98 lines
2.5 KiB
C
98 lines
2.5 KiB
C
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/* Portable pseudorandom number generator
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* Based on a 24,55 Fibonacci generator, using 55/503 rejection
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* c.f.- TAoCP, 3.2.2(7), for j=24,k=55,m=2^64
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*
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* THIS FILE IS PUBLIC DOMAIN CODE.
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*
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* Written by Bob Adolf.
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* Attribution is appreciated but by no means legally required.
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*
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* This function is sufficient for most non-crypto applications.
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* It passes all but one of George Marsaglia's "diehard" randomness tests.
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* (overlapping 5-tuple permutations, which is allegedly buggy)
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*/
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#ifndef __PRNG_H__
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#define __PRNG_H__
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#include <inttypes.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <stdlib.h>
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#define LAG1 (UINT16_C(24))
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#define LAG2 (UINT16_C(55))
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#define RAND_SSIZE ((UINT16_C(1)) << 6)
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#define RAND_SMASK (RAND_SSIZE - 1)
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#define RAND_EXHAUST_LIMIT LAG2
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// 10x is a heuristic, it just needs to be large enough to remove correlation
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#define RAND_REFILL_COUNT ((LAG2 * 10) - RAND_EXHAUST_LIMIT)
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struct prng_rand_t
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{
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uint64_t s[RAND_SSIZE]; // Lags
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uint_fast16_t i; // Location of the current lag
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uint_fast16_t c; // Exhaustion count
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};
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#define PRNG_RAND_MAX UINT64_MAX
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static inline uint64_t prng_rand(struct prng_rand_t* state)
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{
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uint_fast16_t i;
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uint_fast16_t r, new_rands = 0;
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if (!state->c)
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{ // Randomness exhausted, run forward to refill
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new_rands += RAND_REFILL_COUNT + 1;
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state->c = RAND_EXHAUST_LIMIT - 1;
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}
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else
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{
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new_rands = 1;
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state->c--;
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}
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for (r = 0; r < new_rands; r++)
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{
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i = state->i;
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state->s[i & RAND_SMASK] = state->s[(i + RAND_SSIZE - LAG1) & RAND_SMASK]
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+ state->s[(i + RAND_SSIZE - LAG2) & RAND_SMASK];
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state->i++;
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}
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return state->s[i & RAND_SMASK];
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}
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static inline void prng_srand(uint64_t seed, struct prng_rand_t* state)
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{
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uint_fast16_t i;
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// Naive seed
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state->c = RAND_EXHAUST_LIMIT;
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state->i = 0;
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state->s[0] = seed;
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for (i = 1; i < RAND_SSIZE; i++)
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{
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// Arbitrary magic, mostly to eliminate the effect of low-value seeds.
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// Probably could be better, but the run-up obviates any real need to.
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state->s[i] = i * (UINT64_C(2147483647)) + seed;
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}
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// Run forward 10,000 numbers
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for (i = 0; i < 10000; i++)
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{
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prng_rand(state);
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}
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}
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// Clean up our macros
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#undef LAG1
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#undef LAG2
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#undef RAND_SSIZE
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#undef RAND_SMASK
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#undef RAND_EXHAUST_LIMIT
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#undef RAND_REFILL_COUNT
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// PRNG_RAND_MAX is exported
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#endif
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