_IS_64_BIT
    key = (~key) + (key << 21);    // key = (key << 21) - key - 1;
    key = key ^ (key >> 24);
    key = (key + (key << 3)) + (key << 8);    // key * 265
    key = key ^ (key >> 14);
    key = (key + (key << 2)) + (key << 4);    // key * 21
    key = key ^ (key >> 28);
    key = key + (key << 31);
#else
    key = ~key + (key << 15);    // key = (key << 15) - key - 1;
    key = key ^ (key >> 12);
    key = key + (key << 2);
    key = key ^ (key >> 4);
    key = key * 2057;    // key = (key + (key << 3)) + (key << 11);
    key = key ^ (key >> 16);
#endif
    return key;
}

// Combine a new value with an existing hash value
static inline UInt AddToHash(UInt seed, UInt v)
{
    return seed ^ (ShuffleUInt(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2));
}

// Transform a UInt into a signed GAP intermediate integer, shrinking
// the size of the number as required
static inline Obj HashValueToObjInt(UInt uhash)
{
    Int hash = (Int)uhash;
    // Make sure bottom bits are not lost
    hash += hash << 11;
    hash /= 16;
    return INTOBJ_INT(hash);
}

#endif

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