linux/include/linux/hash.h

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

#include <asm/types.h>
#include <linux/compiler.h>

/*
 * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and
 * fs/inode.c.  It's not actually prime any more (the previous primes
 * were actively bad for hashing), but the name remains.
 */
#if BITS_PER_LONG == 32
#define GOLDEN_RATIO_PRIME …
#define hash_long …
#elif BITS_PER_LONG == 64
#define hash_long(val, bits) …
#define GOLDEN_RATIO_PRIME …
#else
#error Wordsize not 32 or 64
#endif

/*
 * This hash multiplies the input by a large odd number and takes the
 * high bits.  Since multiplication propagates changes to the most
 * significant end only, it is essential that the high bits of the
 * product be used for the hash value.
 *
 * Chuck Lever verified the effectiveness of this technique:
 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
 *
 * Although a random odd number will do, it turns out that the golden
 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
 * properties.  (See Knuth vol 3, section 6.4, exercise 9.)
 *
 * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2,
 * which is very slightly easier to multiply by and makes no
 * difference to the hash distribution.
 */
#define GOLDEN_RATIO_32 …
#define GOLDEN_RATIO_64 …

#ifdef CONFIG_HAVE_ARCH_HASH
/* This header may use the GOLDEN_RATIO_xx constants */
#include <asm/hash.h>
#endif

/*
 * The _generic versions exist only so lib/test_hash.c can compare
 * the arch-optimized versions with the generic.
 *
 * Note that if you change these, any <asm/hash.h> that aren't updated
 * to match need to have their HAVE_ARCH_* define values updated so the
 * self-test will not false-positive.
 */
#ifndef HAVE_ARCH__HASH_32
#define __hash_32 …
#endif
static inline u32 __hash_32_generic(u32 val)
{ … }

static inline u32 hash_32(u32 val, unsigned int bits)
{ … }

#ifndef HAVE_ARCH_HASH_64
#define hash_64 …
#endif
static __always_inline u32 hash_64_generic(u64 val, unsigned int bits)
{ … }

static inline u32 hash_ptr(const void *ptr, unsigned int bits)
{ … }

/* This really should be called fold32_ptr; it does no hashing to speak of. */
static inline u32 hash32_ptr(const void *ptr)
{ … }

#endif /* _LINUX_HASH_H */