using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;

namespace UdPunching
{
    public static class TransferCodec
    {
        public static readonly Guid InvalidPeerId = new Guid("ffffffff-ffff-ffff-ffff-ffffffffffff");

        // [0...15] GUID PeerId
        // [16..19] Encrypted data length prefix (LeInt32)
        // [20..END] Encrypted data content

        public static Guid ReadId(byte[] recvData)
        {
            return new Guid(recvData.Take(16).ToArray());
        }

        public static byte[] DecodeData(RSACryptoServiceProvider cryptoProvider, byte[] recvData)
        {
            var len = recvData.ToLeInt32(16);
            return cryptoProvider.Decrypt(recvData.Skip(20).Take(len).ToArray(), false);
        }

        public static byte[] Encode(RSACryptoServiceProvider cryptoProvider, Guid id, byte[] data)
        {
            var idBytes = id.ToByteArray(); // 16
            var encryptedBytes = cryptoProvider.Encrypt(data, false);

            var lst = new List<byte>(idBytes.Length + encryptedBytes.Length);
            lst.AddRange(idBytes);
            lst.AddRange(encryptedBytes.Length.ToLeBytes()); //4
            lst.AddRange(encryptedBytes);

            return lst.ToArray();
        }
    }
}