ported from perforce

RobotAndDonkey.Game/Utils/SRandom.cs

@@ -0,0 +1,100 @@
+using System;
+
+namespace RobotAndDonkey.Game.Utils;
+
+/// <summary>
+///     Implements a XorShift* PRNG, with 64 bits of internal state.
+///     See http://en.wikipedia.org/wiki/Xorshift
+/// </summary>
+public struct SRandom
+{
+    public SRandom(ulong seed)
+    {
+        if (seed == 0)
+            throw new InvalidOperationException("Seed needs to be bigger than zero.");
+
+        Seed = seed;
+    }
+
+    public void Shuffle<T>(Span<T> span)
+    {
+        var n = span.Length;
+
+        for (var i = 0; i < n; i++)
+        {
+            var r = i + Next(n - i);
+            (span[r], span[i]) = (span[i], span[r]);
+        }
+    }
+
+    public ulong Next()
+    {
+        Seed ^= Seed >> 12;
+        Seed ^= Seed << 25;
+        Seed ^= Seed >> 27;
+        return Seed * 2685821657736338717UL; // multiplier taken from wikipedia article on XorShift PRNGs
+    }
+
+    public ulong Next(ulong upperLimit)
+    {
+        if (upperLimit == 0)
+            return 0;
+
+        return Next() % upperLimit;
+    }
+
+    public uint Next(uint upperLimit)
+    {
+        if (upperLimit == 0)
+            return 0;
+
+        return (uint)(Next() % upperLimit);
+    }
+
+    public uint Next(uint lowerLimit, uint upperLimit)
+    {
+        if (upperLimit == 0)
+            return 0;
+
+        if (lowerLimit == upperLimit)
+            return lowerLimit;
+
+        return lowerLimit + (uint)(Next() % (upperLimit - lowerLimit));
+    }
+
+    public int Next(int upperLimit)
+    {
+        if (upperLimit == 0)
+            return 0;
+
+        return (int)(Next() % (uint)(upperLimit & 0x7fffffff));
+    }
+
+    public int Next(int lowerLimit, int upperLimit)
+    {
+        if (upperLimit == 0)
+            return 0;
+
+        if (lowerLimit == upperLimit)
+            return lowerLimit;
+
+        return lowerLimit + (int)(Next() % (uint)((upperLimit - lowerLimit) & 0x7fffffff));
+    }
+
+    public double NextDouble()
+    {
+        return Next(int.MaxValue) * (1.0 / int.MaxValue);
+    }
+
+    public float NextSingle()
+    {
+        return (float)NextDouble();
+    }
+
+    public readonly override string ToString()
+    {
+        return $"0x{Seed:X}";
+    }
+
+    public ulong Seed { get; internal set; }
+}