using System;
using System.Numerics;
using System.Security.Cryptography;
using System.Text;

namespace CipherTest
{
    public class Cipher
    {
        const long KeySize = 64;
        const long NonceSize = 8;
        const long BlockSize = 8;
        const long RoundCount = 16;

        // <Hermione, Don't You Think On It For One Minute>
        static readonly ulong[] Hagrid = { 0x3C4865726D696F6E, 0x652C20446F6E2774, 0x20596F7520546869, 0x6E6B204F6E204974, 0x20466F72204F6E65, 0x204D696E7574653E };

        static readonly Encoding UTF = Encoding.UTF8;

        private readonly RNGCryptoServiceProvider provider;
        private readonly ulong[] key;

        public Cipher(RNGCryptoServiceProvider provider, BigInteger key)
        {
            this.provider = provider;
            byte[] keyBytes = new byte[KeySize], inBytes = key.ToByteArray(true);
            Array.Copy(inBytes, keyBytes, Math.Min(inBytes.Length, KeySize));
            this.key = Array.ConvertAll(keyBytes, Convert.ToUInt64);
        }

        public static void QR(ref ulong a, ref ulong b, ref ulong c, ref ulong d)
        {
            b ^= (a + d).RL(25);
            c ^= (b + a).RL(23);
            d ^= (c + b).RL(19);
            a ^= (d + c).RL(46);
        }

        public static void Round(ulong[] table)
        {
            QR(ref table[0], ref table[4], ref table[8], ref table[12]);
            QR(ref table[5], ref table[9], ref table[13], ref table[1]);
            QR(ref table[10], ref table[14], ref table[2], ref table[6]);
            QR(ref table[15], ref table[3], ref table[7], ref table[11]);

            QR(ref table[0], ref table[1], ref table[2], ref table[3]);
            QR(ref table[5], ref table[6], ref table[7], ref table[4]);
            QR(ref table[10], ref table[11], ref table[8], ref table[9]);
            QR(ref table[15], ref table[12], ref table[13], ref table[14]);
        }

        public ulong KeyBlock(ulong block, ulong nonce)
        {
            ulong[] table = {
                Hagrid[0],  Hagrid[1],  Hagrid[2],  Hagrid[3],
                key[0],     key[1],     key[2],     key[3],
                key[4],     key[5],     key[6],     key[7],
                block,      nonce,      Hagrid[4],  Hagrid[5]
            };
            var original = table[block & 15];

            for (int i = 0; i < RoundCount; i += 2)
            {
                Round(table);
            }

            return original + table[block & 15];
        }

        public byte[] TransformBlock(byte[] bytes, long i, ulong block, long size, ulong nonce)
        {
            var inputBytes = new byte[BlockSize];
            Array.Copy(bytes, i, inputBytes, 0, Math.Min(BlockSize, size - i));
            return BitConverter.GetBytes(KeyBlock(block, nonce) ^ BitConverter.ToUInt64(inputBytes));
        }

        public string Encrypt(string input)
        {
            var plain = UTF.GetBytes(input);
            var nonceBytes = new byte[NonceSize];
            provider.GetBytes(nonceBytes);
            ulong nonce = BitConverter.ToUInt64(nonceBytes);

            long length = plain.Length;
            long size = length + NonceSize;
            var cipher = new byte[size];
            Array.Copy(nonceBytes, cipher, NonceSize);

            ulong j = 0;
            for (long i = 0; i < length; i += BlockSize, ++j)
            {
                Array.Copy(TransformBlock(plain, i, j, length, nonce), 0, cipher, i + NonceSize, Math.Min(BlockSize, length - i));
            }

            return Helpers.BytesToHex(cipher);
        }

        public string? Decrypt(string input)
        {
            var bytes = Helpers.HexToBytes(input);
            if (bytes == null || bytes.Length < NonceSize)
            {
                return null;
            }

            var nonceBytes = new byte[NonceSize];
            Array.Copy(bytes, nonceBytes, NonceSize);
            var nonce = BitConverter.ToUInt64(nonceBytes);

            long size = bytes.Length - NonceSize;
            var cipher = new byte[size];
            Array.Copy(bytes, NonceSize, cipher, 0, size);

            var plain = new byte[size];

            ulong j = 0;
            for (long i = 0; i < size; i += BlockSize, ++j)
            {
                Array.Copy(TransformBlock(cipher, i, j, size, nonce), 0, plain, i, Math.Min(BlockSize, size - i));
            }

            return UTF.GetString(plain);
        }
    }
}