Add xorshift implemetation

src/ImageSharp/Formats/Jxl/Processing/JpegXorShift.cs

@@ -0,0 +1,58 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+internal sealed class JpegXorShift
+{
+    private readonly ulong[] s0 = new ulong[8];
+    private readonly ulong[] s1 = new ulong[8];
+
+    public void XorShift128Plus(ulong seed)
+    {
+        this.s0[0] = SplitMix64(seed + 0x9E3779B97F4A7C15L);
+        this.s1[0] = SplitMix64(this.s0[0]);
+        for (int i = 1; i < 8; ++i)
+        {
+            this.s0[i] = SplitMix64(this.s1[i - 1]);
+            this.s1[i] = SplitMix64(this.s0[i]);
+        }
+    }
+
+    public void XorShift128Plus(uint seed1, uint seed2, uint seed3, uint seed4)
+    {
+        this.s0[0] = SplitMix64((((ulong)seed1 << 32) + seed2) + 0x9E3779B97F4A7C15uL);
+        this.s1[0] = SplitMix64((((ulong)seed3 << 32) + seed4) + 0x9E3779B97F4A7C15uL);
+        for (int i = 1; i < 8; ++i)
+        {
+            this.s0[i] = SplitMix64(this.s0[i - 1]);
+            this.s1[i] = SplitMix64(this.s1[i - 1]);
+        }
+    }
+
+    // TODO: SIMD
+    public void Fill(Span<ulong> randomBits)
+    {
+        for (int i = 0; i < 8; ++i)
+        {
+            ulong s1 = this.s0[i];
+            ulong s0 = this.s1[i];
+            ulong bits = s1 + s0;
+            this.s0[i] = s0;
+            s1 ^= s1 << 23;
+            randomBits[i] = bits;
+            s1 ^= s0 ^ (s1 >> 18) ^ (s0 >> 5);
+            this.s1[i] = s1;
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static ulong SplitMix64(ulong z)
+    {
+        z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9uL;
+        z = (z ^ (z >> 27)) * 0x94D049BB133111EBuL;
+        return z ^ (z >> 31);
+    }
+}