godosa-engine: initial public history

1 files changed, 51 insertions, 0 deletions

diff --git a/src/Godosa.Core/Runs/StateDigest.cs b/src/Godosa.Core/Runs/StateDigest.cs
new file mode 100644
index 0000000..a7adaf2
--- /dev/null
+++ b/src/Godosa.Core/Runs/StateDigest.cs
@@ -0,0 +1,51 @@
+using System.Buffers.Binary;
+using System.Text;
+
+namespace Godosa.Core.Runs;
+
+/// <summary>FNV-1a 64 over state bytes (numbers little-endian, strings UTF-8) for comparing runs.</summary>
+public sealed class StateDigest
+{
+    private const ulong Offset = 14695981039346656037UL, Prime = 1099511628211UL;
+
+    public ulong Value { get; private set; } = Offset;
+    public string Hex => Value.ToString("x16");
+
+    public static ulong Of(ReadOnlySpan<byte> bytes)
+    {
+        var d = new StateDigest();
+        d.Add(bytes);
+        return d.Value;
+    }
+
+    public void Add(ReadOnlySpan<byte> bytes)
+    {
+        var h = Value;
+        foreach (var b in bytes)
+            h = (h ^ b) * Prime;
+        Value = h;
+    }
+
+    public void Add(byte b) => Value = (Value ^ b) * Prime;
+
+    public void Add(uint v)
+    {
+        Span<byte> b = stackalloc byte[4];
+        BinaryPrimitives.WriteUInt32LittleEndian(b, v);
+        Add(b);
+    }
+
+    public void Add(int v) => Add(unchecked((uint)v));
+
+    public void Add(ulong v)
+    {
+        Span<byte> b = stackalloc byte[8];
+        BinaryPrimitives.WriteUInt64LittleEndian(b, v);
+        Add(b);
+    }
+
+    public void Add(long v) => Add(unchecked((ulong)v));
+    public void Add(float v) => Add(BitConverter.SingleToUInt32Bits(v));
+    public void Add(double v) => Add(BitConverter.DoubleToUInt64Bits(v));
+    public void Add(string s) => Add(Encoding.UTF8.GetBytes(s));
+}