//
//  ClientWorldCryptography.cs
//
//  Copyright (c) František Boháček. All rights reserved.
//  Licensed under the MIT license. See LICENSE file in the project root for full license information.

using System.Text;

namespace NosSmooth.Cryptography;

/// <summary>
/// A cryptography used on world server, has to have a session id (encryption key) set from the client.
/// </summary>
public class ClientWorldCryptography : ICryptography
{
    private static readonly char[] Keys = { ' ', '-', '.', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'n' };

    /// <summary>
    /// Gets or sets the encryption key.
    /// </summary>
    public int EncryptionKey { get; set; }

    /// <summary>
    /// Initializes a new instance of the <see cref="ClientWorldCryptography"/> class.
    /// </summary>
    /// <param name="encryptionKey">Encryption key received by LoginServer.</param>
    public ClientWorldCryptography(int encryptionKey = 0)
    {
        EncryptionKey = encryptionKey;
    }

    /// <inheritdoc />
    public string Decrypt(in ReadOnlySpan<byte> bytes, Encoding encoding)
    {
        try
        {
            int index = 0;
            var currentPacket = new List<byte>();

            while (index < bytes.Length)
            {
                byte currentByte = bytes[index++];

                if (currentByte == 0xFF)
                {
                    currentPacket.Add((byte)'\n');
                    continue;
                }

                int length = currentByte & 0x7F;

                if ((currentByte & 0x80) != 0)
                {
                    while (length != 0)
                    {
                        if (index < bytes.Length)
                        {
                            currentByte = bytes[index++];
                            int firstIndex = (currentByte & 0xF0) >> 4;
                            char first = '?';
                            if (firstIndex != 0)
                            {
                                firstIndex--;
                                first = firstIndex != 14 ? Keys[firstIndex] : '\u0000';
                            }

                            if (first != 0x6E)
                            {
                                currentPacket.Add((byte)first);
                            }

                            if (length <= 1)
                            {
                                break;
                            }

                            int secondIndex = currentByte & 0xF;
                            char second = '?';
                            if (secondIndex != 0)
                            {
                                secondIndex--;
                                second = secondIndex != 14 ? Keys[secondIndex] : '\u0000';
                            }

                            if (second != 0x6E)
                            {
                                currentPacket.Add((byte)second);
                            }

                            length -= 2;
                        }
                        else
                        {
                            length--;
                        }
                    }
                }
                else
                {
                    while (length != 0)
                    {
                        if (index < bytes.Length)
                        {
                            currentPacket.Add((byte)(bytes[index] ^ 0xFF));
                            index++;
                        }
                        else if (index == bytes.Length)
                        {
                            currentPacket.Add((byte)'\n');
                            index++;
                        }

                        length--;
                    }
                }
            }

            return string.Concat(currentPacket.Select(x => (char)x));

            // byte[] tmp = Encoding.Convert(encoding, Encoding.UTF8, currentPacket.ToArray());
            // return Encoding.UTF8.GetString(tmp);
        }
        catch
        {
            return string.Empty;
        }
    }

    /// <inheritdoc />
    public byte[] Encrypt(string value, Encoding encoding)
    {
        try
        {
            var output = new List<byte>();

            string mask = new string
            (
                value.Select
                (
                    c =>
                    {
                        sbyte b = (sbyte)c;
                        if (c == '#' || c == '/' || c == '%')
                        {
                            return '0';
                        }

                        if ((b -= 0x20) == 0 || (b += unchecked((sbyte)0xF1)) < 0 || (b -= 0xB) < 0 ||
                            b - unchecked((sbyte)0xC5) == 0)
                        {
                            return '1';
                        }

                        return '0';
                    }
                ).ToArray()
            );

            int packetLength = value.Length;

            int sequenceCounter = 0;
            int currentPosition = 0;

            while (currentPosition <= packetLength)
            {
                int lastPosition = currentPosition;
                while (currentPosition < packetLength && mask[currentPosition] == '0')
                {
                    currentPosition++;
                }

                int sequences;
                int length;

                if (currentPosition != 0)
                {
                    length = currentPosition - lastPosition;
                    sequences = length / 0x7E;
                    for (int i = 0; i < length; i++, lastPosition++)
                    {
                        if (i == sequenceCounter * 0x7E)
                        {
                            if (sequences == 0)
                            {
                                output.Add((byte)(length - i));
                            }
                            else
                            {
                                output.Add(0x7E);
                                sequences--;
                                sequenceCounter++;
                            }
                        }

                        output.Add((byte)((byte)value[lastPosition] ^ 0xFF));
                    }
                }

                if (currentPosition >= packetLength)
                {
                    break;
                }

                lastPosition = currentPosition;
                while (currentPosition < packetLength && mask[currentPosition] == '1')
                {
                    currentPosition++;
                }

                if (currentPosition == 0)
                {
                    continue;
                }

                length = currentPosition - lastPosition;
                sequences = length / 0x7E;
                for (int i = 0; i < length; i++, lastPosition++)
                {
                    if (i == sequenceCounter * 0x7E)
                    {
                        if (sequences == 0)
                        {
                            output.Add((byte)((length - i) | 0x80));
                        }
                        else
                        {
                            output.Add(0x7E | 0x80);
                            sequences--;
                            sequenceCounter++;
                        }
                    }

                    byte currentByte = (byte)value[lastPosition];
                    switch (currentByte)
                    {
                        case 0x20:
                            currentByte = 1;
                            break;
                        case 0x2D:
                            currentByte = 2;
                            break;
                        case 0xFF:
                            currentByte = 0xE;
                            break;
                        default:
                            currentByte -= 0x2C;
                            break;
                    }

                    if (currentByte == 0x00)
                    {
                        continue;
                    }

                    if (i % 2 == 0)
                    {
                        output.Add((byte)(currentByte << 4));
                    }
                    else
                    {
                        output[output.Count - 1] = (byte)(output.Last() | currentByte);
                    }
                }
            }

            output.Add(0xFF);

            sbyte sessionNumber = (sbyte)((EncryptionKey >> 6) & 0xFF & 0x80000003);

            if (sessionNumber < 0)
            {
                sessionNumber = (sbyte)(((sessionNumber - 1) | 0xFFFFFFFC) + 1);
            }

            byte sessionKey = (byte)(EncryptionKey & 0xFF);

            if (EncryptionKey == 0)
            {
                sessionNumber = -1;
            }

            switch (sessionNumber)
            {
                case 0:
                    for (int i = 0; i < output.Count; i++)
                    {
                        output[i] = (byte)(output[i] + sessionKey + 0x40);
                    }

                    break;
                case 1:
                    for (int i = 0; i < output.Count; i++)
                    {
                        output[i] = (byte)(output[i] - (sessionKey + 0x40));
                    }

                    break;
                case 2:
                    for (int i = 0; i < output.Count; i++)
                    {
                        output[i] = (byte)((output[i] ^ 0xC3) + sessionKey + 0x40);
                    }

                    break;
                case 3:
                    for (int i = 0; i < output.Count; i++)
                    {
                        output[i] = (byte)((output[i] ^ 0xC3) - (sessionKey + 0x40));
                    }

                    break;
                default:
                    for (int i = 0; i < output.Count; i++)
                    {
                        output[i] = (byte)(output[i] + 0x0F);
                    }

                    break;
            }

            return output.ToArray();
        }
        catch
        {
            return Array.Empty<byte>();
        }
    }
}