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79 lines
2.7 KiB
Python
79 lines
2.7 KiB
Python
# Copyright 2024 Google LLC
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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try:
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import Queue
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except ImportError:
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# Py3 support
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import queue as Queue
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HDLC_FRAME_START = 0x7e
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HDLC_ESCAPE = 0x7d
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HDLC_ESCAPE_MASK = 0x20
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class HDLCDecoder(object):
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_STATE_SYNC, _STATE_DATA, _STATE_ESCAPE = range(3)
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def __init__(self):
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self._frames = Queue.Queue()
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self._state = self._STATE_SYNC
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self._buffer = bytearray()
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def write(self, data):
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for b in bytearray(data):
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if self._state == self._STATE_SYNC:
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# waiting for the first FRAME_START byte
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if b == HDLC_FRAME_START:
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self._state = self._STATE_DATA
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elif self._state == self._STATE_DATA:
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if b == HDLC_FRAME_START:
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# this is the end of the frame (and the start of the next one)
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if self._buffer:
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self._frames.put_nowait(bytes(self._buffer))
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self._buffer = bytearray()
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elif b == HDLC_ESCAPE:
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# escape the next byte
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self._state = self._STATE_ESCAPE
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else:
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# this a valid byte of data
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self._buffer.append(b)
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elif self._state == self._STATE_ESCAPE:
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if b == HDLC_FRAME_START:
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# invalid byte combination - drop this frame and start the next one
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self._buffer = bytearray()
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else:
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# escape this character
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self._buffer.append(b ^ HDLC_ESCAPE_MASK)
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self._state = self._STATE_DATA
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else:
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assert False, 'Invalid state!'
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def get_frame(self):
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try:
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return self._frames.get_nowait()
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except Queue.Empty:
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return None
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def hdlc_encode_data(data):
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frame = bytearray()
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frame.append(HDLC_FRAME_START)
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for b in bytearray(data):
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if b == HDLC_FRAME_START or b == HDLC_ESCAPE:
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b ^= HDLC_ESCAPE_MASK
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frame.append(HDLC_ESCAPE)
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frame.append(b)
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frame.append(HDLC_FRAME_START)
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return bytes(frame)
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