| # 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 |
| # |
| # https://www.apache.org/licenses/LICENSE-2.0 |
| # |
| # 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.""" |
| |
| import zlib |
| |
| # Special flag character for delimiting HDLC frames. |
| FLAG = 0x7E |
| |
| # Special character for escaping other special characters in a frame. |
| ESCAPE = 0x7D |
| |
| |
| 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') |
