hash.h

#ifndef _LINUX_HASH_H
#define _LINUX_HASH_H
/* Fast hashing routine for ints,  longs and pointers.
   (C) 2002 Nadia Yvette Chambers, 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.
 */

#include <sys/types.h>
#include <asm/bitsperlong.h>

/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
#define GOLDEN_RATIO_PRIME_32 0x9e370001UL

static inline u_int32_t hash_32(u_int32_t val, unsigned int bits)
{
	/* On some cpus multiply is faster, on others gcc will do shifts */
	u_int32_t hash = val * GOLDEN_RATIO_PRIME_32;

	/* High bits are more random, so use them. */
	return hash >> (32 - bits);
}

#endif /* _LINUX_HASH_H */