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hash_id: Change to xorshift*. #455

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merged 1 commit into from
Jan 3, 2024
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hash_id: Change to xorshift*. #455

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11 changes: 7 additions & 4 deletions include/adt/hash.h
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Original file line number Diff line number Diff line change
Expand Up @@ -15,14 +15,17 @@
#define FSM_PHI_32 0x9e3779b9UL
#define FSM_PHI_64 (uint64_t)0x9e3779b97f4a7c15UL

/* A suitable hash function for individual sequentially allocated
* identifiers. See Knuth 6.4, Fibonacci hashing. */

SUPPRESS_EXPECTED_UNSIGNED_INTEGER_OVERFLOW()
static __inline__ uint64_t
hash_id(unsigned id)
{
return FSM_PHI_64 * (uint64_t)(id + (unsigned)1);
/* xorshift* A1(12,25,27),
* from http://vigna.di.unimi.it/ftp/papers/xorshift.pdf */
uint64_t x = id + 1;
x ^= x >> 12; // a
x ^= x << 25; // b
x ^= x >> 27; // c
return x * 2685821657736338717LLU;
}

/* FNV-1a hash function, 32 and 64 bit versions. This is in the public
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