HashCode.cs

// 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.

/*

The xxHash32 implementation is based on the code published by Yann Collet:
https://raw.githubusercontent.com/Cyan4973/xxHash/5c174cfa4e45a42f94082dc0d4539b39696afea1/xxhash.c

  xxHash - Fast Hash algorithm
  Copyright (C) 2012-2016, Yann Collet
  
  BSD 2-Clause License (http://www.opensource.org/licenses/bsd-license.php)
  
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:
  
  * Redistributions of source code must retain the above copyright
  notice, this list of conditions and the following disclaimer.
  * Redistributions in binary form must reproduce the above
  copyright notice, this list of conditions and the following disclaimer
  in the documentation and/or other materials provided with the
  distribution.
  
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  You can contact the author at :
  - xxHash homepage: http://www.xxhash.com
  - xxHash source repository : https://github.com/Cyan4973/xxHash

*/

using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System;
using System.Threading;

namespace ConsoleApp6.SeedThreadLazy
{
    // xxHash32 is used for the hash code.
    // https://github.com/Cyan4973/xxHash

    public struct HashCode
    {
#   if SYSTEM_HASHCODE_TESTVECTORS
        private static readonly uint s_seed = 0;
#   else
        private static int s_seed;
        [ThreadStatic]
        private static int s_threadSeed;
#   endif

        private const uint Prime1 = 2654435761U;
        private const uint Prime2 = 2246822519U;
        private const uint Prime3 = 3266489917U;
        private const uint Prime4 = 668265263U;
        private const uint Prime5 = 374761393U;

        private uint _v1, _v2, _v3, _v4;
        private uint _queue1, _queue2, _queue3;
        private uint _length;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1>(T1 value1)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);

            uint hash = MixEmptyState();
            hash += 4;

            QueueRound(ref hash, hc1);

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2>(T1 value1, T2 value2)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);

            uint hash = MixEmptyState();
            hash += 8;

            QueueRound(ref hash, hc1);
            QueueRound(ref hash, hc2);

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2, T3>(T1 value1, T2 value2, T3 value3)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);
            var hc3 = (uint)(value3?.GetHashCode() ?? 0);

            uint hash = MixEmptyState();
            hash += 12;

            QueueRound(ref hash, hc1);
            QueueRound(ref hash, hc2);
            QueueRound(ref hash, hc3);

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2, T3, T4>(T1 value1, T2 value2, T3 value3, T4 value4)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);
            var hc3 = (uint)(value3?.GetHashCode() ?? 0);
            var hc4 = (uint)(value4?.GetHashCode() ?? 0);

            Initialize(out uint v1, out uint v2, out uint v3, out uint v4);

            Round(ref v1, hc1);
            Round(ref v2, hc2);
            Round(ref v3, hc3);
            Round(ref v4, hc4);

            uint hash = MixState(v1, v2, v3, v4);
            hash += 16;

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2, T3, T4, T5>(T1 value1, T2 value2, T3 value3, T4 value4, T5 value5)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);
            var hc3 = (uint)(value3?.GetHashCode() ?? 0);
            var hc4 = (uint)(value4?.GetHashCode() ?? 0);
            var hc5 = (uint)(value5?.GetHashCode() ?? 0);

            Initialize(out uint v1, out uint v2, out uint v3, out uint v4);

            Round(ref v1, hc1);
            Round(ref v2, hc2);
            Round(ref v3, hc3);
            Round(ref v4, hc4);

            uint hash = MixState(v1, v2, v3, v4);
            hash += 20;

            QueueRound(ref hash, hc5);

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2, T3, T4, T5, T6>(T1 value1, T2 value2, T3 value3, T4 value4, T5 value5, T6 value6)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);
            var hc3 = (uint)(value3?.GetHashCode() ?? 0);
            var hc4 = (uint)(value4?.GetHashCode() ?? 0);
            var hc5 = (uint)(value5?.GetHashCode() ?? 0);
            var hc6 = (uint)(value6?.GetHashCode() ?? 0);

            Initialize(out uint v1, out uint v2, out uint v3, out uint v4);

            Round(ref v1, hc1);
            Round(ref v2, hc2);
            Round(ref v3, hc3);
            Round(ref v4, hc4);

            uint hash = MixState(v1, v2, v3, v4);
            hash += 24;

            QueueRound(ref hash, hc5);
            QueueRound(ref hash, hc6);

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2, T3, T4, T5, T6, T7>(T1 value1, T2 value2, T3 value3, T4 value4, T5 value5, T6 value6, T7 value7)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);
            var hc3 = (uint)(value3?.GetHashCode() ?? 0);
            var hc4 = (uint)(value4?.GetHashCode() ?? 0);
            var hc5 = (uint)(value5?.GetHashCode() ?? 0);
            var hc6 = (uint)(value6?.GetHashCode() ?? 0);
            var hc7 = (uint)(value7?.GetHashCode() ?? 0);

            Initialize(out uint v1, out uint v2, out uint v3, out uint v4);

            Round(ref v1, hc1);
            Round(ref v2, hc2);
            Round(ref v3, hc3);
            Round(ref v4, hc4);

            uint hash = MixState(v1, v2, v3, v4);
            hash += 28;

            QueueRound(ref hash, hc5);
            QueueRound(ref hash, hc6);
            QueueRound(ref hash, hc7);

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Combine<T1, T2, T3, T4, T5, T6, T7, T8>(T1 value1, T2 value2, T3 value3, T4 value4, T5 value5, T6 value6, T7 value7, T8 value8)
        {
            var hc1 = (uint)(value1?.GetHashCode() ?? 0);
            var hc2 = (uint)(value2?.GetHashCode() ?? 0);
            var hc3 = (uint)(value3?.GetHashCode() ?? 0);
            var hc4 = (uint)(value4?.GetHashCode() ?? 0);
            var hc5 = (uint)(value5?.GetHashCode() ?? 0);
            var hc6 = (uint)(value6?.GetHashCode() ?? 0);
            var hc7 = (uint)(value7?.GetHashCode() ?? 0);
            var hc8 = (uint)(value8?.GetHashCode() ?? 0);

            Initialize(out uint v1, out uint v2, out uint v3, out uint v4);

            Round(ref v1, hc1);
            Round(ref v2, hc2);
            Round(ref v3, hc3);
            Round(ref v4, hc4);

            Round(ref v1, hc5);
            Round(ref v2, hc6);
            Round(ref v3, hc7);
            Round(ref v4, hc8);

            uint hash = MixState(v1, v2, v3, v4);
            hash += 32;

            MixFinal(ref hash);
            return (int)hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static uint Rol(uint value, int count)
            => (value << count) | (value >> (32 - count));

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static uint CreateSeed()
        {
            int seed = s_threadSeed;
            if (s_threadSeed == 0)
            {
                seed = s_seed;
                if (seed == 0)
                {
                    var random = new Random();
                    do
                    {
                        seed = random.Next(int.MinValue, int.MaxValue);
                    } while (seed == 0);
                    seed = Interlocked.CompareExchange(ref s_seed, seed, 0);
                }
                s_threadSeed = seed;
            }

            return (uint)seed;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void Initialize(out uint v1, out uint v2, out uint v3, out uint v4)
        {
            uint seed = CreateSeed();

            v1 = (uint)seed + Prime1 + Prime2;
            v2 = (uint)seed + Prime2;
            v3 = (uint)seed + 0;
            v4 = (uint)seed - Prime1;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void Round(ref uint hash, uint input)
        {
            hash += input * Prime2;
            hash = Rol(hash, 13);
            hash *= Prime1;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void QueueRound(ref uint hash, uint queuedValue)
        {
            hash += queuedValue * Prime3;
            hash = Rol(hash, 17) * Prime4;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static uint MixState(uint v1, uint v2, uint v3, uint v4)
        {
            return Rol(v1, 1) + Rol(v2, 7) + Rol(v3, 12) + Rol(v4, 18);
        }

        private static uint MixEmptyState()
        {
            return CreateSeed() + Prime5;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void MixFinal(ref uint hash)
        {
            hash ^= hash >> 15;
            hash *= Prime2;
            hash ^= hash >> 13;
            hash *= Prime3;
            hash ^= hash >> 16;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Add<T>(T value)
        {
            Add(value?.GetHashCode() ?? 0);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Add<T>(T value, IEqualityComparer<T> comparer)
        {
            var cmp = comparer ?? EqualityComparer<T>.Default;
            Add(cmp.GetHashCode(value));
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private void Add(int value)
        {
            // Note that x & 0x3 is like mod 3, but faster.

            var val = (uint)value;
            uint position = _length & 0x3;

            // xxHash works as follows:
            // 0. Initialize immediately. We can't do this in a struct (no default
            //    ctor).
            // 1. Accumulate blocks of length 16 (4 uints) into 4 accumulators.
            // 2. Accumulate remaining blocks of length 4 (1 uint) into the hash.
            // 3. Accumulate remaining blocks of length 1 into the hash.

            // There is no need for *3 as this type only accepts ints. _queue1,
            // _queue2 and _queue3 are basically a buffer so that when ToHashCode is
            // called we can execute *2 correctly. That's what first three case
            // statements do.

            // We need to initialize the xxHash32 state (_v1 -> _v4) lazily (see *0)
            // and the last place that can be done if you look at the original code
            // is just before the first block of 16 bytes is mixed in. The xxHash32
            // state is never used for streams containing fewer than 16 bytes.

            // A bloom filter is used to determine whether the default case statement
            // has even been executed. To do that we check if the length is smaller
            // than 4 (_length ^ position will be non-zero if this is the case). The
            // case statement is for values larger than 2, so the check only succeeds
            // on exactly 3.

            // To see what's really going on here, have a look at the Combine methods.

            if (position == 0)
                _queue1 = val;
            else if (position == 1)
                _queue2 = val;
            else if (position == 2)
                _queue3 = val;
            else
            {
                // length smaller than 4?
                if ((_length ^ position) == 0)
                    Initialize(out _v1, out _v2, out _v3, out _v4);

                Round(ref _v1, _queue1);
                Round(ref _v2, _queue2);
                Round(ref _v3, _queue3);
                Round(ref _v4, val);
            }

            // Throw for more than uint.MaxValue fields.
            _length = checked(_length + 1);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public int ToHashCode()
        {
            uint position = _length & 0x3;

            // If the length is less than 3, _v1 -> _v4 don't contain
            // anything yet. xxHash32 treats this differently.

            uint hash = (_length ^ position) == 0
                ? MixEmptyState()
                : MixState(_v1, _v2, _v3, _v4);

            // Multiply by 4 because we've been counting in ints, not
            // bytes.

            hash += _length * 4;

            // Mix what remains in the queue

            if (position > 0)
            {
                QueueRound(ref hash, _queue1);
                if (position > 1)
                {
                    QueueRound(ref hash, _queue2);
                    if (position > 2)
                        QueueRound(ref hash, _queue3);
                }
            }

            MixFinal(ref hash);

            return (int)hash;
        }

#       pragma warning disable 0809
        // Obsolete member 'memberA' overrides non-obsolete member 'memberB'. 
        // Disallowing GetHashCode is by design

        // * We decided to not override GetHashCode() to produce the hash code 
        //   as this would be weird, both naming-wise as well as from a behavioral 
        //   standpoint (GetHashCode() should return the object's hash code, not 
        //   the one being computed).

        // * Even though ToHashCode() can be called safely multiple times on this
        //   implementation, it is not part of the contract. If the implementation
        //   has to change in the future we don't want to worry about people who
        //   might have incorrectly used this type.

        [Obsolete("Use ToHashCode to retrieve the computed hash code.", error: true)]
        [EditorBrowsable(EditorBrowsableState.Never)]
        [MethodImpl(MethodImplOptions.NoInlining)]
        public override int GetHashCode() => throw new NotSupportedException();
#       pragma warning restore 0809

    }
}