| #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 <stdint.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 __WORDSIZE == 32 |

| #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32 |

| #define hash_long(val, bits) hash_32(val, bits) |

| #elif __WORDSIZE == 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 uint64_t hash_64(const uint64_t val, const unsigned int bits) |

| { |

| uint64_t hash = val; |

| |

| /* 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; |

| |

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

| return hash >> (64 - bits); |

| } |

| |

| static inline uint32_t hash_32(uint32_t val, unsigned int bits) |

| { |

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

| uint32_t 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 */ |