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

// Copied from https://github.com/dotnet/runtime/blob/f1d463f46268382a8287d71aea929dadaa5dfef5/src/libraries/System.IO.Hashing/src/System/IO/Hashing/XxHash64.State.cs#L1C13-L1C13
// Remove once we can actually add a reference to System.IO.Hashing v8.0.0

// Implemented from the specification at
// https://github.com/Cyan4973/xxHash/blob/f9155bd4c57e2270a4ffbb176485e5d713de1c9b/doc/xxhash_spec.md

using System.Runtime.CompilerServices;
using static System.IO.Hashing.XxHashShared;

namespace System.IO.Hashing;

internal sealed partial class XxHash64
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    internal static ulong Avalanche(ulong hash)
    {
        hash ^= hash >> 33;
        hash *= Prime64_2;
        hash ^= hash >> 29;
        hash *= Prime64_3;
        hash ^= hash >> 32;
        return hash;
    }

#if false
    private struct State
    {
        private ulong _acc1;
        private ulong _acc2;
        private ulong _acc3;
        private ulong _acc4;
        private readonly ulong _smallAcc;
        private bool _hadFullStripe;

        internal State(ulong seed)
        {
            _acc1 = seed + unchecked(Prime64_1 + Prime64_2);
            _acc2 = seed + Prime64_2;
            _acc3 = seed;
            _acc4 = seed - Prime64_1;

            _smallAcc = seed + Prime64_5;
            _hadFullStripe = false;
        }

        internal void ProcessStripe(ReadOnlySpan<byte> source)
        {
            Debug.Assert(source.Length >= StripeSize);
            source = source.Slice(0, StripeSize);

            _acc1 = ApplyRound(_acc1, source);
            _acc2 = ApplyRound(_acc2, source.Slice(sizeof(ulong)));
            _acc3 = ApplyRound(_acc3, source.Slice(2 * sizeof(ulong)));
            _acc4 = ApplyRound(_acc4, source.Slice(3 * sizeof(ulong)));

            _hadFullStripe = true;
        }

        private static ulong MergeAccumulator(ulong acc, ulong accN)
        {
            acc ^= ApplyRound(0, accN);
            acc *= Prime64_1;
            acc += Prime64_4;

            return acc;
        }

        private readonly ulong Converge()
        {
            ulong acc =
                BitOperations.RotateLeft(_acc1, 1) +
                BitOperations.RotateLeft(_acc2, 7) +
                BitOperations.RotateLeft(_acc3, 12) +
                BitOperations.RotateLeft(_acc4, 18);

            acc = MergeAccumulator(acc, _acc1);
            acc = MergeAccumulator(acc, _acc2);
            acc = MergeAccumulator(acc, _acc3);
            acc = MergeAccumulator(acc, _acc4);

            return acc;
        }

        private static ulong ApplyRound(ulong acc, ReadOnlySpan<byte> lane)
        {
            return ApplyRound(acc, BinaryPrimitives.ReadUInt64LittleEndian(lane));
        }

        private static ulong ApplyRound(ulong acc, ulong lane)
        {
            acc += lane * Prime64_2;
            acc = BitOperations.RotateLeft(acc, 31);
            acc *= Prime64_1;

            return acc;
        }

        // Inliner may decide to inline this method into HashToUInt64() with help of PGO and
        // can run out of "time budget" producing non-inlined simple calls such as Span.Slice.
        // TODO: Remove NoInlining when https://github.com/dotnet/runtime/issues/85531 is fixed.
        [MethodImpl(MethodImplOptions.NoInlining)]
        internal readonly ulong Complete(long length, ReadOnlySpan<byte> remaining)
        {
            ulong acc = _hadFullStripe ? Converge() : _smallAcc;

            acc += (ulong)length;

            while (remaining.Length >= sizeof(ulong))
            {
                ulong lane = BinaryPrimitives.ReadUInt64LittleEndian(remaining);
                acc ^= ApplyRound(0, lane);
                acc = BitOperations.RotateLeft(acc, 27);
                acc *= Prime64_1;
                acc += Prime64_4;

                remaining = remaining.Slice(sizeof(ulong));
            }

            // Doesn't need to be a while since it can occur at most once.
            if (remaining.Length >= sizeof(uint))
            {
                ulong lane = BinaryPrimitives.ReadUInt32LittleEndian(remaining);
                acc ^= lane * Prime64_1;
                acc = BitOperations.RotateLeft(acc, 23);
                acc *= Prime64_2;
                acc += Prime64_3;

                remaining = remaining.Slice(sizeof(uint));
            }

            for (int i = 0; i < remaining.Length; i++)
            {
                ulong lane = remaining[i];
                acc ^= lane * Prime64_5;
                acc = BitOperations.RotateLeft(acc, 11);
                acc *= Prime64_1;
            }

            return Avalanche(acc);
        }
    }
#endif
}