using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Text;

namespace ZeroLevel.Services.HashFunctions
{
    public static class XXH3_64
    {
        private const ulong PRIME64_1 = 11400714785074694791UL;
        private const ulong PRIME64_2 = 14029467366897019727UL;
        private const ulong PRIME64_3 = 1609587929392839161UL;
        private const ulong PRIME64_4 = 9650029242287828579UL;
        private const ulong PRIME64_5 = 2870177450012600261UL;

        [StructLayout(LayoutKind.Sequential)]
        private struct QuadUlong
        {
            public ulong v1;
            public ulong v2;
            public ulong v3;
            public ulong v4;
        }

        public static ulong Hash(string line)
        {
            return Hash(new ReadOnlySpan<byte>(Encoding.UTF8.GetBytes(line)));
        }

        public static ulong Hash(in ReadOnlySpan<byte> buffer)
        {
            unchecked
            {
                var remainingBytes = buffer;
                var bulkVals = remainingBytes.PopAll<QuadUlong>();

                var h64 = !bulkVals.IsEmpty ? BulkStride(bulkVals) : PRIME64_5;

                h64 += (uint)buffer.Length;

                var ulongSpan = remainingBytes.PopAll<ulong>();
                for (int i = 0; i < ulongSpan.Length; i++)
                {
                    var val = ulongSpan[i] * PRIME64_2;
                    val = RotateLeft(val, 31);
                    val *= PRIME64_1;
                    h64 ^= val;
                    h64 = RotateLeft(h64, 27) * PRIME64_1;
                    h64 += PRIME64_4;
                }

                ref byte remaining = ref MemoryMarshal.GetReference(remainingBytes);
                if (remainingBytes.Length >= sizeof(uint))
                {
                    h64 ^= Unsafe.As<byte, uint>(ref remaining) * PRIME64_1;
                    h64 = RotateLeft(h64, 23) * PRIME64_2;
                    h64 += PRIME64_3;
                    remaining = ref Unsafe.Add(ref remaining, sizeof(uint));
                }

                switch (remainingBytes.Length % sizeof(uint))
                {
                    case 3:
                        h64 = RotateLeft(h64 ^ remaining * PRIME64_5, 11) * PRIME64_1;
                        remaining = ref Unsafe.Add(ref remaining, 1);
                        goto case 2;
                    case 2:
                        h64 = RotateLeft(h64 ^ remaining * PRIME64_5, 11) * PRIME64_1;
                        remaining = ref Unsafe.Add(ref remaining, 1);
                        goto case 1;
                    case 1:
                        h64 = RotateLeft(h64 ^ remaining * PRIME64_5, 11) * PRIME64_1;
                        break;
                }

                h64 ^= h64 >> 33;
                h64 *= PRIME64_2;
                h64 ^= h64 >> 29;
                h64 *= PRIME64_3;
                h64 ^= h64 >> 32;

                return h64;
            }
        }

        [MethodImpl(MethodImplOptions.NoInlining)]
        private static ulong BulkStride(in ReadOnlySpan<QuadUlong> bulkVals)
        {
            unchecked
            {
                ulong acc1 = 0 + PRIME64_1 + PRIME64_2;
                ulong acc2 = 0 + PRIME64_2;
                ulong acc3 = 0 + 0;
                ulong acc4 = 0 - PRIME64_1;

                for (int i = 0; i < bulkVals.Length; i++)
                {
                    ref readonly QuadUlong val = ref bulkVals[i];

                    acc1 += val.v1 * PRIME64_2;
                    acc2 += val.v2 * PRIME64_2;
                    acc3 += val.v3 * PRIME64_2;
                    acc4 += val.v4 * PRIME64_2;

                    acc1 = RotateLeft(acc1, 31);
                    acc2 = RotateLeft(acc2, 31);
                    acc3 = RotateLeft(acc3, 31);
                    acc4 = RotateLeft(acc4, 31);

                    acc1 *= PRIME64_1;
                    acc2 *= PRIME64_1;
                    acc3 *= PRIME64_1;
                    acc4 *= PRIME64_1;
                }

                return MergeValues(acc1, acc2, acc3, acc4);
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static ulong RotateLeft(ulong val, int bits) => (val << bits) | (val >> (64 - bits));

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static ulong MergeValues(ulong v1, ulong v2, ulong v3, ulong v4)
        {
            var acc = RotateLeft(v1, 1) + RotateLeft(v2, 7) + RotateLeft(v3, 12) + RotateLeft(v4, 18);
            acc = MergeAccumulator(acc, v1);
            acc = MergeAccumulator(acc, v2);
            acc = MergeAccumulator(acc, v3);
            acc = MergeAccumulator(acc, v4);
            return acc;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static ulong MergeAccumulator(ulong accMain, ulong accN)
        {
            accN = (accN * PRIME64_2);
            accN = RotateLeft(accN, 31);
            accN = accN * PRIME64_1;
            accMain ^= accN;
            accMain *= PRIME64_1;
            return accMain + PRIME64_4;
        }
    }
}