using System;
using System.Security.Cryptography;
namespace DTLib
{
    //
    // честно взятый с гитхаба алгоритм хеширования
    // выдаёт хеш в виде массива четырёх байтов
    //
    sealed class XXHash32 : HashAlgorithm
    {
        private const uint PRIME32_1 = 2654435761U;
        private const uint PRIME32_2 = 2246822519U;
        private const uint PRIME32_3 = 3266489917U;
        private const uint PRIME32_4 = 668265263U;
        private const uint PRIME32_5 = 374761393U;
        private static readonly Func<byte[], int, uint> FuncGetLittleEndianUInt32;
        private static readonly Func<uint, uint> FuncGetFinalHashUInt32;
        private uint _Seed32;
        private uint _ACC32_1;
        private uint _ACC32_2;
        private uint _ACC32_3;
        private uint _ACC32_4;
        private uint _Hash32;
        private int _RemainingLength;
        private long _TotalLength = 0;
        private int _CurrentIndex;
        private byte[] _CurrentArray;

        static XXHash32()
        {
            if(BitConverter.IsLittleEndian)
            {
                FuncGetLittleEndianUInt32=new Func<byte[], int, uint>((x, i) =>
              {
                  unsafe
                  {
                      fixed(byte* array = x)
                      {
                          return *(uint*)(array+i);
                      }
                  }
              });
                FuncGetFinalHashUInt32=new Func<uint, uint>(i => (i&0x000000FFU)<<24|(i&0x0000FF00U)<<8|(i&0x00FF0000U)>>8|(i&0xFF000000U)>>24);
            }
            else
            {
                FuncGetLittleEndianUInt32=new Func<byte[], int, uint>((x, i) =>
              {
                  unsafe
                  {
                      fixed(byte* array = x)
                      {
                          return (uint)(array[i++]|(array[i++]<<8)|(array[i++]<<16)|(array[i]<<24));
                      }
                  }
              });
                FuncGetFinalHashUInt32=new Func<uint, uint>(i => i);
            }
        }

        // Creates an instance of <see cref="XXHash32"/> class by default seed(0).
        // <returns></returns>
        public static new XXHash32 Create() => new();

        // Initializes a new instance of the <see cref="XXHash32"/> class by default seed(0).
        public XXHash32() => Initialize(0);

        // Initializes a new instance of the <see cref="XXHash32"/> class, and sets the <see cref="Seed"/> to the specified value.
        // <param name="seed">Represent the seed to be used for xxHash32 computing.</param>
        public XXHash32(uint seed) => Initialize(seed);

        // Gets the <see cref="uint"/> value of the computed hash code.
        // <exception cref="InvalidOperationException">Hash computation has not yet completed.</exception>
        public uint HashUInt32 => State==0 ? _Hash32 : throw new InvalidOperationException("Hash computation has not yet completed.");

        // Gets or sets the value of seed used by xxHash32 algorithm.
        // <exception cref="InvalidOperationException">Hash computation has not yet completed.</exception>
        public uint Seed
        {
            get => _Seed32;
            set
            {
                if(value!=_Seed32)
                {
                    if(State!=0)
                        throw new InvalidOperationException("Hash computation has not yet completed.");
                    _Seed32=value;
                    Initialize();
                }
            }
        }

        // Initializes this instance for new hash computing.
        public override void Initialize()
        {
            _ACC32_1=_Seed32+PRIME32_1+PRIME32_2;
            _ACC32_2=_Seed32+PRIME32_2;
            _ACC32_3=_Seed32+0;
            _ACC32_4=_Seed32-PRIME32_1;
        }

        // Routes data written to the object into the hash algorithm for computing the hash.
        // <param name="array">The input to compute the hash code for.</param>
        // <param name="ibStart">The offset into the byte array from which to begin using data.</param>
        // <param name="cbSize">The number of bytes in the byte array to use as data.</param>
        protected override void HashCore(byte[] array, int ibStart, int cbSize)
        {
            if(State!=1)
                State=1;
            int size = cbSize-ibStart;
            _RemainingLength=size&15;
            if(cbSize>=16)
            {
                int limit = size-_RemainingLength;
                do
                {
                    _ACC32_1=Round32(_ACC32_1, FuncGetLittleEndianUInt32(array, ibStart));
                    ibStart+=4;
                    _ACC32_2=Round32(_ACC32_2, FuncGetLittleEndianUInt32(array, ibStart));
                    ibStart+=4;
                    _ACC32_3=Round32(_ACC32_3, FuncGetLittleEndianUInt32(array, ibStart));
                    ibStart+=4;
                    _ACC32_4=Round32(_ACC32_4, FuncGetLittleEndianUInt32(array, ibStart));
                    ibStart+=4;
                } while(ibStart<limit);
            }
            _TotalLength+=cbSize;
            if(_RemainingLength!=0)
            {
                _CurrentArray=array;
                _CurrentIndex=ibStart;
            }
        }

        // Finalizes the hash computation after the last data is processed by the cryptographic stream object.
        // <returns>The computed hash code.</returns>
        protected override byte[] HashFinal()
        {
            if(_TotalLength>=16)
            {
                _Hash32=RotateLeft32(_ACC32_1, 1)+RotateLeft32(_ACC32_2, 7)+RotateLeft32(_ACC32_3, 12)+RotateLeft32(_ACC32_4, 18);
            }
            else
            {
                _Hash32=_Seed32+PRIME32_5;
            }
            _Hash32+=(uint)_TotalLength;
            while(_RemainingLength>=4)
            {
                _Hash32=RotateLeft32(_Hash32+FuncGetLittleEndianUInt32(_CurrentArray, _CurrentIndex)*PRIME32_3, 17)*PRIME32_4;
                _CurrentIndex+=4;
                _RemainingLength-=4;
            }
            unsafe
            {
                fixed(byte* arrayPtr = _CurrentArray)
                {
                    while(_RemainingLength-->=1)
                    {
                        _Hash32=RotateLeft32(_Hash32+arrayPtr[_CurrentIndex++]*PRIME32_5, 11)*PRIME32_1;
                    }
                }
            }
            _Hash32=(_Hash32^(_Hash32>>15))*PRIME32_2;
            _Hash32=(_Hash32^(_Hash32>>13))*PRIME32_3;
            _Hash32^=_Hash32>>16;
            _TotalLength=State=0;
            return BitConverter.GetBytes(FuncGetFinalHashUInt32(_Hash32));
        }

        private static uint Round32(uint input, uint value) => RotateLeft32(input+(value*PRIME32_2), 13)*PRIME32_1;

        private static uint RotateLeft32(uint value, int count) => (value<<count)|(value>>(32-count));

        private void Initialize(uint seed)
        {
            HashSizeValue=32;
            _Seed32=seed;
            Initialize();
        }
    }
}