/// <summary>
/// Returns the number of bytes to read from the stream.
/// </summary>
/// <param name="sessionState">Instance of <see cref="T:NavtelecomProtocol.SessionState" />.</param>
/// <param name="receiveBuffer">A read-only collection of bytes received from a stream.</param>
/// <param name="sendBuffer"><see cref="T:SharpStructures.MemoryBuffer" /> with data to be sent to the client.</param>
/// <param name="token">The token to monitor for cancellation requests.</param>
/// <returns>A task that represents the asynchronous operation. Contains the total number of bytes to read from the socket. Zero bytes to stop reading and send the response.</returns>
public Task <int> GetPendingBytesAsync(SessionState sessionState, IReadOnlyList <byte> receiveBuffer, MemoryBuffer sendBuffer, CancellationToken token)
{
switch (receiveBuffer.Count)
{
case 1:
return(Task.FromResult(HeaderLength - 1));
case HeaderLength:
var reader = new BinaryListReader(receiveBuffer, true);
if (!reader.ReadBytes(HeaderPreamble.Length).Select(x => (char)x).SequenceEqual(HeaderPreamble))
{
throw new ArgumentException("NTCB header preamble does not match.");
}
sessionState.ReceiverId = reader.ReadUInt32();
sessionState.SenderId = reader.ReadUInt32();
var payloadLength = reader.ReadUInt16();
reader.ReadByte();
var headerChecksum = reader.ReadByte();
if (BinaryUtilities.GetXorSum(receiveBuffer.Take(HeaderLength - 1)) != headerChecksum)
{
throw new ArgumentException("NTCB header checksum does not match.");
}
return(Task.FromResult((int)payloadLength));
default:
if (BinaryUtilities.GetXorSum(receiveBuffer.Skip(HeaderLength)) != receiveBuffer[14])
{
throw new ArgumentException("NTCB body checksum does not match.");
}
var bodyProcessor =
_bodyProcessors.GetValueOrDefault(receiveBuffer.Skip(HeaderLength).Select(x => (char)x));
if (bodyProcessor == null)
{
throw new ArgumentException("Unknown NTCB message type.");
}
var bodyReader = new BinaryListReader(receiveBuffer, true);
bodyReader.SetPosition(HeaderLength);
sendBuffer.AllocateSpace(HeaderLength);
var writer = new MemoryBufferWriter(sendBuffer, true);
bodyProcessor.ProcessBody(sessionState, bodyReader, writer);
var responseLength = sendBuffer.Position - HeaderLength;
sendBuffer.SetPosition(0);
writer.Write(HeaderPreamble.Select(x => (byte)x).ToArray());
writer.Write(sessionState.SenderId);
writer.Write(sessionState.ReceiverId);
writer.Write((ushort)responseLength);
writer.Write(BinaryUtilities.GetXorSum(sendBuffer.Array.Skip(HeaderLength).Take(responseLength)));
writer.Write(BinaryUtilities.GetXorSum(sendBuffer.Array.Take(HeaderLength - 1)));
sendBuffer.SetPosition(responseLength + HeaderLength);
return(Task.FromResult(0));
}
}