#ifndef _LINUX_HASH_H
  #define _LINUX_HASH_H

  /* Fast hashing routine for ints,  longs and pointers.

     (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.
   */

  
  #include <asm/types.h>

  /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */

  #define GOLDEN_RATIO_PRIME_32 0x9e370001UL

  /*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */

  #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
  
  #if BITS_PER_LONG == 32
  #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_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_PRIME_64

  #else

  #error Wordsize not 32 or 64

  #endif

  static inline u64 hash_64(u64 val, unsigned int bits)

  {

  	u64 hash = val;

  	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */

  	u64 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;

  
  	/* High bits are more random, so use them. */
  	return hash >> (64 - bits);
  }
  
  static inline u32 hash_32(u32 val, unsigned int bits)
  {

  	/* On some cpus multiply is faster, on others gcc will do shifts */

  	u32 hash = val * GOLDEN_RATIO_PRIME_32;

  
  	/* High bits are more random, so use them. */

  	return hash >> (32 - bits);

  }

  static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
  {
  	return hash_long((unsigned long)ptr, bits);
  }
  #endif /* _LINUX_HASH_H */