blob: f12b79b30b1cde4798fccac0850009a5299821e8 [file] [log] [blame]
# Copyright 2020 The Pigweed Authors
# Licensed under the Apache License, Version 2.0 (the "License"); you may not
# use this file except in compliance with the License. You may obtain a copy of
# the License at
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations under
# the License.
"""Module for low-level HDLC protocol features."""
from typing import Tuple
import zlib
# Special flag character for delimiting HDLC frames.
FLAG = 0x7E
# Special character for escaping other special characters in a frame.
# Characters allowed after a 0x7d escape character.
# Maximum allowed HDLC address (uint64_t in C++).
MAX_ADDRESS = 2**64 - 1
def escape(byte: int) -> int:
"""Escapes or unescapes a byte, which should have been preceeded by 0x7d."""
return byte ^ 0x20
def frame_check_sequence(data: bytes) -> bytes:
return zlib.crc32(data).to_bytes(4, 'little')
def encode_address(address: int) -> bytes:
"""Encodes an HDLC address as a one-terminated LSB varint."""
result = bytearray()
while True:
result += bytes([(address & 0x7f) << 1])
address >>= 7
if address == 0:
result[-1] |= 0x1
return result
def decode_address(frame: bytes) -> Tuple[int, int]:
"""Decodes an HDLC address from a frame, returning it and its size."""
result = 0
length = 0
while length < len(frame):
byte = frame[length]
result |= (byte >> 1) << (length * 7)
length += 1
if byte & 0x1 == 0x1:
if result > MAX_ADDRESS:
return -1, 0
return result, length
class UFrameControl:
def __init__(self, frame_type: int):
self._data: bytes = bytes([0x03 | frame_type])
def data(self):
return self._data
def unnumbered_information(cls):
return UFrameControl(0x00)