import crc16
import struct

class PacketWriter(object):
  MAX_PAYLOAD = 1584
  MIN_LEN = 6
  MAX_LEN = 1590
  SOF = 0x01
  OFFSET_SOF = 0
  OFFSET_LENGTH = 1
  OFFSET_CMD = 3
  OFFSET_PAYLOAD = 4

  def __init__(self):
    self._packet = None

  def Clear(self):
    self._packet = None

  def NewSOF(self, v):
    self._packet[0] = chr(v)

  def PacketString(self):
    return ''.join(self._packet)
 
  def AppendCrc(self):
    self.SetLength()
    ps = self.PacketString()
    crc = crc16.crc16(ps, 0, len(ps))
    for x in struct.pack('H', crc):
      self._packet.append(x)

  def SetLength(self):
    self._packet[1] = chr(len(self._packet) + 2)

  def _Add(self, x):
    try:
      len(x)
      for y in x:
        self._Add(y)
    except:
      self._packet.append(x)

  def ComposePacket(self, command, payload=None):
    assert self._packet is None
    self._packet = ["\x01", None, "\x00", chr(command)]
    if payload:
      self._Add(payload)
    self.AppendCrc()