godosa-engine: initial public history

1 files changed, 14 insertions, 0 deletions

diff --git a/src/Godosa.Core/Runs/SeedStreams.cs b/src/Godosa.Core/Runs/SeedStreams.cs
new file mode 100644
index 0000000..385d732
--- /dev/null
+++ b/src/Godosa.Core/Runs/SeedStreams.cs
@@ -0,0 +1,14 @@
+namespace Godosa.Core.Runs;
+
+/// <summary>Independent per-purpose seeds from one run seed: SplitMix64 output number <c>stream + 1</c> of
+/// <paramref name="seed"/> (audio draws then never shift the game's stream).</summary>
+public static class SeedStreams
+{
+    public static ulong Of(ulong seed, int stream)
+    {
+        var z = unchecked(seed + (ulong)(stream + 1) * 0x9E3779B97F4A7C15UL);
+        z = unchecked((z ^ (z >> 30)) * 0xBF58476D1CE4E5B9UL);
+        z = unchecked((z ^ (z >> 27)) * 0x94D049BB133111EBUL);
+        return z ^ (z >> 31);
+    }
+}