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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using System.Diagnostics;
using System.Runtime.CompilerServices;
using Internal.Runtime.CompilerServices;
namespace System
{
internal static class Marvin
{
/// <summary>
/// Convenience method to compute a Marvin hash and collapse it into a 32-bit hash.
/// </summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int ComputeHash32(ref byte data, int count, ulong seed)
{
long hash64 = ComputeHash(ref data, count, seed);
return ((int)(hash64 >> 32)) ^ (int)hash64;
}
/// <summary>
/// Computes a 64-hash using the Marvin algorithm.
/// </summary>
public static long ComputeHash(ref byte data, int count, ulong seed)
{
uint ucount = (uint)count;
uint p0 = (uint)seed;
uint p1 = (uint)(seed >> 32);
int byteOffset = 0; // declared as signed int so we don't have to cast everywhere (it's passed to Unsafe.Add() and used for nothing else.)
while (ucount >= 8)
{
p0 += Unsafe.As<byte, uint>(ref Unsafe.Add(ref data, byteOffset));
Block(ref p0, ref p1);
p0 += Unsafe.As<byte, uint>(ref Unsafe.Add(ref data, byteOffset + 4));
Block(ref p0, ref p1);
byteOffset += 8;
ucount -= 8;
}
switch (ucount)
{
case 4:
p0 += Unsafe.As<byte, uint>(ref Unsafe.Add(ref data, byteOffset));
Block(ref p0, ref p1);
goto case 0;
case 0:
p0 += 0x80u;
break;
case 5:
p0 += Unsafe.As<byte, uint>(ref Unsafe.Add(ref data, byteOffset));
byteOffset += 4;
Block(ref p0, ref p1);
goto case 1;
case 1:
p0 += 0x8000u | Unsafe.Add(ref data, byteOffset);
break;
case 6:
p0 += Unsafe.As<byte, uint>(ref Unsafe.Add(ref data, byteOffset));
byteOffset += 4;
Block(ref p0, ref p1);
goto case 2;
case 2:
p0 += 0x800000u | Unsafe.As<byte, ushort>(ref Unsafe.Add(ref data, byteOffset));
break;
case 7:
p0 += Unsafe.As<byte, uint>(ref Unsafe.Add(ref data, byteOffset));
byteOffset += 4;
Block(ref p0, ref p1);
goto case 3;
case 3:
p0 += 0x80000000u | (((uint)(Unsafe.Add(ref data, byteOffset + 2))) << 16)| (uint)(Unsafe.As<byte, ushort>(ref Unsafe.Add(ref data, byteOffset)));
break;
default:
Debug.Fail("Should not get here.");
break;
}
Block(ref p0, ref p1);
Block(ref p0, ref p1);
return (((long)p1) << 32) | p0;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static void Block(ref uint rp0, ref uint rp1)
{
uint p0 = rp0;
uint p1 = rp1;
p1 ^= p0;
p0 = _rotl(p0, 20);
p0 += p1;
p1 = _rotl(p1, 9);
p1 ^= p0;
p0 = _rotl(p0, 27);
p0 += p1;
p1 = _rotl(p1, 19);
rp0 = p0;
rp1 = p1;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static uint _rotl(uint value, int shift)
{
// This is expected to be optimized into a single rol (or ror with negated shift value) instruction
return (value << shift) | (value >> (32 - shift));
}
public static ulong DefaultSeed => s_defaultSeed;
private static ulong s_defaultSeed = GenerateSeed();
private static ulong GenerateSeed()
{
#if MONO
return 839433921;
#else
ulong seed;
unsafe
{
Interop.GetRandomBytes((byte*)&seed, sizeof(ulong));
}
return seed;
#endif
}
}
}
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