52 lines
1.6 KiB
C
52 lines
1.6 KiB
C
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/* Single-header LCG random generator and faster-than-modulo rand_until, rand_between */
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/* LICENCE: Feel free to use it but please mention MagosIT Kft or its youtube channel */
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#ifndef FAST_RAND_H
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#define FAST_RAND_H
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#include <stdint.h>
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#ifndef NO_CSTDLIB
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#include <stdlib.h>
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#endif /* NO_CSTDLIB */
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/* Currently a single integer is enough */
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typedef uint32_t rand_state;
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/** Creates a random number generator state with given seed */
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static inline rand_state init_rand_with(uint32_t seed) {
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return seed;
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}
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#ifndef NO_CSTDLIB
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/** Creates a random number generator state with arc4random() which does not need seeding as it uses system etropy */
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static inline rand_state init_rand() {
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return arc4random();
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}
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#endif /* NO_CSTDLIB */
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// 32-bit LCG
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static inline uint32_t lcg(uint32_t *state) {
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*state = *state * 1664525u + 1013904223u;
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return *state;
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}
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/** Pick a "reasonably random" number in [0, until-1] without modulus */
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static inline uint32_t rand_until(uint32_t *state, uint32_t until) {
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uint32_t rand = lcg(state);
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// Multiply by "until", take the upper 32 bits of the 64-bit result
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return (uint32_t)(((uint64_t)rand * until) >> 32);
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}
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/**
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* Pick a "reasonably random" number in [from, to) without modulus.
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*
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* @param state The random state - See: init_rand() and init_rand_with(seed)
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* @param from The smallest possible value
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* @param to The biggest possible value + 1
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* @returns A value in [from, to) interval
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*/
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static inline uint32_t rand_between(uint32_t *state, uint32_t from, uint32_t to) {
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return from + rand_until(state, to - from);
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}
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#endif /* FAST_RAND_H */
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