[linux.git] / include / linux / hash.h

#ifndef _LINUX_HASH_H
#define _LINUX_HASH_H
/* Fast hashing routine for a long.
   (C) 2002 William Lee Irwin III, IBM */

/*
 * Knuth recommends primes in approximately golden ratio to the maximum
 * integer representable by a machine word for multiplicative hashing.
 * Chuck Lever verified the effectiveness of this technique:
 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
 *
 * These primes are chosen to be bit-sparse, that is operations on
 * them can use shifts and additions instead of multiplications for
 * machines where multiplications are slow.
 */
#if BITS_PER_LONG == 32
/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
#define GOLDEN_RATIO_PRIME 0x9e370001UL
#elif BITS_PER_LONG == 64
/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
#else
#error Define GOLDEN_RATIO_PRIME for your wordsize.
#endif

static inline unsigned long hash_long(unsigned long val, unsigned int bits)
{
	unsigned long hash = val;

#if BITS_PER_LONG == 64
	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
	unsigned long n = hash;
	n <<= 18;
	hash -= n;
	n <<= 33;
	hash -= n;
	n <<= 3;
	hash += n;
	n <<= 3;
	hash -= n;
	n <<= 4;
	hash += n;
	n <<= 2;
	hash += n;
#else
	/* On some cpus multiply is faster, on others gcc will do shifts */
	hash *= GOLDEN_RATIO_PRIME;
#endif

	/* High bits are more random, so use them. */
	return hash >> (BITS_PER_LONG - bits);
}
	
static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
{
	return hash_long((unsigned long)ptr, bits);
}
#endif /* _LINUX_HASH_H */
Commit	Line	Data
1da177e4 LT	1	#ifndef _LINUX_HASH_H
	2	#define _LINUX_HASH_H
	3	/* Fast hashing routine for a long.
	4	(C) 2002 William Lee Irwin III, IBM */
	5
	6	/*
	7	* Knuth recommends primes in approximately golden ratio to the maximum
	8	* integer representable by a machine word for multiplicative hashing.
	9	* Chuck Lever verified the effectiveness of this technique:
	10	* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
	11	*
	12	* These primes are chosen to be bit-sparse, that is operations on
	13	* them can use shifts and additions instead of multiplications for
	14	* machines where multiplications are slow.
	15	*/
	16	#if BITS_PER_LONG == 32
	17	/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
	18	#define GOLDEN_RATIO_PRIME 0x9e370001UL
	19	#elif BITS_PER_LONG == 64
	20	/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
	21	#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
	22	#else
	23	#error Define GOLDEN_RATIO_PRIME for your wordsize.
	24	#endif
	25
	26	static inline unsigned long hash_long(unsigned long val, unsigned int bits)
	27	{
	28	unsigned long hash = val;
	29
	30	#if BITS_PER_LONG == 64
	31	/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
	32	unsigned long n = hash;
	33	n <<= 18;
	34	hash -= n;
	35	n <<= 33;
	36	hash -= n;
	37	n <<= 3;
	38	hash += n;
	39	n <<= 3;
	40	hash -= n;
	41	n <<= 4;
	42	hash += n;
	43	n <<= 2;
	44	hash += n;
	45	#else
	46	/* On some cpus multiply is faster, on others gcc will do shifts */
	47	hash *= GOLDEN_RATIO_PRIME;
	48	#endif
	49
	50	/* High bits are more random, so use them. */
	51	return hash >> (BITS_PER_LONG - bits);
	52	}
	53
	54	static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
	55	{
	56	return hash_long((unsigned long)ptr, bits);
	57	}
	58	#endif /* _LINUX_HASH_H */