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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.
 */

#ifdef __WORDSIZE
#define BITS_PER_LONG   __WORDSIZE
#else
#define BITS_PER_LONG   32
#endif

#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 */

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