Decompress stream to account for different possible encodings (closes: #51)

Recompressing the image stream makes the comparison robust against output
preserving changes in the zlib compress output bitstream
(e.g. between different zlib implementations/versions/releases).
Without this, some img2pdf 0.3.2 tests fail on Fedora 29/aarch64.
See also:
josch/img2pdf#51
https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/R7GD4L5Z6HELCDAL2RDESWR2F3ZXHWVX/

src/tests/__init__.py

@@ -17,6 +17,51 @@ if PY3:
 else:
     PdfReaderIO = BytesIO
 
+# Recompressing the image stream makes the comparison robust against output
+# preserving changes in the zlib compress output bitstream
+# (e.g. between different zlib implementations/versions/releases).
+# Without this, some img2pdf 0.3.2 tests fail on Fedora 29/aarch64.
+# See also:
+# https://gitlab.mister-muffin.de/josch/img2pdf/issues/51
+# https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/R7GD4L5Z6HELCDAL2RDESWR2F3ZXHWVX/
+def recompress_last_stream(bs):
+    length_pos = bs.rindex(b'/Length')
+    li = length_pos + 8
+    lj = bs.index(b' ', li)
+    n = int(bs[li:lj])
+    stream_pos = bs.index(b'\nstream\n', lj)
+    si = stream_pos + 8
+    sj = si + n
+    startx_pos = bs.rindex(b'\nstartxref\n')
+    xi = startx_pos + 11
+    xj = bs.index(b'\n', xi)
+    m = int(bs[xi:xj])
+
+    unc_t = zlib.decompress(bs[si:sj])
+    t = zlib.compress(unc_t)
+
+    new_len = str(len(t)).encode('ascii')
+    u = (lj-li) + n
+    v = len(new_len) + len(t)
+    off = v - u
+
+    rs = (bs[:li] + new_len + bs[lj:si] + t + bs[sj:xi]
+            + str(m+off).encode('ascii') + bs[xj:])
+
+    return rs
+
+def compare_pdf(outx, outy):
+    if b'/FlateDecode' in outx:
+        x = recompress_last_stream(outx)
+        y = recompress_last_stream(outy)
+        if x != y:
+            print('original outx:\n{}\nouty:\n{}\n'.format(outx, outy), file=sys.stderr)
+            print('recompressed outx:\n{}\nouty:\n{}\n'.format(x, y), file=sys.stderr)
+            return False
+    else:
+        if outx != outy:
+            print('original outx:\n{}\nouty:\n{}\n'.format(outx, outy), file=sys.stderr)
+    return True
 
 # convert +set date:create +set date:modify -define png:exclude-chunk=time
 
@@ -669,7 +714,7 @@ def test_suite():
             ywriter.trailer = y
             xwriter.write(outx)
             ywriter.write(outy)
-            self.assertEqual(outx.getvalue(), outy.getvalue())
+            self.assertEqual(compare_pdf(outx.getvalue(), outy.getvalue()), True)
             # the python-pil version 2.3.0-1ubuntu3 in Ubuntu does not have the
             # close() method
             try: