OASIS file size increases significantly after only reading in and writing out (even for maximum compression level)

[fnJeff]

This is the same testcase as #277. While testing that fix, I was curious if it was related to the compression level, so I wrote out an OASIS using all 10 compression levels from the initial OASIS. What I saw was unexpected. I believe that the tool I write out the original OASIS also uses a default compression level of 6, same as the gdstk default. But for some reason, all OASIS files written out are more than 2x the size of the original, and the compression level 0 OASIS is more like 5x (which may be normal).

I've attached the original testcase, as well as the 10 versions saved from it in case it helps to debug the reason behind the increase. But below you can see the file sizes. The one without the number suffix is the one written out with the default compression level, which you can see matches 6 as expected.

-rw-r----- 1 xxxx xxxx    11133 Nov 20 14:17 test.oasis
-rw-r----- 1 xxxx xxxx    24793 Nov 26 11:36 test.saved.oasis
-rw-r----- 1 xxxx xxxx    54746 Nov 26 12:17 test.saved.oasis.0
-rw-r----- 1 xxxx xxxx    25992 Nov 26 12:17 test.saved.oasis.1
-rw-r----- 1 xxxx xxxx    25866 Nov 26 12:17 test.saved.oasis.2
-rw-r----- 1 xxxx xxxx    25687 Nov 26 12:17 test.saved.oasis.3
-rw-r----- 1 xxxx xxxx    25147 Nov 26 12:17 test.saved.oasis.4
-rw-r----- 1 xxxx xxxx    24862 Nov 26 12:17 test.saved.oasis.5
-rw-r----- 1 xxxx xxxx    24793 Nov 26 12:17 test.saved.oasis.6
-rw-r----- 1 xxxx xxxx    24757 Nov 26 12:17 test.saved.oasis.7
-rw-r----- 1 xxxx xxxx    24760 Nov 26 12:17 test.saved.oasis.8
-rw-r----- 1 xxxx xxxx    24760 Nov 26 12:17 test.saved.oasis.9

test.oasis.tar.gz