From 3ee4bff8ac3eb14e9d0619efbc6b6aaf467a7e75 Mon Sep 17 00:00:00 2001 From: John Kaniarz Date: Wed, 19 Jun 2024 12:49:54 -0400 Subject: [PATCH] Upgraded constants after statistical testing. --- src/stdlib/SDL_random.c | 29 +++++++++++++++++------------ 1 file changed, 17 insertions(+), 12 deletions(-) diff --git a/src/stdlib/SDL_random.c b/src/stdlib/SDL_random.c index 83be9ae6a1..2f3cac502e 100644 --- a/src/stdlib/SDL_random.c +++ b/src/stdlib/SDL_random.c @@ -54,24 +54,29 @@ float SDL_rand_float(void) return (SDL_rand() >> (32-24)) * 0x1p-24f; } -/* A fast psuedo-random number generator. - * Not suitable for cryptography or gambling - */ Uint32 SDL_rand_r(Uint64 *state) { if (!state) { return 0; } - // Multiplier from Table 6 of - // Steele GL, Vigna S. Computationally easy, spectrally good multipliers - // for congruential pseudorandom number generators. - // Softw Pract Exper. 2022;52(2):443-458. doi: 10.1002/spe.3030 + // The C and A parameters of this LCG have been chosen based on hundreds + // of core-hours of testing with PractRand and TestU01's Crush. + // Using a 32-bit A improves performance on 32-bit architectures. + // C can be any odd number, but < 256 generates smaller code on ARM32 + // These values perform as well as a full 64-bit implementation against + // Crush and PractRand. Plus, their worst-case performance is better + // than common 64-bit constants when tested against PractRand using seeds + // with only a single bit set. - // 32-bit 'a' improves performance on 32-bit architectures - // 'c' can be any odd number, but < 256 generates smaller code on some arch - *state = *state * 0xf9b25d65ul + 0xFD; + // We tested all 32-bit and 33-bit A with all C < 256 from a v2 of: + // Steele GL, Vigna S. Computationally easy, spectrally good multipliers + // for congruential pseudorandom number generators. + // Softw Pract Exper. 2022;52(2):443-458. doi: 10.1002/spe.3030 + // https://arxiv.org/abs/2001.05304v2 - // Only return top 32 bits because they have a longer period - return (Uint32)(*state >> 32); + *state = *state * 0xff1cd035ul + 0x05; + + // Only return top 32 bits because they have a longer period + return (Uint32)(*state >> 32); }