// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

// Ported from dotnet/runtime (release/9.0):
// src/libraries/System.IO.Hashing/src/System/IO/Hashing/XxHashShared.cs
// Adapted for the Unity editor (Mono, netstandard2.1 API surface):
// - SIMD intrinsics paths (#if NET) removed; the scalar fallbacks are kept.
// - System.Runtime.CompilerServices.Unsafe and System.Numerics.BitOperations
//   usages replaced to avoid extra assembly dependencies.

using System;
using System.Buffers.Binary;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Unity.PlasticSCM.Editor.Hashing
{
    /// <summary>Shared implementation of the XXH3 hash algorithm for the <see cref="XxHash128"/> version.</summary>
    internal static unsafe class XxHashShared
    {
        public const int StripeLengthBytes = 64;
        public const int SecretLengthBytes = 192;
        public const int SecretSizeMin = 136;
        public const int SecretLastAccStartBytes = 7;
        public const int SecretConsumeRateBytes = 8;
        public const int SecretMergeAccsStartBytes = 11;
        public const int NumStripesPerBlock = (SecretLengthBytes - StripeLengthBytes) / SecretConsumeRateBytes;
        public const int AccumulatorCount = StripeLengthBytes / sizeof(ulong);
        public const int MidSizeMaxBytes = 240;
        public const int InternalBufferStripes = InternalBufferLengthBytes / StripeLengthBytes;
        public const int InternalBufferLengthBytes = 256;

        /// <summary>The default secret for when no seed is provided.</summary>
        /// <remarks>This is the same as a custom secret derived from a seed of 0.</remarks>
        public static ReadOnlySpan<byte> DefaultSecret => new byte[]
        {
            0xb8, 0xfe, 0x6c, 0x39, 0x23, 0xa4, 0x4b, 0xbe, // DefaultSecretUInt64_0
            0x7c, 0x01, 0x81, 0x2c, 0xf7, 0x21, 0xad, 0x1c, // DefaultSecretUInt64_1
            0xde, 0xd4, 0x6d, 0xe9, 0x83, 0x90, 0x97, 0xdb, // DefaultSecretUInt64_2
            0x72, 0x40, 0xa4, 0xa4, 0xb7, 0xb3, 0x67, 0x1f, // DefaultSecretUInt64_3
            0xcb, 0x79, 0xe6, 0x4e, 0xcc, 0xc0, 0xe5, 0x78, // DefaultSecretUInt64_4
            0x82, 0x5a, 0xd0, 0x7d, 0xcc, 0xff, 0x72, 0x21, // DefaultSecretUInt64_5
            0xb8, 0x08, 0x46, 0x74, 0xf7, 0x43, 0x24, 0x8e, // DefaultSecretUInt64_6
            0xe0, 0x35, 0x90, 0xe6, 0x81, 0x3a, 0x26, 0x4c, // DefaultSecretUInt64_7
            0x3c, 0x28, 0x52, 0xbb, 0x91, 0xc3, 0x00, 0xcb, // DefaultSecretUInt64_8
            0x88, 0xd0, 0x65, 0x8b, 0x1b, 0x53, 0x2e, 0xa3, // DefaultSecretUInt64_9
            0x71, 0x64, 0x48, 0x97, 0xa2, 0x0d, 0xf9, 0x4e, // DefaultSecretUInt64_10
            0x38, 0x19, 0xef, 0x46, 0xa9, 0xde, 0xac, 0xd8, // DefaultSecretUInt64_11
            0xa8, 0xfa, 0x76, 0x3f, 0xe3, 0x9c, 0x34, 0x3f, // DefaultSecretUInt64_12
            0xf9, 0xdc, 0xbb, 0xc7, 0xc7, 0x0b, 0x4f, 0x1d, // DefaultSecretUInt64_13
            0x8a, 0x51, 0xe0, 0x4b, 0xcd, 0xb4, 0x59, 0x31, // DefaultSecretUInt64_14
            0xc8, 0x9f, 0x7e, 0xc9, 0xd9, 0x78, 0x73, 0x64, // DefaultSecretUInt64_15
            0xea, 0xc5, 0xac, 0x83, 0x34, 0xd3, 0xeb, 0xc3, // DefaultSecretUInt64_16
            0xc5, 0x81, 0xa0, 0xff, 0xfa, 0x13, 0x63, 0xeb, // DefaultSecretUInt64_17
            0x17, 0x0d, 0xdd, 0x51, 0xb7, 0xf0, 0xda, 0x49, // DefaultSecretUInt64_18
            0xd3, 0x16, 0x55, 0x26, 0x29, 0xd4, 0x68, 0x9e, // DefaultSecretUInt64_19
            0x2b, 0x16, 0xbe, 0x58, 0x7d, 0x47, 0xa1, 0xfc, // DefaultSecretUInt64_20
            0x8f, 0xf8, 0xb8, 0xd1, 0x7a, 0xd0, 0x31, 0xce, // DefaultSecretUInt64_21
            0x45, 0xcb, 0x3a, 0x8f, 0x95, 0x16, 0x04, 0x28, // DefaultSecretUInt64_22
            0xaf, 0xd7, 0xfb, 0xca, 0xbb, 0x4b, 0x40, 0x7e, // DefaultSecretUInt64_23
        };

        // Cast of DefaultSecret byte[] => ulong[] (See above for the correspondence)
        public const ulong DefaultSecretUInt64_0 = 0xBE4BA423396CFEB8;
        public const ulong DefaultSecretUInt64_1 = 0x1CAD21F72C81017C;
        public const ulong DefaultSecretUInt64_2 = 0xDB979083E96DD4DE;
        public const ulong DefaultSecretUInt64_3 = 0x1F67B3B7A4A44072;
        public const ulong DefaultSecretUInt64_4 = 0x78E5C0CC4EE679CB;
        public const ulong DefaultSecretUInt64_5 = 0x2172FFCC7DD05A82;
        public const ulong DefaultSecretUInt64_6 = 0x8E2443F7744608B8;
        public const ulong DefaultSecretUInt64_7 = 0x4C263A81E69035E0;
        public const ulong DefaultSecretUInt64_8 = 0xCB00C391BB52283C;
        public const ulong DefaultSecretUInt64_9 = 0xA32E531B8B65D088;
        public const ulong DefaultSecretUInt64_10 = 0x4EF90DA297486471;
        public const ulong DefaultSecretUInt64_11 = 0xD8ACDEA946EF1938;
        public const ulong DefaultSecretUInt64_12 = 0x3F349CE33F76FAA8;
        public const ulong DefaultSecretUInt64_13 = 0x1D4F0BC7C7BBDCF9;
        public const ulong DefaultSecretUInt64_14 = 0x3159B4CD4BE0518A;
        public const ulong DefaultSecretUInt64_15 = 0x647378D9C97E9FC8;

        // Cast of DefaultSecret offset by 3 bytes, byte[] => ulong[]
        public const ulong DefaultSecret3UInt64_0 = 0x81017CBE4BA42339;
        public const ulong DefaultSecret3UInt64_1 = 0x6DD4DE1CAD21F72C;
        public const ulong DefaultSecret3UInt64_2 = 0xA44072DB979083E9;
        public const ulong DefaultSecret3UInt64_3 = 0xE679CB1F67B3B7A4;
        public const ulong DefaultSecret3UInt64_4 = 0xD05A8278E5C0CC4E;
        public const ulong DefaultSecret3UInt64_5 = 0x4608B82172FFCC7D;
        public const ulong DefaultSecret3UInt64_6 = 0x9035E08E2443F774;
        public const ulong DefaultSecret3UInt64_7 = 0x52283C4C263A81E6;
        public const ulong DefaultSecret3UInt64_8 = 0x65D088CB00C391BB;
        public const ulong DefaultSecret3UInt64_9 = 0x486471A32E531B8B;
        public const ulong DefaultSecret3UInt64_10 = 0xEF19384EF90DA297;
        public const ulong DefaultSecret3UInt64_11 = 0x76FAA8D8ACDEA946;
        public const ulong DefaultSecret3UInt64_12 = 0xBBDCF93F349CE33F;
        public const ulong DefaultSecret3UInt64_13 = 0xE0518A1D4F0BC7C7;

        public const ulong Prime64_1 = 0x9E3779B185EBCA87UL;
        public const ulong Prime64_2 = 0xC2B2AE3D27D4EB4FUL;
        public const ulong Prime64_3 = 0x165667B19E3779F9UL;
        public const ulong Prime64_4 = 0x85EBCA77C2B2AE63UL;
        public const ulong Prime64_5 = 0x27D4EB2F165667C5UL;

        public const uint Prime32_1 = 0x9E3779B1U;
        public const uint Prime32_2 = 0x85EBCA77U;
        public const uint Prime32_3 = 0xC2B2AE3DU;
        public const uint Prime32_4 = 0x27D4EB2FU;
        public const uint Prime32_5 = 0x165667B1U;

#if DEBUG
        static XxHashShared()
        {
            // Make sure DefaultSecret is the custom secret derived from a seed of 0.
            byte* secret = stackalloc byte[SecretLengthBytes];
            DeriveSecretFromSeed(secret, 0);
            Debug.Assert(new Span<byte>(secret, SecretLengthBytes).SequenceEqual(DefaultSecret));

            // Validate some relationships.
            Debug.Assert(InternalBufferLengthBytes % StripeLengthBytes == 0);

            ReadOnlySpan<ulong> defaultSecretUInt64 = MemoryMarshal.Cast<byte, ulong>(DefaultSecret);
            Debug.Assert(ReadLE64(defaultSecretUInt64[0]) == DefaultSecretUInt64_0);
            Debug.Assert(ReadLE64(defaultSecretUInt64[1]) == DefaultSecretUInt64_1);
            Debug.Assert(ReadLE64(defaultSecretUInt64[2]) == DefaultSecretUInt64_2);
            Debug.Assert(ReadLE64(defaultSecretUInt64[3]) == DefaultSecretUInt64_3);
            Debug.Assert(ReadLE64(defaultSecretUInt64[4]) == DefaultSecretUInt64_4);
            Debug.Assert(ReadLE64(defaultSecretUInt64[5]) == DefaultSecretUInt64_5);
            Debug.Assert(ReadLE64(defaultSecretUInt64[6]) == DefaultSecretUInt64_6);
            Debug.Assert(ReadLE64(defaultSecretUInt64[7]) == DefaultSecretUInt64_7);
            Debug.Assert(ReadLE64(defaultSecretUInt64[8]) == DefaultSecretUInt64_8);
            Debug.Assert(ReadLE64(defaultSecretUInt64[9]) == DefaultSecretUInt64_9);
            Debug.Assert(ReadLE64(defaultSecretUInt64[10]) == DefaultSecretUInt64_10);
            Debug.Assert(ReadLE64(defaultSecretUInt64[11]) == DefaultSecretUInt64_11);
            Debug.Assert(ReadLE64(defaultSecretUInt64[12]) == DefaultSecretUInt64_12);
            Debug.Assert(ReadLE64(defaultSecretUInt64[13]) == DefaultSecretUInt64_13);
            Debug.Assert(ReadLE64(defaultSecretUInt64[14]) == DefaultSecretUInt64_14);
            Debug.Assert(ReadLE64(defaultSecretUInt64[15]) == DefaultSecretUInt64_15);

            ReadOnlySpan<ulong> defaultSecret3UInt64 = MemoryMarshal.Cast<byte, ulong>(DefaultSecret.Slice(3));
            Debug.Assert(ReadLE64(defaultSecret3UInt64[0]) == DefaultSecret3UInt64_0);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[1]) == DefaultSecret3UInt64_1);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[2]) == DefaultSecret3UInt64_2);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[3]) == DefaultSecret3UInt64_3);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[4]) == DefaultSecret3UInt64_4);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[5]) == DefaultSecret3UInt64_5);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[6]) == DefaultSecret3UInt64_6);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[7]) == DefaultSecret3UInt64_7);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[8]) == DefaultSecret3UInt64_8);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[9]) == DefaultSecret3UInt64_9);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[10]) == DefaultSecret3UInt64_10);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[11]) == DefaultSecret3UInt64_11);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[12]) == DefaultSecret3UInt64_12);
            Debug.Assert(ReadLE64(defaultSecret3UInt64[13]) == DefaultSecret3UInt64_13);

            static ulong ReadLE64(ulong data) => BitConverter.IsLittleEndian ? data : BinaryPrimitives.ReverseEndianness(data);
        }
#endif

        public static void Initialize(ref State state, ulong seed)
        {
            state.Seed = (ulong)seed;

            fixed (byte* secret = state.Secret)
            {
                if (seed == 0)
                {
                    DefaultSecret.CopyTo(new Span<byte>(secret, SecretLengthBytes));
                }
                else
                {
                    DeriveSecretFromSeed(secret, (ulong)seed);
                }
            }

            Reset(ref state);
        }

        public static void Reset(ref State state)
        {
            state.BufferedCount = 0;
            state.StripesProcessedInCurrentBlock = 0;
            state.TotalLength = 0;

            fixed (ulong* accumulators = state.Accumulators)
            {
                InitializeAccumulators(accumulators);
            }
        }

        /// <summary>"This is a stronger avalanche, preferable when input has not been previously mixed."</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong Rrmxmx(ulong hash, uint length)
        {
            hash ^= RotateLeft(hash, 49) ^ RotateLeft(hash, 24);
            hash *= 0x9FB21C651E98DF25;
            hash ^= (hash >> 35) + length;
            hash *= 0x9FB21C651E98DF25;
            return XorShift(hash, 28);
        }

        public static void HashInternalLoop(ulong* accumulators, byte* source, uint length, byte* secret)
        {
            Debug.Assert(length > 240);

            const int StripesPerBlock = (SecretLengthBytes - StripeLengthBytes) / SecretConsumeRateBytes;
            const int BlockLen = StripeLengthBytes * StripesPerBlock;
            int blocksNum = (int)((length - 1) / BlockLen);

            Accumulate(accumulators, source, secret, StripesPerBlock, true, blocksNum);
            int offset = BlockLen * blocksNum;

            int stripesNumber = (int)((length - 1 - offset) / StripeLengthBytes);
            Accumulate(accumulators, source + offset, secret, stripesNumber);
            Accumulate512(accumulators, source + length - StripeLengthBytes, secret + (SecretLengthBytes - StripeLengthBytes - SecretLastAccStartBytes));
        }

        public static void ConsumeStripes(ulong* accumulators, ref ulong stripesSoFar, ulong stripesPerBlock, byte* source, ulong stripes, byte* secret)
        {
            Debug.Assert(stripes <= stripesPerBlock); // can handle max 1 scramble per invocation
            Debug.Assert(stripesSoFar < stripesPerBlock);

            ulong stripesToEndOfBlock = stripesPerBlock - stripesSoFar;
            if (stripesToEndOfBlock <= stripes)
            {
                // need a scrambling operation
                ulong stripesAfterBlock = stripes - stripesToEndOfBlock;
                Accumulate(accumulators, source, secret + ((int)stripesSoFar * SecretConsumeRateBytes), (int)stripesToEndOfBlock);
                ScrambleAccumulators(accumulators, secret + (SecretLengthBytes - StripeLengthBytes));
                Accumulate(accumulators, source + ((int)stripesToEndOfBlock * StripeLengthBytes), secret, (int)stripesAfterBlock);
                stripesSoFar = stripesAfterBlock;
            }
            else
            {
                Accumulate(accumulators, source, secret + ((int)stripesSoFar * SecretConsumeRateBytes), (int)stripes);
                stripesSoFar += stripes;
            }
        }

        public static void Append(ref State state, ReadOnlySpan<byte> source)
        {
            Debug.Assert(state.BufferedCount <= InternalBufferLengthBytes);

            state.TotalLength += (uint)source.Length;

            fixed (byte* buffer = state.Buffer)
            {
                // Small input: just copy the data to the buffer.
                if (source.Length <= InternalBufferLengthBytes - state.BufferedCount)
                {
                    source.CopyTo(new Span<byte>(buffer + state.BufferedCount, source.Length));
                    state.BufferedCount += (uint)source.Length;
                    return;
                }

                fixed (byte* secret = state.Secret)
                fixed (ulong* accumulators = state.Accumulators)
                fixed (byte* sourcePtr = &MemoryMarshal.GetReference(source))
                {
                    // Internal buffer is partially filled (always, except at beginning). Complete it, then consume it.
                    int sourceIndex = 0;
                    if (state.BufferedCount != 0)
                    {
                        int loadSize = InternalBufferLengthBytes - (int)state.BufferedCount;

                        source.Slice(0, loadSize).CopyTo(new Span<byte>(buffer + state.BufferedCount, loadSize));
                        sourceIndex = loadSize;

                        ConsumeStripes(accumulators, ref state.StripesProcessedInCurrentBlock, NumStripesPerBlock, buffer, InternalBufferStripes, secret);
                        state.BufferedCount = 0;
                    }
                    Debug.Assert(sourceIndex < source.Length);

                    // Large input to consume: ingest per full block.
                    if (source.Length - sourceIndex > NumStripesPerBlock * StripeLengthBytes)
                    {
                        ulong stripes = (ulong)(source.Length - sourceIndex - 1) / StripeLengthBytes;
                        Debug.Assert(NumStripesPerBlock >= state.StripesProcessedInCurrentBlock);

                        // Join to current block's end.
                        ulong stripesToEnd = NumStripesPerBlock - state.StripesProcessedInCurrentBlock;
                        Debug.Assert(stripesToEnd <= stripes);
                        Accumulate(accumulators, sourcePtr + sourceIndex, secret + ((int)state.StripesProcessedInCurrentBlock * SecretConsumeRateBytes), (int)stripesToEnd);
                        ScrambleAccumulators(accumulators, secret + (SecretLengthBytes - StripeLengthBytes));
                        state.StripesProcessedInCurrentBlock = 0;
                        sourceIndex += (int)stripesToEnd * StripeLengthBytes;
                        stripes -= stripesToEnd;

                        // Consume entire blocks.
                        while (stripes >= NumStripesPerBlock)
                        {
                            Accumulate(accumulators, sourcePtr + sourceIndex, secret, NumStripesPerBlock);
                            ScrambleAccumulators(accumulators, secret + (SecretLengthBytes - StripeLengthBytes));
                            sourceIndex += NumStripesPerBlock * StripeLengthBytes;
                            stripes -= NumStripesPerBlock;
                        }

                        // Consume complete stripes in the last partial block.
                        Accumulate(accumulators, sourcePtr + sourceIndex, secret, (int)stripes);
                        sourceIndex += (int)stripes * StripeLengthBytes;
                        Debug.Assert(sourceIndex < source.Length);  // at least some bytes left
                        state.StripesProcessedInCurrentBlock = stripes;

                        // Copy the last stripe into the end of the buffer so it is available to GetCurrentHashCore when processing the "stripe from the end".
                        source.Slice(sourceIndex - StripeLengthBytes, StripeLengthBytes).CopyTo(new Span<byte>(buffer + InternalBufferLengthBytes - StripeLengthBytes, StripeLengthBytes));
                    }
                    else if (source.Length - sourceIndex > InternalBufferLengthBytes)
                    {
                        // Content to consume <= block size. Consume source by a multiple of internal buffer size.
                        do
                        {
                            ConsumeStripes(accumulators, ref state.StripesProcessedInCurrentBlock, NumStripesPerBlock, sourcePtr + sourceIndex, InternalBufferStripes, secret);
                            sourceIndex += InternalBufferLengthBytes;
                        }
                        while (source.Length - sourceIndex > InternalBufferLengthBytes);

                        // Copy the last stripe into the end of the buffer so it is available to GetCurrentHashCore when processing the "stripe from the end".
                        source.Slice(sourceIndex - StripeLengthBytes, StripeLengthBytes).CopyTo(new Span<byte>(buffer + InternalBufferLengthBytes - StripeLengthBytes, StripeLengthBytes));
                    }

                    // Buffer the remaining input.
                    Span<byte> remaining = new Span<byte>(buffer, source.Length - sourceIndex);
                    Debug.Assert(sourceIndex < source.Length);
                    Debug.Assert(remaining.Length <= InternalBufferLengthBytes);
                    Debug.Assert(state.BufferedCount == 0);
                    source.Slice(sourceIndex).CopyTo(remaining);
                    state.BufferedCount = (uint)remaining.Length;
                }
            }
        }

        public static void CopyAccumulators(ref State state, ulong* accumulators)
        {
            fixed (ulong* stateAccumulators = state.Accumulators)
            {
                for (int i = 0; i < 8; i++)
                {
                    accumulators[i] = stateAccumulators[i];
                }
            }
        }

        public static void DigestLong(ref State state, ulong* accumulators, byte* secret)
        {
            Debug.Assert(state.BufferedCount > 0);

            fixed (byte* buffer = state.Buffer)
            {
                byte* accumulateData;
                if (state.BufferedCount >= StripeLengthBytes)
                {
                    uint stripes = (state.BufferedCount - 1) / StripeLengthBytes;
                    ulong stripesSoFar = state.StripesProcessedInCurrentBlock;

                    ConsumeStripes(accumulators, ref stripesSoFar, NumStripesPerBlock, buffer, stripes, secret);

                    accumulateData = buffer + state.BufferedCount - StripeLengthBytes;
                }
                else
                {
                    byte* lastStripe = stackalloc byte[StripeLengthBytes];
                    int catchupSize = StripeLengthBytes - (int)state.BufferedCount;

                    new ReadOnlySpan<byte>(buffer + InternalBufferLengthBytes - catchupSize, catchupSize).CopyTo(new Span<byte>(lastStripe, StripeLengthBytes));
                    new ReadOnlySpan<byte>(buffer, (int)state.BufferedCount).CopyTo(new Span<byte>(lastStripe + catchupSize, (int)state.BufferedCount));
                    accumulateData = lastStripe;
                }

                Accumulate512(accumulators, accumulateData, secret + (SecretLengthBytes - StripeLengthBytes - SecretLastAccStartBytes));
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void InitializeAccumulators(ulong* accumulators)
        {
            accumulators[0] = Prime32_3;
            accumulators[1] = Prime64_1;
            accumulators[2] = Prime64_2;
            accumulators[3] = Prime64_3;
            accumulators[4] = Prime64_4;
            accumulators[5] = Prime32_2;
            accumulators[6] = Prime64_5;
            accumulators[7] = Prime32_1;
        }

        public static ulong MergeAccumulators(ulong* accumulators, byte* secret, ulong start)
        {
            ulong result64 = start;

            result64 += Multiply64To128ThenFold(accumulators[0] ^ ReadUInt64LE(secret), accumulators[1] ^ ReadUInt64LE(secret + 8));
            result64 += Multiply64To128ThenFold(accumulators[2] ^ ReadUInt64LE(secret + 16), accumulators[3] ^ ReadUInt64LE(secret + 24));
            result64 += Multiply64To128ThenFold(accumulators[4] ^ ReadUInt64LE(secret + 32), accumulators[5] ^ ReadUInt64LE(secret + 40));
            result64 += Multiply64To128ThenFold(accumulators[6] ^ ReadUInt64LE(secret + 48), accumulators[7] ^ ReadUInt64LE(secret + 56));

            return Avalanche(result64);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong Mix16Bytes(byte* source, ulong secretLow, ulong secretHigh, ulong seed) =>
            Multiply64To128ThenFold(
                ReadUInt64LE(source) ^ (secretLow + seed),
                ReadUInt64LE(source + sizeof(ulong)) ^ (secretHigh - seed));

        /// <summary>Calculates a 32-bit to 64-bit long multiply.</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong Multiply32To64(uint v1, uint v2) => (ulong)v1 * v2;

        /// <summary>"This is a fast avalanche stage, suitable when input bits are already partially mixed."</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong Avalanche(ulong hash)
        {
            hash = XorShift(hash, 37);
            hash *= 0x165667919E3779F9;
            hash = XorShift(hash, 32);
            return hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong Multiply64To128(ulong left, ulong right, out ulong lower)
        {
            ulong lowerLow = Multiply32To64((uint)left, (uint)right);
            ulong higherLow = Multiply32To64((uint)(left >> 32), (uint)right);
            ulong lowerHigh = Multiply32To64((uint)left, (uint)(right >> 32));
            ulong higherHigh = Multiply32To64((uint)(left >> 32), (uint)(right >> 32));

            ulong cross = (lowerLow >> 32) + (higherLow & 0xFFFFFFFF) + lowerHigh;
            ulong upper = (higherLow >> 32) + (cross >> 32) + higherHigh;
            lower = (cross << 32) | (lowerLow & 0xFFFFFFFF);
            return upper;
        }

        /// <summary>Calculates a 64-bit to 128-bit multiply, then XOR folds it.</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong Multiply64To128ThenFold(ulong left, ulong right)
        {
            ulong upper = Multiply64To128(left, right, out ulong lower);
            return lower ^ upper;
        }

        public static void DeriveSecretFromSeed(byte* destinationSecret, ulong seed)
        {
            fixed (byte* defaultSecret = &MemoryMarshal.GetReference(DefaultSecret))
            {
                for (int i = 0; i < SecretLengthBytes; i += sizeof(ulong) * 2)
                {
                    WriteUInt64LE(destinationSecret + i, ReadUInt64LE(defaultSecret + i) + seed);
                    WriteUInt64LE(destinationSecret + i + 8, ReadUInt64LE(defaultSecret + i + 8) - seed);
                }
            }
        }

        /// <summary>Optimized version of looping over <see cref="Accumulate512"/>.</summary>
        [MethodImpl(MethodImplOptions.NoInlining)]
        private static void Accumulate(ulong* accumulators, byte* source, byte* secret, int stripesToProcess, bool scramble = false, int blockCount = 1)
        {
            byte* secretForScramble = secret + (SecretLengthBytes - StripeLengthBytes);

            for (int j = 0; j < blockCount; j++)
            {
                for (int i = 0; i < stripesToProcess; i++)
                {
                    Accumulate512Inlined(accumulators, source, secret + (i * SecretConsumeRateBytes));
                    source += StripeLengthBytes;
                }

                if (scramble)
                {
                    ScrambleAccumulators(accumulators, secretForScramble);
                }
            }
        }

        [MethodImpl(MethodImplOptions.NoInlining)]
        public static void Accumulate512(ulong* accumulators, byte* source, byte* secret)
        {
            Accumulate512Inlined(accumulators, source, secret);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void Accumulate512Inlined(ulong* accumulators, byte* source, byte* secret)
        {
            for (int i = 0; i < AccumulatorCount; i++)
            {
                ulong sourceVal = ReadUInt64LE(source + (8 * i));
                ulong sourceKey = sourceVal ^ ReadUInt64LE(secret + (i * 8));

                accumulators[i ^ 1] += sourceVal; // swap adjacent lanes
                accumulators[i] += Multiply32To64((uint)sourceKey, (uint)(sourceKey >> 32));
            }
        }

        private static void ScrambleAccumulators(ulong* accumulators, byte* secret)
        {
            for (int i = 0; i < AccumulatorCount; i++)
            {
                ulong xorShift = XorShift(*accumulators, 47);
                ulong xorWithKey = xorShift ^ ReadUInt64LE(secret);
                *accumulators = xorWithKey * Prime32_1;

                accumulators++;
                secret += sizeof(ulong);
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong XorShift(ulong value, int shift)
        {
            Debug.Assert(shift >= 0 && shift < 64);
            return value ^ (value >> shift);
        }

        // Replacements for System.Numerics.BitOperations.RotateLeft, which is not
        // available in the Unity editor's netstandard2.1 profile.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static uint RotateLeft(uint value, int offset) =>
            (value << offset) | (value >> (32 - offset));

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong RotateLeft(ulong value, int offset) =>
            (value << offset) | (value >> (64 - offset));

        // Plain pointer dereferences instead of Unsafe.ReadUnaligned/WriteUnaligned:
        // every platform the Unity editor runs on (x64/arm64) supports unaligned
        // scalar loads and stores, and this avoids a dependency on the
        // System.Runtime.CompilerServices.Unsafe assembly.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static uint ReadUInt32LE(byte* data) =>
            BitConverter.IsLittleEndian ?
                *(uint*)data :
                BinaryPrimitives.ReverseEndianness(*(uint*)data);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong ReadUInt64LE(byte* data) =>
            BitConverter.IsLittleEndian ?
                *(ulong*)data :
                BinaryPrimitives.ReverseEndianness(*(ulong*)data);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void WriteUInt64LE(byte* data, ulong value)
        {
            if (!BitConverter.IsLittleEndian)
            {
                value = BinaryPrimitives.ReverseEndianness(value);
            }
            *(ulong*)data = value;
        }

        [StructLayout(LayoutKind.Auto)]
        public struct State
        {
            /// <summary>The accumulators. Length is <see cref="AccumulatorCount"/>.</summary>
            internal fixed ulong Accumulators[AccumulatorCount];

            /// <summary>Used to store a custom secret generated from a seed. Length is <see cref="SecretLengthBytes"/>.</summary>
            internal fixed byte Secret[SecretLengthBytes];

            /// <summary>The internal buffer. Length is <see cref="InternalBufferLengthBytes"/>.</summary>
            internal fixed byte Buffer[InternalBufferLengthBytes];

            /// <summary>The amount of memory in <see cref="Buffer"/>.</summary>
            internal uint BufferedCount;

            /// <summary>Number of stripes processed in the current block.</summary>
            internal ulong StripesProcessedInCurrentBlock;

            /// <summary>Total length hashed.</summary>
            internal ulong TotalLength;

            /// <summary>The seed employed (possibly 0).</summary>
            internal ulong Seed;
        };
    }
}