initial commit, 4.5 stable

thirdparty/sdl/stdlib/SDL_random.c

@@ -0,0 +1,115 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2025 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+// This file contains portable random functions for SDL
+
+static Uint64 SDL_rand_state;
+static bool SDL_rand_initialized = false;
+
+void SDL_srand(Uint64 seed)
+{
+    if (!seed) {
+        seed = SDL_GetPerformanceCounter();
+    }
+    SDL_rand_state = seed;
+    SDL_rand_initialized = true;
+}
+
+Sint32 SDL_rand(Sint32 n)
+{
+    if (!SDL_rand_initialized) {
+        SDL_srand(0);
+    }
+
+    return SDL_rand_r(&SDL_rand_state, n);
+}
+
+float SDL_randf(void)
+{
+    if (!SDL_rand_initialized) {
+        SDL_srand(0);
+    }
+
+    return SDL_randf_r(&SDL_rand_state);
+}
+
+Uint32 SDL_rand_bits(void)
+{
+    if (!SDL_rand_initialized) {
+        SDL_srand(0);
+    }
+
+    return SDL_rand_bits_r(&SDL_rand_state);
+}
+
+Uint32 SDL_rand_bits_r(Uint64 *state)
+{
+    if (!state) {
+        return 0;
+    }
+
+    // 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.
+
+    // 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
+
+    *state = *state * 0xff1cd035ul + 0x05;
+
+    // Only return top 32 bits because they have a longer period
+    return (Uint32)(*state >> 32);
+}
+
+Sint32 SDL_rand_r(Uint64 *state, Sint32 n)
+{
+    // Algorithm: get 32 bits from SDL_rand_bits() and treat it as a 0.32 bit
+    // fixed point number. Multiply by the 31.0 bit n to get a 31.32 bit
+    // result. Shift right by 32 to get the 31 bit integer that we want.
+
+    if (n < 0) {
+        // The algorithm looks like it works for numbers < 0 but it has an
+        // infinitesimal chance of returning a value out of range.
+        // Returning -SDL_rand(abs(n)) blows up at INT_MIN instead.
+        // It's easier to just say no.
+        return 0;
+    }
+
+    // On 32-bit arch, the compiler will optimize to a single 32-bit multiply
+    Uint64 val = (Uint64)SDL_rand_bits_r(state) * n;
+    return (Sint32)(val >> 32);
+}
+
+float SDL_randf_r(Uint64 *state)
+{
+    // Note: its using 24 bits because float has 23 bits significand + 1 implicit bit
+    return (SDL_rand_bits_r(state) >> (32 - 24)) * 0x1p-24f;
+}
+