text: #ifndef _LINUX_HASH_H
  #define _LINUX_HASH_H
  
  #include <inttypes.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.
   */
  
  #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
  
  /*
   * The above primes are actively bad for hashing, since they are
   * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
   * real problems. Besides, the "prime" part is pointless for the
   * multiplicative hash.
   *
   * 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.
   *
   * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
   * (See Knuth vol 3, section 6.4, exercise 9.)
   */
  #define GOLDEN_RATIO_32 0x61C88647
  #define GOLDEN_RATIO_64 0x61C8864680B583EBull
  
  static inline unsigned long __hash_long(uint64_t val)
  {
  	uint64_t hash = val;
  
  #if BITS_PER_LONG == 64
  	hash *= GOLDEN_RATIO_64;
  #else
  	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  	uint64_t 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;
  #endif
  
  	return hash;
  }
  
  static inline unsigned long hash_long(unsigned long val, unsigned int bits)
  {
  	/* High bits are more random, so use them. */
  	return __hash_long(val) >> (BITS_PER_LONG - bits);
  }
  
  static inline uint64_t __hash_u64(uint64_t val)
  {
  	return val * GOLDEN_RATIO_64;
  }
  	
  static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
  {
  	return hash_long((uintptr_t)ptr, bits);
  }
  
  /*
   * Bob Jenkins jhash
   */
  
  #define JHASH_INITVAL	GOLDEN_RATIO_32
  
  static inline uint32_t rol32(uint32_t word, uint32_t shift)
  {
  	return (word << shift) | (word >> (32 - shift));
  }
  
  /* __jhash_mix -- mix 3 32-bit values reversibly. */
  #define __jhash_mix(a, b, c)			\
  {						\
  	a -= c;  a ^= rol32(c, 4);  c += b;	\
  	b -= a;  b ^= rol32(a, 6);  a += c;	\
  	c -= b;  c ^= rol32(b, 8);  b += a;	\
  	a -= c;  a ^= rol32(c, 16); c += b;	\
  	b -= a;  b ^= rol32(a, 19); a += c;	\
  	c -= b;  c ^= rol32(b, 4);  b += a;	\
  }
  
  /* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
  #define __jhash_final(a, b, c)			\
  {						\
  	c ^= b; c -= rol32(b, 14);		\
  	a ^= c; a -= rol32(c, 11);		\
  	b ^= a; b -= rol32(a, 25);		\
  	c ^= b; c -= rol32(b, 16);		\
  	a ^= c; a -= rol32(c, 4);		\
  	b ^= a; b -= rol32(a, 14);		\
  	c ^= b; c -= rol32(b, 24);		\
  }
  
  static inline uint32_t jhash(const void *key, uint32_t length, uint32_t initval)
  {
  	const uint8_t *k = key;
  	uint32_t a, b, c;
  
  	/* Set up the internal state */
  	a = b = c = JHASH_INITVAL + length + initval;
  
  	/* All but the last block: affect some 32 bits of (a,b,c) */
  	while (length > 12) {
  		a += *k;
  		b += *(k + 4);
  		c += *(k + 8);
  		__jhash_mix(a, b, c);
  		length -= 12;
  		k += 12;
  	}
  
  	/* Last block: affect all 32 bits of (c) */
  	/* All the case statements fall through */
  	switch (length) {
  	case 12: c += (uint32_t) k[11] << 24;
  	case 11: c += (uint32_t) k[10] << 16;
  	case 10: c += (uint32_t) k[9] << 8;
  	case 9:  c += k[8];
  	case 8:  b += (uint32_t) k[7] << 24;
  	case 7:  b += (uint32_t) k[6] << 16;
  	case 6:  b += (uint32_t) k[5] << 8;
  	case 5:  b += k[4];
  	case 4:  a += (uint32_t) k[3] << 24;
  	case 3:  a += (uint32_t) k[2] << 16;
  	case 2:  a += (uint32_t) k[1] << 8;
  	case 1:  a += k[0];
  		 __jhash_final(a, b, c);
  	case 0: /* Nothing left to add */
  		break;
  	}
  
  	return c;
  }
  
  #endif /* _LINUX_HASH_H */
  