1 files changed, 109 insertions, 0 deletions
diff --git a/Spv.Generator/DeterministicHashCode.cs b/Spv.Generator/DeterministicHashCode.cs
new file mode 100644
index 00000000..1bf0b468
--- /dev/null
+++ b/Spv.Generator/DeterministicHashCode.cs
@@ -0,0 +1,109 @@
+using System;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+
+namespace Spv.Generator
+{
+    /// <summary>
+    /// Similar to System.HashCode, but without introducing random values.
+    /// The same primes and shifts are used.
+    /// </summary>
+    internal static class DeterministicHashCode
+    {
+        private const uint Prime1 = 2654435761U;
+        private const uint Prime2 = 2246822519U;
+        private const uint Prime3 = 3266489917U;
+        private const uint Prime4 = 668265263U;
+
+        public static int GetHashCode(string value)
+        {
+            uint hash = (uint)value.Length + Prime1;
+
+            for (int i = 0; i < value.Length; i++)
+            {
+                hash += (hash << 7) ^ value[i];
+            }
+
+            return (int)MixFinal(hash);
+        }
+
+        public static int Combine<T>(ReadOnlySpan<T> values)
+        {
+            uint hashCode = Prime2;
+            hashCode += 4 * (uint)values.Length;
+
+            foreach (T value in values)
+            {
+                uint hc = (uint)(value?.GetHashCode() ?? 0);
+                hashCode = MixStep(hashCode, hc);
+            }
+
+            return (int)MixFinal(hashCode);
+        }
+
+        public static int Combine<T1, T2>(T1 value1, T2 value2)
+        {
+            uint hc1 = (uint)(value1?.GetHashCode() ?? 0);
+            uint hc2 = (uint)(value2?.GetHashCode() ?? 0);
+
+            uint hash = Prime2;
+            hash += 8;
+
+            hash = MixStep(hash, hc1);
+            hash = MixStep(hash, hc2);
+
+            return (int)MixFinal(hash);
+        }
+
+        public static int Combine<T1, T2, T3>(T1 value1, T2 value2, T3 value3)
+        {
+            uint hc1 = (uint)(value1?.GetHashCode() ?? 0);
+            uint hc2 = (uint)(value2?.GetHashCode() ?? 0);
+            uint hc3 = (uint)(value3?.GetHashCode() ?? 0);
+
+            uint hash = Prime2;
+            hash += 12;
+
+            hash = MixStep(hash, hc1);
+            hash = MixStep(hash, hc2);
+            hash = MixStep(hash, hc3);
+
+            return (int)MixFinal(hash);
+        }
+
+        public static int Combine<T1, T2, T3, T4>(T1 value1, T2 value2, T3 value3, T4 value4)
+        {
+            uint hc1 = (uint)(value1?.GetHashCode() ?? 0);
+            uint hc2 = (uint)(value2?.GetHashCode() ?? 0);
+            uint hc3 = (uint)(value3?.GetHashCode() ?? 0);
+            uint hc4 = (uint)(value4?.GetHashCode() ?? 0);
+
+            uint hash = Prime2;
+            hash += 16;
+
+            hash = MixStep(hash, hc1);
+            hash = MixStep(hash, hc2);
+            hash = MixStep(hash, hc3);
+            hash = MixStep(hash, hc4);
+
+            return (int)MixFinal(hash);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static uint MixStep(uint hashCode, uint mixValue)
+        {
+            return BitOperations.RotateLeft(hashCode + mixValue * Prime3, 17) * Prime4;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static uint MixFinal(uint hash)
+        {
+            hash ^= hash >> 15;
+            hash *= Prime2;
+            hash ^= hash >> 13;
+            hash *= Prime3;
+            hash ^= hash >> 16;
+            return hash;
+        }
+    }
+}