The new jhash implementation

/* jhash.h: Jenkins hash support.

 *

 * Copyright (C) 1996 Bob Jenkins (bob_jenkins@burtleburtle.net)

 * Copyright (C) 2006. Bob Jenkins (bob_jenkins@burtleburtle.net)

 *

 * http://burtleburtle.net/bob/hash/

 *

 * These are the credits from Bob's sources:

 *

 * lookup2.c, by Bob Jenkins, December 1996, Public Domain.

 * hash(), hash2(), hash3, and mix() 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 has no warranty.

 * lookup3.c, by Bob Jenkins, May 2006, Public Domain.

 *

 * Copyright (C) 2003 David S. Miller (davem@redhat.com)

 * 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 surely my fault.  -DaveM

 * 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)

/* NOTE: Arguments are modified. */

/* __jhash_mix -- mix 3 32-bit values reversibly. */

#define __jhash_mix(a, b, c)			\

{						\

  a -= b; a -= c; a ^= (c>>13); \

  b -= c; b -= a; b ^= (a<<8); \

  c -= a; c -= b; c ^= (b>>13); \

  a -= b; a -= c; a ^= (c>>12);  \

  b -= c; b -= a; b ^= (a<<16); \

  c -= a; c -= b; c ^= (b>>5); \

  a -= b; a -= c; a ^= (c>>3);  \

  b -= c; b -= a; b ^= (a<<10); \

  c -= a; c -= b; c ^= (b>>15); \

	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);		\

}

/* The golden ration: an arbitrary value */

#define JHASH_GOLDEN_RATIO	0x9e3779b9

/* An arbitrary initial parameter */

#define JHASH_INITVAL		0xdeadbeef

/* The most generic version, hashes an arbitrary sequence

 * of bytes.  No alignment or length assumptions are made about

 * the input key.

/* 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, len;

	u32 a, b, c;

	const u8 *k = key;

	len = length;

	a = b = JHASH_GOLDEN_RATIO;

	c = initval;

	while (len >= 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));

	/* 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;

		len -= 12;

	}

	c += length;

	switch (len) {

	case 11: c += ((u32)k[10]<<24);

	case 10: c += ((u32)k[9]<<16);

	case 9 : c += ((u32)k[8]<<8);

	case 8 : b += ((u32)k[7]<<24);

	case 7 : b += ((u32)k[6]<<16);

	case 6 : b += ((u32)k[5]<<8);

	/* 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 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_mix(a,b,c);

		 __jhash_final(a, b, c);

	case 0: /* Nothing left to add */

		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.

/* 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, len;

	u32 a, b, c;

	a = b = JHASH_GOLDEN_RATIO;

	c = initval;

	len = length;

	/* Set up the internal state */

	a = b = c = JHASH_INITVAL + (length<<2) + initval;

	while (len >= 3) {

	/* Handle most of the key */

	while (length > 3) {

		a += k[0];

		b += k[1];

		c += k[2];

		__jhash_mix(a, b, c);

		k += 3; len -= 3;

		length -= 3;

		k += 3;

	}

	c += length * 4;

	switch (len) {

	/* 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_mix(a,b,c);

		__jhash_final(a, b, c);

	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).

 *

 * NOTE: In particular the "c += length; __jhash_mix(a,b,c);" normally

 *       done at the end is not done here.

 */

/* 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_GOLDEN_RATIO;

	b += JHASH_GOLDEN_RATIO;

	a += JHASH_INITVAL;

	b += JHASH_INITVAL;

	c += initval;

	__jhash_mix(a, b, c);

	__jhash_final(a, b, c);

	return c;

}