Added SDL_srand(), SDL_rand(), and SDL_rand_r() (thanks @JKaniarz!)

These are simple random functions that should not be used for serious random number generation.

Fixes https://github.com/libsdl-org/SDL/issues/4968

src/stdlib/SDL_random.c

@@ -0,0 +1,80 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2024 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;
+
+void SDL_srand(Uint64 seed)
+{
+    SDL_rand_state = seed;
+}
+
+Uint32 SDL_rand(void)
+{
+	return SDL_rand_r(&SDL_rand_state);
+}
+
+/*
+ * Return a number between [0, n)
+ * Fast but slightly biased. Don't run your casino with this.
+ */
+Sint32 SDL_rand_n(Sint32 n)
+{
+	// On 32-bit arch, the compiler will optimize to a single 32-bit multiply
+	Uint64 val = (Uint64)SDL_rand() * n;
+	return (Sint32)(val >> 32);
+}
+
+/*
+ * Random float in range [0,1)
+ */
+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;
+    }
+
+    if (!*state) {
+        *state = SDL_GetPerformanceCounter();
+    }
+
+	// 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
+
+	// 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;
+
+	// Only return top 32 bits because they have a longer period
+	return (Uint32)(*state >> 32);
+}