Meta/jbig2_to_pdf.py: Allow jbig2 files with random-access organization

jbig2 data in PDFs is in the embedded organization, which is like the
sequential organization with the file header removed.

That means jbig2 files using the random-access organization need to
be transformed to be supported. A random-access jbig2 has all segment
headers at the start, followed by the data of all segments. Decode
all headers and rewrite them to the sequential organization, where
each segment header is followed by that segment's data.

The motivation is that almost all of the jbig2 files in
ghostpdl/test/jbig2 use the random-access organization.

Meta/jbig2_to_pdf.py

@@ -9,15 +9,80 @@ Usage is a bit clunky (use Build/lagom/bin/file to get the dimensions):
 % open foo.pdf
 """
 
+from dataclasses import dataclass
 import argparse
-import sys
+import struct
 import textwrap
 
 
+EndOfFile = 51
+
+
 def dedent(b):
     return textwrap.dedent(b.decode('latin1')).encode('latin1')
 
 
+@dataclass
+class SegmentHeader:
+    segment_header_size: int
+    type: int
+    bytes: bytes
+    data_size: int
+
+
+def read_segment_header(data, offset):
+    segment_number, = struct.unpack_from('>I', data, offset)
+    flags = data[offset + 4]
+    segment_page_association_size_is_32_bits = (flags & 0b100_0000) != 0
+    type = (flags & 0b11_1111)
+
+    referred_segments_count = data[offset + 5] >> 5
+    if referred_segments_count > 4:
+        raise Exception('cannot handle more than 4 referred-to segments')
+
+    if segment_number <= 256:
+        ref_size = 1
+    elif segment_number <= 65536:
+        ref_size = 2
+    else:
+        ref_size = 4
+    segment_header_size = 4 + 1 + 1 + ref_size * referred_segments_count
+
+    if segment_page_association_size_is_32_bits:
+        segment_header_size += 4
+    else:
+        segment_header_size += 1
+
+    data_size, = struct.unpack_from('>I', data, offset + segment_header_size)
+    if data_size == 0xffff_ffff:
+        raise Exception('cannot handle indeterminate length')
+    segment_header_size += 4
+
+    bytes = data[offset:offset + segment_header_size]
+    return SegmentHeader(segment_header_size, type, bytes, data_size)
+
+
+def random_access_to_sequential(data):
+    offset = 0
+
+    segment_headers = []
+    while True:
+        segment_header = read_segment_header(data, offset)
+        segment_headers.append(segment_header)
+
+        offset += segment_header.segment_header_size
+        if segment_header.type == EndOfFile:
+            break
+
+    out_data = bytes()
+    for segment_header in segment_headers:
+        out_data += segment_header.bytes
+        out_data += data[offset:offset + segment_header.data_size]
+        offset += segment_header.data_size
+
+    return out_data
+
+
 def main():
     parser = argparse.ArgumentParser(
         epilog=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
@@ -36,13 +101,14 @@ def main():
 
     # strip jbig2 header
     image_data = image_data[8:]
-    if image_data[0] & 1 == 0:
-        print('random-access jbig2 does not work', file=sys.stderr)
-        sys.exit(1)
+    is_random_access = image_data[0] & 1 == 0
     if image_data[0] & 2 == 0:
         image_data = image_data[4:]
     image_data = image_data[1:]
 
+    if is_random_access:
+        image_data = random_access_to_sequential(image_data)
+
     start = dedent(b'''\
               %PDF-1.4
               %\265\266