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79 lines
2.9 KiB
C
79 lines
2.9 KiB
C
/*
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Simple DirectMedia Layer
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Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>
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This software is provided 'as-is', without any express or implied
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warranty. In no event will the authors be held liable for any damages
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arising from the use of this software.
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Permission is granted to anyone to use this software for any purpose,
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including commercial applications, and to alter it and redistribute it
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freely, subject to the following restrictions:
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1. The origin of this software must not be misrepresented; you must not
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claim that you wrote the original software. If you use this software
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in a product, an acknowledgment in the product documentation would be
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appreciated but is not required.
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2. Altered source versions must be plainly marked as such, and must not be
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misrepresented as being the original software.
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3. This notice may not be removed or altered from any source distribution.
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*/
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#include "SDL_internal.h"
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/* This file contains portable random functions for SDL */
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static Uint64 SDL_rand_state;
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static SDL_bool SDL_rand_initialized = SDL_FALSE;
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void SDL_srand(Uint64 seed)
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{
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if (!seed) {
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seed = SDL_GetPerformanceCounter();
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}
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SDL_rand_state = seed;
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SDL_rand_initialized = SDL_TRUE;
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}
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Uint32 SDL_rand_bits(void)
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{
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if (!SDL_rand_initialized) {
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SDL_srand(0);
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}
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// The C and A parameters of this LCG have been chosen based on hundreds
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// of core-hours of testing with PractRand and TestU01's Crush.
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// Using a 32-bit A improves performance on 32-bit architectures.
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// C can be any odd number, but < 256 generates smaller code on ARM32
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// These values perform as well as a full 64-bit implementation against
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// Crush and PractRand. Plus, their worst-case performance is better
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// than common 64-bit constants when tested against PractRand using seeds
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// with only a single bit set.
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// We tested all 32-bit and 33-bit A with all C < 256 from a v2 of:
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// Steele GL, Vigna S. Computationally easy, spectrally good multipliers
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// for congruential pseudorandom number generators.
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// Softw Pract Exper. 2022;52(2):443-458. doi: 10.1002/spe.3030
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// https://arxiv.org/abs/2001.05304v2
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SDL_rand_state = SDL_rand_state * 0xff1cd035ul + 0x05;
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// Only return top 32 bits because they have a longer period
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return (Uint32)(SDL_rand_state >> 32);
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}
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Sint32 SDL_rand_n(Sint32 n)
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{
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// Algorithm: get 32 bits from SDL_rand_bits() and treat it as a 0.32 bit
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// fixed point number. Multiply by the 31.0 bit n to get a 31.32 bit
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// result. Shift right by 32 to get the 31 bit integer that we want.
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// On 32-bit arch, the compiler will optimize to a single 32-bit multiply
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Uint64 val = (Uint64)SDL_rand_bits() * n;
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return (Sint32)(val >> 32);
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}
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float SDL_rand_float(void)
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{
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// Note: its using 24 bits because float has 23 bits significand + 1 implicit bit
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return (SDL_rand_bits() >> (32 - 24)) * 0x1p-24f;
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}
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