jhash.h

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#ifndef _LINUX_JHASH_H
#define _LINUX_JHASH_H

/* jhash.h: Jenkins hash support.
 *
 * Copyright (C) 2006. Bob Jenkins ([email protected])
 *
 * http://burtleburtle.net/bob/hash/
 *
 * These are the credits from Bob's sources:
 *
 * lookup3.c, by Bob Jenkins, May 2006, Public Domain.
 *
 * These are functions for producing 32-bit hashes for hash table lookup.
 * hashword(), hashlittle(), hashlittle2(), hashbig(), mix(), and final()
 * are externally useful functions.  Routines to test the hash are included
 * if SELF_TEST is defined.  You can use this free for any purpose.  It's in
 * the public domain.  It has no warranty.
 *
 * Copyright (C) 2009-2010 Jozsef Kadlecsik ([email protected])
 *
 * I've modified Bob's hash to be useful in the Linux kernel, and
 * any bugs present are my fault.
 * Jozsef
 */
#include <linux/bitops.h>
#include <linux/unaligned/packed_struct.h>

/* Best hash sizes are of power of two */
#define jhash_size(n)   ((u32)1<<(n))
/* Mask the hash value, i.e (value & jhash_mask(n)) instead of (value % n) */
#define jhash_mask(n)   (jhash_size(n)-1)

/* __jhash_mix -- mix 3 32-bit values reversibly. */
#define __jhash_mix(a, b, c)            \
{                       \
    a -= c;  a ^= rol32(c, 4);  c += b; \
    b -= a;  b ^= rol32(a, 6);  a += c; \
    c -= b;  c ^= rol32(b, 8);  b += a; \
    a -= c;  a ^= rol32(c, 16); c += b; \
    b -= a;  b ^= rol32(a, 19); a += c; \
    c -= b;  c ^= rol32(b, 4);  b += a; \
}

/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
#define __jhash_final(a, b, c)          \
{                       \
    c ^= b; c -= rol32(b, 14);      \
    a ^= c; a -= rol32(c, 11);      \
    b ^= a; b -= rol32(a, 25);      \
    c ^= b; c -= rol32(b, 16);      \
    a ^= c; a -= rol32(c, 4);       \
    b ^= a; b -= rol32(a, 14);      \
    c ^= b; c -= rol32(b, 24);      \
}

/* An arbitrary initial parameter */
#define JHASH_INITVAL       0xdeadbeef

/* jhash - hash an arbitrary key
 * @k: sequence of bytes as key
 * @length: the length of the key
 * @initval: the previous hash, or an arbitray value
 *
 * The generic version, hashes an arbitrary sequence of bytes.
 * No alignment or length assumptions are made about the input key.
 *
 * Returns the hash value of the key. The result depends on endianness.
 */
static inline u32 jhash(const void *key, u32 length, u32 initval)
{
    u32 a, b, c;
    const u8 *k = key;

    /* Set up the internal state */
    a = b = c = JHASH_INITVAL + length + initval;

    /* All but the last block: affect some 32 bits of (a,b,c) */
    while (length > 12) {
        a += __get_unaligned_cpu32(k);
        b += __get_unaligned_cpu32(k + 4);
        c += __get_unaligned_cpu32(k + 8);
        __jhash_mix(a, b, c);
        length -= 12;
        k += 12;
    }
    /* Last block: affect all 32 bits of (c) */
    /* All the case statements fall through */
    switch (length) {
    case 12: c += (u32)k[11]<<24;
    case 11: c += (u32)k[10]<<16;
    case 10: c += (u32)k[9]<<8;
    case 9:  c += k[8];
    case 8:  b += (u32)k[7]<<24;
    case 7:  b += (u32)k[6]<<16;
    case 6:  b += (u32)k[5]<<8;
    case 5:  b += k[4];
    case 4:  a += (u32)k[3]<<24;
    case 3:  a += (u32)k[2]<<16;
    case 2:  a += (u32)k[1]<<8;
    case 1:  a += k[0];
         __jhash_final(a, b, c);
    case 0: /* Nothing left to add */
        break;
    }

    return c;
}

/* jhash2 - hash an array of u32's
 * @k: the key which must be an array of u32's
 * @length: the number of u32's in the key
 * @initval: the previous hash, or an arbitray value
 *
 * Returns the hash value of the key.
 */
static inline u32 jhash2(const u32 *k, u32 length, u32 initval)
{
    u32 a, b, c;

    /* Set up the internal state */
    a = b = c = JHASH_INITVAL + (length<<2) + initval;

    /* Handle most of the key */
    while (length > 3) {
        a += k[0];
        b += k[1];
        c += k[2];
        __jhash_mix(a, b, c);
        length -= 3;
        k += 3;
    }

    /* Handle the last 3 u32's: all the case statements fall through */
    switch (length) {
    case 3: c += k[2];
    case 2: b += k[1];
    case 1: a += k[0];
        __jhash_final(a, b, c);
    case 0: /* Nothing left to add */
        break;
    }

    return c;
}


/* jhash_3words - hash exactly 3, 2 or 1 word(s) */
static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
{
    a += JHASH_INITVAL;
    b += JHASH_INITVAL;
    c += initval;

    __jhash_final(a, b, c);

    return c;
}

static inline u32 jhash_2words(u32 a, u32 b, u32 initval)
{
    return jhash_3words(a, b, 0, initval);
}

static inline u32 jhash_1word(u32 a, u32 initval)
{
    return jhash_3words(a, 0, 0, initval);
}

#endif /* _LINUX_JHASH_H */