'''This Protocol subclass handles the lower level of the on-wire format.
It decodes the packet length, separates message type and payload, and
verifies checksum.
It does not decode the Protocol Buffer messages.
'''

from twisted.protocols import basic
from twisted.python import log

class PacketFormat(basic.Int16StringReceiver):
    '''Handles a simple packet format:
        (LEN LEN) TYPE PAYLOAD CHECKSUM
    '''
    def getChecksum(self, string):
        '''Compute a checksum by XORing all bytes in
        the string. Returns integer.
        '''
        checksum = 0xFF
        for b in string:
            checksum ^= ord(b)
        return checksum
    
    def stringReceived(self, string):
        if len(string) < 2:
            self.protocolError("packet too short")
            return
        
        if self.getChecksum(string) != 0:
            self.protocolError("checksum wrong")
            return
        
        msgtype = ord(string[0])
        payload = string[1:-1]
        
        log.msg("Received: " + repr(msgtype) + " " + repr(payload))
        self.messageReceived(msgtype, payload)
    
    def messageReceived(self, msgtype, payload):
        '''Override this to handle received messages.'''
        raise NotImplementedError
    
    def protocolError(self, error):
        '''Override this to handle protocol errors.'''
        print error
        log.err(error)
        self.transport.loseConnection()
    
    def sendMessage(self, msgtype, payload):
        if hasattr(payload, "SerializeToString"):
            payload = payload.SerializeToString()
        
        message = chr(msgtype) + payload
        message += chr(self.getChecksum(message))
        log.msg("Sending: " + repr(message))
        self.sendString(message)