hash.h

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#pragma once
#include <osmocom/core/log2.h>
/* Fast hashing routine for ints,  longs and pointers.
   (C) 2002 Nadia Yvette Chambers, IBM */
 
#include <limits.h>
#if ULONG_MAX == 4294967295
#define BITS_PER_LONG 32
#else
#define BITS_PER_LONG 64
#endif
 
/*
 * 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 GOLDEN_RATIO_32
#define hash_long(val, bits) hash_32(val, bits)
#elif BITS_PER_LONG == 64
#define hash_long(val, bits) hash_64(val, bits)
#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64
#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 0x61C88647
#define GOLDEN_RATIO_64 0x61C8864680B583EBull
 
/*
 * 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 __hash_32_generic
#endif
static inline uint32_t __hash_32_generic(uint32_t val)
{
        return val * GOLDEN_RATIO_32;
}
 
#ifndef HAVE_ARCH_HASH_32
#define hash_32 hash_32_generic
#endif
static inline uint32_t hash_32_generic(uint32_t val, unsigned int bits)
{
        /* High bits are more random, so use them. */
        return __hash_32(val) >> (32 - bits);
}
 
#ifndef HAVE_ARCH_HASH_64
#define hash_64 hash_64_generic
#endif
static __always_inline uint32_t hash_64_generic(uint64_t val, unsigned int bits)
{
#if BITS_PER_LONG == 64
        /* 64x64-bit multiply is efficient on all 64-bit processors */
        return val * GOLDEN_RATIO_64 >> (64 - bits);
#else
        /* Hash 64 bits using only 32x32-bit multiply. */
        return hash_32((uint32_t)val ^ __hash_32(val >> 32), bits);
#endif
}
 
static inline uint32_t hash_ptr(const void *ptr, unsigned int bits)
{
        return hash_long((unsigned long)ptr, bits);
}
 
/* This really should be called fold32_ptr; it does no hashing to speak of. */
static inline uint32_t hash32_ptr(const void *ptr)
{
        unsigned long val = (unsigned long)ptr;
 
#if BITS_PER_LONG == 64
        val ^= (val >> 32);
#endif
        return (uint32_t)val;
}