img: add _ChunkParser._iter_chunk_offsets()

Author: Steve Canny

docx/image/png.py

@@ -253,7 +253,16 @@ def _iter_chunk_offsets(self):
         in the PNG image stream. Iteration stops after the IEND chunk is
         returned.
         """
-        raise NotImplementedError
+        chunk_offset = 8
+        while True:
+            chunk_data_len = self._stream_rdr.read_long(chunk_offset)
+            chunk_type = self._stream_rdr.read_str(4, chunk_offset, 4)
+            data_offset = chunk_offset + 8
+            yield chunk_type, data_offset
+            if chunk_type == 'IEND':
+                break
+            # incr offset for chunk len long, chunk type, chunk data, and CRC
+            chunk_offset += (4 + 4 + chunk_data_len + 4)
 
 
 def _ChunkFactory(chunk_type, stream_rdr, offset):

tests/image/test_png.py

@@ -355,6 +355,12 @@ def it_can_iterate_over_the_chunks_in_its_png_stream(self, iter_fixture):
         ]
         assert chunks == chunk_lst
 
+    def it_iterates_over_the_chunk_offsets_to_help_parse(
+            self, iter_offsets_fixture):
+        chunk_parser, expected_chunk_offsets = iter_offsets_fixture
+        chunk_offsets = [co for co in chunk_parser._iter_chunk_offsets()]
+        assert chunk_offsets == expected_chunk_offsets
+
     # fixtures -------------------------------------------------------
 
     @pytest.fixture
@@ -408,6 +414,17 @@ def iter_fixture(
             offsets, chunk_lst
         )
 
+    @pytest.fixture
+    def iter_offsets_fixture(self):
+        bytes_ = b'-filler-\x00\x00\x00\x00IHDRxxxx\x00\x00\x00\x00IEND'
+        stream_rdr = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        chunk_parser = _ChunkParser(stream_rdr)
+        expected_chunk_offsets = [
+            (PNG_CHUNK_TYPE.IHDR, 16),
+            (PNG_CHUNK_TYPE.IEND, 28),
+        ]
+        return chunk_parser, expected_chunk_offsets
+
     @pytest.fixture
     def StreamReader_(self, request, stream_rdr_):
         return class_mock(