With luck this will be the final post on the Bellard-LZSS compressor, allowing us to sign off on the last piece of the puzzle required to write a PIC encoder for the Railroad Tycoon Deluxe (RRDX) variant of the MicroProse PIC file format PIC⁹³. Last time we left off having found a few bugs, and some implementation variances that resulted in our output being different than that of the reference file. We finished off by having our primary test file pass a full validation. However upon looking at a few others we were still failing on those. So in this post we pick-up where the last one left off and try to find whatever other bugs and variances remain.

In my last post, we wrapped up writing our Bellard-LZSS compressor, to facilitate eventually writing a PIC⁹³ encoder for Railroad Tycoon Deluxe (RRDX) from MicroProse. In this post we will take the next steps, debugging and validating the code which we have written. I have no doubt it will take some tweaking to get the compressor to generate the original compressed stream that we have as a reference, I just hope we are not too far off right out of the gate. So with that said, lets get into it.

In my last post on the PIC⁹³ version of the MicroProse PIC file format we left off having successfully decompressed the LZSS compressed image data within the file and reconstituting the image. That part was easy, as the code was pretty much already written for us in the form of the unlzexe github repository. It wasn’t easy getting to the point of identifying that Bellard-LZSS was actually the compression used. In the end we got there. In this post we’re going to try and reverse the process. This could prove to be far more challenging as we don’t have a full reference to look back onto.