from struct import *
import struct

from conpot.protocols.s7comm.exceptions import ParseException


class TPKT:
    # References: rfc2126 section-4.3, rfc1006# section-6
    # Packet format:
    # +--------+--------+----------------+-----------....---------------+
    # |version |reserved| packet length  |             TPDU             |
    # +----------------------------------------------....---------------+
    # <8 bits> <8 bits> <   16 bits    > <       variable length       >

    def __init__(self, version=3, payload=''):
        self.payload = payload
        self.version = version
        self.reserved = 0
        self.packet_length = len(payload) + 4

    def pack(self):

        return pack('!BBH', self.version, self.reserved, self.packet_length) + str(self.payload)

    def parse(self, packet):

        try:
            # try to extract the header by pattern to find malformed header data
            header = unpack('!BBH', packet[:4])
        except struct.error:
            raise ParseException('s7comm', 'malformed packet header structure')

        # extract header data and payload
        self.version = header[0]
        self.reserved = header[1]
        self.packet_length = header[2]
        self.payload = packet[4:4 + header[2]]

        return self
