Implement bitonal decoder

src/PIL/PpmImagePlugin.py

@@ -142,6 +142,108 @@ class PpmImageFile(ImageFile.ImageFile):
 # --------------------------------------------------------------------
 
 
+class PpmPlainDecoder(ImageFile.PyDecoder):
+    _pulls_fd = True
+
+    def _read_block(self, block_size=10 ** 6):
+        return bytearray(self.fd.read(block_size))
+        # return self.fd.read(block_size)
+
+    def _find_comment_end(self, block, start=0):
+        a = block.find(b"\n", start)
+        b = block.find(b"\r", start)
+        return min(a, b) if a * b > 0 else max(a, b)  # lowest nonnegative index (or -1)
+
+    def _ignore_comments(self, block):
+        """
+        Deletes comments from block. If comment does not end in this
+        block, raises a flag.
+        """
+
+        comment_spans = False
+        while True:
+            comment_start = block.find(b"#")  # look for next comment
+            if comment_start == -1:  # no comment found
+                break
+            comment_end = self._find_comment_end(block, comment_start)
+            if comment_end != -1:  # comment ends in this block
+                block = (
+                    block[:comment_start] + block[comment_end + 1 :]
+                )  # delete comment
+            else:  # last comment continues to next block(s)
+                block = block[:comment_start]
+                comment_spans = True
+                break
+        return block, comment_spans
+
+    def _decode_bitonal(self):
+        """
+        The reason this is a separate method is that in the plain PBM
+        format all data tokens are exactly one byte, and so the
+        inter-token whitespace is optional.
+        """
+        decoded_data = bytearray()
+        total_tokens = self.size
+
+        comment_spans = False
+        tokens_read = 0
+        while True:
+            block = self._read_block()  # read next block
+            if not block:
+                raise ValueError("Reached EOF while reading data")
+
+            while block and comment_spans:
+                comment_end = self._find_comment_end(block)
+                if comment_end != -1:  # comment ends in this block
+                    comment_spans = False
+                    block = block[comment_end + 1 :]  # delete tail of previous comment
+                else:  # comment spans whole block
+                    block = self._read_block()
+
+            block, comment_spans = self._ignore_comments(block)
+
+            tokens = b"".join(block.split())
+
+            for token in tokens:
+                if token in (48, 49):
+                    tokens_read += 1
+                else:
+                    raise ValueError(f"Invalid token for this mode: {bytes([token])}")
+
+                decoded_data.append(token)
+                if tokens_read == total_tokens:  # finished!
+                    invert = bytes.maketrans(b"01", b"\xFF\x00")
+                    decoded_data = decoded_data.translate(invert)
+                    return decoded_data
+
+    def _decode_blocks(self, channels=1, depth=8):
+        raise NotImplementedError
+
+    def decode(self, buffer):
+        self.size = self.state.xsize * self.state.ysize
+        rawmode = self.args[0]
+
+        if self.mode == "1":
+            decoded_data = self._decode_bitonal()
+            rawmode = "1;8"
+        elif self.mode == "L":
+            decoded_data = self._decode_blocks(channels=1, depth=8)
+        elif self.mode == "I":
+            if rawmode == "I;16B":
+                decoded_data = self._decode_blocks(channels=1, depth=16)
+            elif rawmode == "I;32B":
+                decoded_data = self._decode_blocks(channels=1, depth=32)
+        elif self.mode == "RGB":
+            decoded_data = self._decode_blocks(channels=3, depth=8)
+
+        self.set_as_raw(bytes(decoded_data), rawmode)
+        return -1, 0
+
+
+#
+# --------------------------------------------------------------------
+
+
 def _save(im, fp, filename):
     if im.mode == "1":
         rawmode, head = "1;I", b"P4"
@@ -177,7 +279,7 @@ def _save(im, fp, filename):
 #
 # --------------------------------------------------------------------
 
-
+Image.register_decoder("plain", PpmPlainDecoder)
 Image.register_open(PpmImageFile.format, PpmImageFile, _accept)
 Image.register_save(PpmImageFile.format, _save)