jhash.h

#ifndef _LINUX_JHASH_H
#define _LINUX_JHASH_H

/* jhash.h: Jenkins hash support.
 *
 * Copyright (C) 2006. Bob Jenkins (bob_jenkins@burtleburtle.net)
 *
 * 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 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
 */

#define __rot(x,k) (((x)<<(k)) | ((x)>>(32-(k))))

/* __jhash_mix - mix 3 32-bit values reversibly. */
#define __jhash_mix(a,b,c) \
{ \
  a -= c;  a ^= __rot(c, 4);  c += b; \
  b -= a;  b ^= __rot(a, 6);  a += c; \
  c -= b;  c ^= __rot(b, 8);  b += a; \
  a -= c;  a ^= __rot(c,16);  c += b; \
  b -= a;  b ^= __rot(a,19);  a += c; \
  c -= b;  c ^= __rot(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 -= __rot(b,14); \
  a ^= c; a -= __rot(c,11); \
  b ^= a; b -= __rot(a,25); \
  c ^= b; c -= __rot(b,16); \
  a ^= c; a -= __rot(c,4);  \
  b ^= a; b -= __rot(a,14); \
  c ^= b; c -= __rot(b,24); \
}

/* The golden ration: an arbitrary value */
#define JHASH_GOLDEN_RATIO	0xdeadbeef

/* The most generic version, hashes an arbitrary sequence
 * of bytes.  No alignment or length assumptions are made about
 * the input 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_GOLDEN_RATIO + length + initval;

	/* all but the last block: affect some 32 bits of (a,b,c) */
	while (length > 12) {
    		a += (k[0] + ((u32)k[1]<<8) + ((u32)k[2]<<16) + ((u32)k[3]<<24));
		b += (k[4] + ((u32)k[5]<<8) + ((u32)k[6]<<16) + ((u32)k[7]<<24));
		c += (k[8] + ((u32)k[9]<<8) + ((u32)k[10]<<16) + ((u32)k[11]<<24));
		__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 :
		break;
	}

	return c;
}

/* A special optimized version that handles 1 or more of u32s.
 * The length parameter here is the number of u32s in the key.
 */
static inline u32 jhash2(u32 *k, u32 length, u32 initval)
{
	u32 a, b, c;

	/* Set up the internal state */
	a = b = c = JHASH_GOLDEN_RATIO + (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:     /* case 0: nothing left to add */
		break;
	}

	return c;
}

/* A special ultra-optimized versions that knows they are hashing exactly
 * 3, 2 or 1 word(s).
 */
static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
{
	a += JHASH_GOLDEN_RATIO + initval;
	b += JHASH_GOLDEN_RATIO + initval;
	c += JHASH_GOLDEN_RATIO + initval;

	__jhash_final(a, b, c);

	return c;
}

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

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

#endif /* _LINUX_JHASH_H */