hash.h revision e43606c2b13ad7fc1af2bbe4a61cf8480ee3a532 
1#ifndef _LINUX_HASH_H
2#define _LINUX_HASH_H
3
4#include <inttypes.h>
5#include "arch/arch.h"
6
7/* Fast hashing routine for a long.
8   (C) 2002 William Lee Irwin III, IBM */
9
10/*
11 * Knuth recommends primes in approximately golden ratio to the maximum
12 * integer representable by a machine word for multiplicative hashing.
13 * Chuck Lever verified the effectiveness of this technique:
14 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
15 *
16 * These primes are chosen to be bit-sparse, that is operations on
17 * them can use shifts and additions instead of multiplications for
18 * machines where multiplications are slow.
19 */
20
21#if BITS_PER_LONG == 32
22/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
23#define GOLDEN_RATIO_PRIME 0x9e370001UL
24#elif BITS_PER_LONG == 64
25/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
26#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
27#else
28#error Define GOLDEN_RATIO_PRIME for your wordsize.
29#endif
30
31static inline unsigned long hash_long(unsigned long val, unsigned int bits)
32{
33	unsigned long hash = val;
34
35#if BITS_PER_LONG == 64
36	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
37	unsigned long n = hash;
38	n <<= 18;
39	hash -= n;
40	n <<= 33;
41	hash -= n;
42	n <<= 3;
43	hash += n;
44	n <<= 3;
45	hash -= n;
46	n <<= 4;
47	hash += n;
48	n <<= 2;
49	hash += n;
50#else
51	/* On some cpus multiply is faster, on others gcc will do shifts */
52	hash *= GOLDEN_RATIO_PRIME;
53#endif
54
55	/* High bits are more random, so use them. */
56	return hash >> (BITS_PER_LONG - bits);
57}
58
59static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
60{
61	return hash_long((uintptr_t)ptr, bits);
62}
63
64/*
65 * Bob Jenkins jhash
66 */
67
68#define JHASH_INITVAL	GOLDEN_RATIO_PRIME
69
70static inline uint32_t rol32(uint32_t word, uint32_t shift)
71{
72	return (word << shift) | (word >> (32 - shift));
73}
74
75/* __jhash_mix -- mix 3 32-bit values reversibly. */
76#define __jhash_mix(a, b, c)			\
77{						\
78	a -= c;  a ^= rol32(c, 4);  c += b;	\
79	b -= a;  b ^= rol32(a, 6);  a += c;	\
80	c -= b;  c ^= rol32(b, 8);  b += a;	\
81	a -= c;  a ^= rol32(c, 16); c += b;	\
82	b -= a;  b ^= rol32(a, 19); a += c;	\
83	c -= b;  c ^= rol32(b, 4);  b += a;	\
84}
85
86/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
87#define __jhash_final(a, b, c)			\
88{						\
89	c ^= b; c -= rol32(b, 14);		\
90	a ^= c; a -= rol32(c, 11);		\
91	b ^= a; b -= rol32(a, 25);		\
92	c ^= b; c -= rol32(b, 16);		\
93	a ^= c; a -= rol32(c, 4);		\
94	b ^= a; b -= rol32(a, 14);		\
95	c ^= b; c -= rol32(b, 24);		\
96}
97
98static inline uint32_t jhash(const void *key, uint32_t length, uint32_t initval)
99{
100	const uint8_t *k = key;
101	uint32_t a, b, c;
102
103	/* Set up the internal state */
104	a = b = c = JHASH_INITVAL + length + initval;
105
106	/* All but the last block: affect some 32 bits of (a,b,c) */
107	while (length > 12) {
108		a += *k;
109		b += *(k + 4);
110		c += *(k + 8);
111		__jhash_mix(a, b, c);
112		length -= 12;
113		k += 12;
114	}
115
116	/* Last block: affect all 32 bits of (c) */
117	/* All the case statements fall through */
118	switch (length) {
119	case 12: c += (uint32_t) k[11] << 24;
120	case 11: c += (uint32_t) k[10] << 16;
121	case 10: c += (uint32_t) k[9] << 8;
122	case 9:  c += k[8];
123	case 8:  b += (uint32_t) k[7] << 24;
124	case 7:  b += (uint32_t) k[6] << 16;
125	case 6:  b += (uint32_t) k[5] << 8;
126	case 5:  b += k[4];
127	case 4:  a += (uint32_t) k[3] << 24;
128	case 3:  a += (uint32_t) k[2] << 16;
129	case 2:  a += (uint32_t) k[1] << 8;
130	case 1:  a += k[0];
131		 __jhash_final(a, b, c);
132	case 0: /* Nothing left to add */
133		break;
134	}
135
136	return c;
137}
138
139#endif /* _LINUX_HASH_H */
140