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packetwriter.py
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packetwriter.py
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#########################################################################
# This source file is from the openaps/dexcom_reader project.
#
# https://github.com/openaps/dexcom_reader
#
# It is under an MIT licence described in the 3 paragraphs below:
#
#########################################################################
#
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included
# in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
# OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
# ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
# OTHER DEALINGS IN THE SOFTWARE.
#
#########################################################################
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()