jhash.h

#ifndef _LINUX_JHASH_H
#define _LINUX_JHASH_H
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"

/* jhash.h: Jenkins hash support.
 *
 * Copyright (C) 2006. Bob Jenkins (bob_jenkins@burtleburtle.net)
 *
 * https://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 (kadlec@blackhole.kfki.hu)
 *
 * 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>

#include <stdint.h>

/**
 * rol32 - rotate a 32-bit value left
 * @word: value to rotate
 * @shift: bits to roll
 */
static inline uint32_t rol32(uint32_t word, unsigned int shift)
{
    return (word << shift) | (word >> ((-shift) & 31));
}

/* 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 uint32_t jhash(const void *key, uint32_t length, uint32_t initval)
{
    uint32_t a, b, c;
    const uint8_t *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 += *k;
        b += *(k + 4);
        c += *(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 += (uint32_t)k[11]<<24;
        case 11: c += (uint32_t)k[10]<<16;
        case 10: c += (uint32_t)k[9]<<8;
        case 9:  c += k[8];
        case 8:  b += (uint32_t)k[7]<<24;
        case 7:  b += (uint32_t)k[6]<<16;
        case 6:  b += (uint32_t)k[5]<<8;
        case 5:  b += k[4];
        case 4:  a += (uint32_t)k[3]<<24;
        case 3:  a += (uint32_t)k[2]<<16;
        case 2:  a += (uint32_t)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 uint32_t's
 * @k: the key which must be an array of uint32_t's
 * @length: the number of uint32_t's in the key
 * @initval: the previous hash, or an arbitray value
 *
 * Returns the hash value of the key.
 */
static inline uint32_t jhash2(const uint32_t *k, uint32_t length, uint32_t initval)
{
    uint32_t 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_nwords - hash exactly 3, 2 or 1 word(s) */
static inline uint32_t __jhash_nwords(uint32_t a, uint32_t b, uint32_t c, uint32_t initval)
{
    a += initval;
    b += initval;
    c += initval;

    __jhash_final(a, b, c);

    return c;
}

static inline uint32_t jhash_3words(uint32_t a, uint32_t b, uint32_t c, uint32_t initval)
{
    return __jhash_nwords(a, b, c, initval + JHASH_INITVAL + (3 << 2));
}

static inline uint32_t jhash_2words(uint32_t a, uint32_t b, uint32_t initval)
{
    return __jhash_nwords(a, b, 0, initval + JHASH_INITVAL + (2 << 2));
}

static inline uint32_t jhash_1word(uint32_t a, uint32_t initval)
{
    return __jhash_nwords(a, 0, 0, initval + JHASH_INITVAL + (1 << 2));
}
#pragma GCC diagnostic pop
#endif /* _LINUX_JHASH_H */