Merge pull request #10983 from ekrctb/pseudo-random

Add `StatelessRNG` to be used for randomly-looking visual elements

osu.Game/Utils/StatelessRNG.cs

@@ -0,0 +1,79 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+
+namespace osu.Game.Utils
+{
+    /// <summary>
+    /// Provides a fast stateless function that can be used in randomly-looking visual elements.
+    /// </summary>
+    public static class StatelessRNG
+    {
+        private static ulong mix(ulong x)
+        {
+            unchecked
+            {
+                x ^= x >> 33;
+                x *= 0xff51afd7ed558ccd;
+                x ^= x >> 33;
+                x *= 0xc4ceb9fe1a85ec53;
+                x ^= x >> 33;
+                return x;
+            }
+        }
+
+        /// <summary>
+        /// Generate a random 64-bit unsigned integer from given seed.
+        /// </summary>
+        /// <param name="seed">
+        /// The seed value of this random number generator.
+        /// </param>
+        /// <param name="series">
+        /// The series number.
+        /// Different values are computed for the same seed in different series.
+        /// </param>
+        public static ulong NextULong(int seed, int series = 0)
+        {
+            unchecked
+            {
+                var combined = ((ulong)(uint)series << 32) | (uint)seed;
+                // The xor operation is to not map (0, 0) to 0.
+                return mix(combined ^ 0x12345678);
+            }
+        }
+
+        /// <summary>
+        /// Generate a random integer in range [0, maxValue) from given seed.
+        /// </summary>
+        /// <param name="maxValue">
+        /// The number of possible results.
+        /// </param>
+        /// <param name="seed">
+        /// The seed value of this random number generator.
+        /// </param>
+        /// <param name="series">
+        /// The series number.
+        /// Different values are computed for the same seed in different series.
+        /// </param>
+        public static int NextInt(int maxValue, int seed, int series = 0)
+        {
+            if (maxValue <= 0) throw new ArgumentOutOfRangeException(nameof(maxValue));
+
+            return (int)(NextULong(seed, series) % (ulong)maxValue);
+        }
+
+        /// <summary>
+        /// Compute a random floating point value between 0 and 1 (excluding 1) from given seed and series number.
+        /// </summary>
+        /// <param name="seed">
+        /// The seed value of this random number generator.
+        /// </param>
+        /// <param name="series">
+        /// The series number.
+        /// Different values are computed for the same seed in different series.
+        /// </param>
+        public static float NextSingle(int seed, int series = 0) =>
+            (float)(NextULong(seed, series) & ((1 << 24) - 1)) / (1 << 24); // float has 24-bit precision
+    }
+}