There’s something of the “standard literature” in undergraduate algorithms: graph traversals, greedy and dynamic programming, sorts and data structures, and so on. One class of algorithms that seems oddly excluded, at least from my experience, is compression algorithms. So I wanted to explore more of that, in this case by implementing LZW.

I implemented (I think…) a more-or-less standard LZW algorithm here. Continuing my theme of bridging computer science and practice (started (?) with my Bloom filter write-up I wrote it in C, and

Some take-aways:

• I didn’t really bother with literate coding. I think that was ultimately the right thing to do. Uh oh.
• My biggest frustration with example implementations online (that I can find, mainly from Wikipedia: there’s an old Dr. Dobbs article, and then a million Rosetta Code implementations all doing the same thing) is that it doesn’t seem to reflect a practical implementation. A linear scan over the dictionary? I know (better than most, I think…) that linear scans are great, but it’s hard to believe that’s the right thing here.
• The Sedgwick implementation for his undergrad book seems OK.
• I think I can make an interesting short-course on how I iteratively improved performance.
• I really dropped the ball implementing clear-codes, that was a mess and a half. But it’s exactly the complications of “really” implementing clear-codes, variable-length coding, and so on that I am frustrated being skimmed over by academic resources! Absent my efforts (such that they are) it seems the students’ options are to ignore them entirely, or dive into gzip’s source base.

I have a PDF with more thoughts here: https://github.com/agorenst/lzw/blob/master/lzw.pdf ; you can see how I’m trying to get the framing.