From a7194e1bc347a204661c939517067cf931c997b6 Mon Sep 17 00:00:00 2001
From: ekrctb <ekrctb@gmail.com>
Date: Fri, 27 Nov 2020 15:41:35 +0900
Subject: [PATCH] add stateless RNG

---
 osu.Game/Utils/StatelessRNG.cs | 44 ++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 osu.Game/Utils/StatelessRNG.cs

diff --git a/osu.Game/Utils/StatelessRNG.cs b/osu.Game/Utils/StatelessRNG.cs
new file mode 100644
index 0000000000..8d08b26ca4
--- /dev/null
+++ b/osu.Game/Utils/StatelessRNG.cs
@@ -0,0 +1,44 @@
+// 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.
+
+namespace osu.Game.Utils
+{
+    /// Provides a fast stateless function that can be used in randomly-looking visual elements.
+    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;
+            }
+        }
+
+        /// Compute an integer from given seed and series number.
+        /// <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 Get(int seed, int series = 0) =>
+            unchecked(mix(((ulong)(uint)series << 32) | ((uint)seed ^ 0x12345678)));
+
+        /// Compute a floating point value between 0 and 1 (excluding 1) from given seed and series number.
+        /// <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 GetSingle(int seed, int series = 0) =>
+            (float)(Get(seed, series) & ((1 << 24) - 1)) / (1 << 24); // float has 24-bit precision
+    }
+}