Import of the watch repository from Pebble

tools/hdlc.py

@@ -0,0 +1,79 @@
+# Copyright 2024 Google LLC
+#
+# 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
+#
+#     http://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.
+
+try:
+    import Queue
+except ImportError:
+    # Py3 support
+    import queue as Queue
+
+HDLC_FRAME_START = 0x7e
+HDLC_ESCAPE = 0x7d
+HDLC_ESCAPE_MASK = 0x20
+
+
+class HDLCDecoder(object):
+    _STATE_SYNC, _STATE_DATA, _STATE_ESCAPE = range(3)
+
+    def __init__(self):
+        self._frames = Queue.Queue()
+        self._state = self._STATE_SYNC
+        self._buffer = bytearray()
+
+    def write(self, data):
+        for b in bytearray(data):
+            if self._state == self._STATE_SYNC:
+                # waiting for the first FRAME_START byte
+                if b == HDLC_FRAME_START:
+                    self._state = self._STATE_DATA
+            elif self._state == self._STATE_DATA:
+                if b == HDLC_FRAME_START:
+                    # this is the end of the frame (and the start of the next one)
+                    if self._buffer:
+                        self._frames.put_nowait(bytes(self._buffer))
+                    self._buffer = bytearray()
+                elif b == HDLC_ESCAPE:
+                    # escape the next byte
+                    self._state = self._STATE_ESCAPE
+                else:
+                    # this a valid byte of data
+                    self._buffer.append(b)
+            elif self._state == self._STATE_ESCAPE:
+                if b == HDLC_FRAME_START:
+                    # invalid byte combination - drop this frame and start the next one
+                    self._buffer = bytearray()
+                else:
+                    # escape this character
+                    self._buffer.append(b ^ HDLC_ESCAPE_MASK)
+                self._state = self._STATE_DATA
+            else:
+                assert False, 'Invalid state!'
+
+    def get_frame(self):
+        try:
+            return self._frames.get_nowait()
+        except Queue.Empty:
+            return None
+
+
+def hdlc_encode_data(data):
+    frame = bytearray()
+    frame.append(HDLC_FRAME_START)
+    for b in bytearray(data):
+        if b == HDLC_FRAME_START or b == HDLC_ESCAPE:
+            b ^= HDLC_ESCAPE_MASK
+            frame.append(HDLC_ESCAPE)
+        frame.append(b)
+    frame.append(HDLC_FRAME_START)
+    return bytes(frame)