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framing.py
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framing.py
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import binascii
import functools
import itertools
import logging
import struct
from . import common
log = logging.getLogger(__name__)
def _checksum_func(x):
''' The result will be unsigned on Python 2/3. '''
return binascii.crc32(bytes(x)) & 0xFFFFFFFF
class Checksum:
fmt = '>L' # unsigned longs (32-bit)
size = struct.calcsize(fmt)
def encode(self, payload):
checksum = _checksum_func(payload)
return struct.pack(self.fmt, checksum) + payload
def decode(self, data):
received, = struct.unpack(self.fmt, bytes(data[:self.size]))
payload = data[self.size:]
expected = _checksum_func(payload)
if received != expected:
log.warning('Invalid checksum: %08x != %08x', received, expected)
raise ValueError('Invalid checksum')
else:
log.debug('Good checksum: %08x', received)
return payload
class Framer:
block_size = 250
prefix_fmt = '>B'
prefix_len = struct.calcsize(prefix_fmt)
checksum = Checksum()
EOF = b''
def _pack(self, block):
frame = self.checksum.encode(block)
return bytearray(struct.pack(self.prefix_fmt, len(frame)) + frame)
def encode(self, data):
for block in common.iterate(data=data, size=self.block_size,
func=bytearray, truncate=False):
yield self._pack(block=block)
yield self._pack(block=self.EOF)
def decode(self, data):
data = iter(data)
while True:
length, = _take_fmt(data, self.prefix_fmt)
frame = _take_len(data, length)
block = self.checksum.decode(frame)
if block == self.EOF:
log.debug('EOF frame detected')
return
yield block
def _take_fmt(data, fmt):
length = struct.calcsize(fmt)
chunk = bytearray(itertools.islice(data, length))
if len(chunk) < length:
raise ValueError('missing prefix data')
return struct.unpack(fmt, bytes(chunk))
def _take_len(data, length):
chunk = bytearray(itertools.islice(data, length))
if len(chunk) < length:
raise ValueError('missing payload data')
return chunk
def chain_wrapper(func):
@functools.wraps(func)
def wrapped(*args, **kwargs):
result = func(*args, **kwargs)
return itertools.chain.from_iterable(result)
return wrapped
class BitPacker:
byte_size = 8
def __init__(self):
bits_list = []
for index in range(2 ** self.byte_size):
bits = [index & (2 ** k) for k in range(self.byte_size)]
bits_list.append(tuple((1 if b else 0) for b in bits))
self.to_bits = dict((i, bits) for i, bits in enumerate(bits_list))
self.to_byte = dict((bits, i) for i, bits in enumerate(bits_list))
@chain_wrapper
def encode(data, framer=None):
converter = BitPacker()
framer = framer or Framer()
for frame in framer.encode(data):
for byte in frame:
yield converter.to_bits[byte]
@chain_wrapper
def _to_bytes(bits):
converter = BitPacker()
for chunk in common.iterate(data=bits, size=8,
func=tuple, truncate=True):
yield [converter.to_byte[chunk]]
def decode_frames(bits, framer=None):
framer = framer or Framer()
for frame in framer.decode(_to_bytes(bits)):
yield bytes(frame)