1# Copyright 2020 The Pigweed Authors 2# 3# Licensed under the Apache License, Version 2.0 (the "License"); you may not 4# use this file except in compliance with the License. You may obtain a copy of 5# the License at 6# 7# https://www.apache.org/licenses/LICENSE-2.0 8# 9# Unless required by applicable law or agreed to in writing, software 10# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT 11# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 12# License for the specific language governing permissions and limitations under 13# the License. 14"""Module for low-level HDLC protocol features.""" 15 16 17import zlib 18 19# Special flag character for delimiting HDLC frames. 20FLAG = 0x7E 21 22# Special character for escaping other special characters in a frame. 23ESCAPE = 0x7D 24 25# Characters allowed after a 0x7d escape character. 26VALID_ESCAPED_BYTES = 0x5D, 0x5E 27 28# Maximum allowed HDLC address (uint64_t in C++). 29MAX_ADDRESS = 2**64 - 1 30 31 32def escape(byte: int) -> int: 33 """Escapes or unescapes a byte, which should have been preceeded by 0x7d.""" 34 return byte ^ 0x20 35 36 37def frame_check_sequence(data: bytes) -> bytes: 38 return zlib.crc32(data).to_bytes(4, 'little') 39 40 41def encode_address(address: int) -> bytes: 42 """Encodes an HDLC address as a one-terminated LSB varint.""" 43 result = bytearray() 44 45 while True: 46 result += bytes([(address & 0x7F) << 1]) 47 48 address >>= 7 49 if address == 0: 50 break 51 52 result[-1] |= 0x1 53 return result 54 55 56def decode_address(frame: bytes) -> tuple[int, int]: 57 """Decodes an HDLC address from a frame, returning it and its size.""" 58 result = 0 59 length = 0 60 61 while length < len(frame): 62 byte = frame[length] 63 result |= (byte >> 1) << (length * 7) 64 length += 1 65 66 if byte & 0x1 == 0x1: 67 break 68 69 if result > MAX_ADDRESS: 70 return -1, 0 71 72 return result, length 73 74 75class UFrameControl: 76 def __init__(self, frame_type: int): 77 self._data: bytes = bytes([0x03 | frame_type]) 78 79 @property 80 def data(self): 81 return self._data 82 83 @classmethod 84 def unnumbered_information(cls): 85 return UFrameControl(0x00) 86