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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;
}
}
}
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