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using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using static System.Numerics.BitOperations;
namespace Spacebar.Offload.Extensions;
// https://github.com/JeremyEspresso/MurmurHash/blob/master/src/MurmurHash/MurmurHash3.cs
// Changes: Default seed to 0
public static class MurmurHash3
{
/// <summary>
/// Hashes the <paramref name="bytes"/> into a MurmurHash3 as a <see cref="uint"/>.
/// </summary>
/// <param name="bytes">The span.</param>
/// <param name="seed">The seed for this algorithm.</param>
/// <returns>The MurmurHash3 as a <see cref="uint"/></returns>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static uint Hash32(ref ReadOnlySpan<byte> bytes, uint seed = 0)
{
ref byte bp = ref MemoryMarshal.GetReference(bytes);
ref uint endPoint = ref Unsafe.Add(ref Unsafe.As<byte, uint>(ref bp), bytes.Length >> 2);
if (bytes.Length >= 4)
{
do
{
seed = RotateLeft(seed ^ RotateLeft(Unsafe.ReadUnaligned<uint>(ref bp) * 3432918353U, 15) * 461845907U, 13) * 5 - 430675100;
bp = ref Unsafe.Add(ref bp, 4);
} while (Unsafe.IsAddressLessThan(ref Unsafe.As<byte, uint>(ref bp), ref endPoint));
}
var remainder = bytes.Length & 3;
if (remainder > 0)
{
uint num = 0;
if (remainder > 2) num ^= Unsafe.Add(ref endPoint, 2) << 16;
if (remainder > 1) num ^= Unsafe.Add(ref endPoint, 1) << 8;
num ^= endPoint;
seed ^= RotateLeft(num * 3432918353U, 15) * 461845907U;
}
seed ^= (uint)bytes.Length;
seed = (uint)((seed ^ (seed >> 16)) * -2048144789);
seed = (uint)((seed ^ (seed >> 13)) * -1028477387);
return seed ^ seed >> 16;
}
}
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