164 lines
4.1 KiB
C
164 lines
4.1 KiB
C
#ifndef _LINUX_HASH_H
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#define _LINUX_HASH_H
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#include <inttypes.h>
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#include "arch/arch.h"
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/* Fast hashing routine for a long.
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(C) 2002 William Lee Irwin III, IBM */
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/*
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* Knuth recommends primes in approximately golden ratio to the maximum
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* integer representable by a machine word for multiplicative hashing.
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* Chuck Lever verified the effectiveness of this technique:
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* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
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*
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* These primes are chosen to be bit-sparse, that is operations on
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* them can use shifts and additions instead of multiplications for
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* machines where multiplications are slow.
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*/
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#if BITS_PER_LONG == 32
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/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
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#define GOLDEN_RATIO_PRIME 0x9e370001UL
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#elif BITS_PER_LONG == 64
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/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
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#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
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#else
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#error Define GOLDEN_RATIO_PRIME for your wordsize.
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#endif
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/*
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* The above primes are actively bad for hashing, since they are
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* too sparse. The 32-bit one is mostly ok, the 64-bit one causes
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* real problems. Besides, the "prime" part is pointless for the
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* multiplicative hash.
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*
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* Although a random odd number will do, it turns out that the golden
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* ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
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* properties.
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*
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* These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
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* (See Knuth vol 3, section 6.4, exercise 9.)
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*/
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#define GOLDEN_RATIO_32 0x61C88647
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#define GOLDEN_RATIO_64 0x61C8864680B583EBull
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static inline unsigned long __hash_long(uint64_t val)
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{
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uint64_t hash = val;
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#if BITS_PER_LONG == 64
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hash *= GOLDEN_RATIO_64;
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#else
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/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
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uint64_t n = hash;
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n <<= 18;
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hash -= n;
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n <<= 33;
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hash -= n;
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n <<= 3;
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hash += n;
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n <<= 3;
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hash -= n;
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n <<= 4;
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hash += n;
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n <<= 2;
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hash += n;
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#endif
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return hash;
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}
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static inline unsigned long hash_long(unsigned long val, unsigned int bits)
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{
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/* High bits are more random, so use them. */
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return __hash_long(val) >> (BITS_PER_LONG - bits);
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}
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static inline uint64_t __hash_u64(uint64_t val)
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{
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return val * GOLDEN_RATIO_64;
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}
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static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
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{
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return hash_long((uintptr_t)ptr, bits);
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}
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/*
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* Bob Jenkins jhash
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*/
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#define JHASH_INITVAL GOLDEN_RATIO_32
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static inline uint32_t rol32(uint32_t word, uint32_t shift)
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{
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return (word << shift) | (word >> (32 - shift));
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}
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/* __jhash_mix -- mix 3 32-bit values reversibly. */
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#define __jhash_mix(a, b, c) \
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{ \
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a -= c; a ^= rol32(c, 4); c += b; \
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b -= a; b ^= rol32(a, 6); a += c; \
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c -= b; c ^= rol32(b, 8); b += a; \
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a -= c; a ^= rol32(c, 16); c += b; \
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b -= a; b ^= rol32(a, 19); a += c; \
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c -= b; c ^= rol32(b, 4); b += a; \
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}
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/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
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#define __jhash_final(a, b, c) \
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{ \
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c ^= b; c -= rol32(b, 14); \
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a ^= c; a -= rol32(c, 11); \
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b ^= a; b -= rol32(a, 25); \
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c ^= b; c -= rol32(b, 16); \
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a ^= c; a -= rol32(c, 4); \
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b ^= a; b -= rol32(a, 14); \
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c ^= b; c -= rol32(b, 24); \
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}
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static inline uint32_t jhash(const void *key, uint32_t length, uint32_t initval)
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{
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const uint8_t *k = key;
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uint32_t a, b, c;
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/* Set up the internal state */
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a = b = c = JHASH_INITVAL + length + initval;
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/* All but the last block: affect some 32 bits of (a,b,c) */
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while (length > 12) {
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a += *k;
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b += *(k + 4);
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c += *(k + 8);
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__jhash_mix(a, b, c);
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length -= 12;
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k += 12;
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}
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/* Last block: affect all 32 bits of (c) */
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/* All the case statements fall through */
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switch (length) {
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case 12: c += (uint32_t) k[11] << 24;
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case 11: c += (uint32_t) k[10] << 16;
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case 10: c += (uint32_t) k[9] << 8;
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case 9: c += k[8];
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case 8: b += (uint32_t) k[7] << 24;
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case 7: b += (uint32_t) k[6] << 16;
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case 6: b += (uint32_t) k[5] << 8;
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case 5: b += k[4];
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case 4: a += (uint32_t) k[3] << 24;
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case 3: a += (uint32_t) k[2] << 16;
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case 2: a += (uint32_t) k[1] << 8;
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case 1: a += k[0];
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__jhash_final(a, b, c);
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case 0: /* Nothing left to add */
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break;
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}
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return c;
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}
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#endif /* _LINUX_HASH_H */
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