For my chess tactics trainer at https://www.checkmatechamp.net/ https://www.checkmatechamp.net/ , I tried a lot of different techniques to compress a small set

For my chess tactics trainer at https://www.checkmatechamp.net/ https://www.checkmatechamp.net/ , I tried a lot of different techniques to compress a small set of positions (about 800). I was not concerned with storing the positions in a fixed size, so I was mainly trying things that would make the entropy of each compressed position smaller. I eventually fit these into a file that is about 14 KB in size when gzipped. Each position takes about 17.5 bytes, excluding the HTTP headers. If you go to the page and scroll through the positions, you'll see that each position loads instantly. That's because all the positions are in that 14 KB file which is loaded initially. I didn't include the castling or en passant info in that file, but I suspect it wouldn't add more than one byte per position on average. I only considered techniques that would not cause the decoder to become overly complicated. One "non-standard" technique that I used was inverting the color of all the pieces on one side of the board. Specifically, on the lower half of the board, I change all the white pieces to black and vice versa. This greatly lowers the average entropy of a typical position, since for most of the game, each player keeps most of his pieces on his side of the board. It is also easy to handle in the decoder in one line (if row < 4, then piece = -piece). I have meaning to write a blog post about it, but I haven't gotten around to it yet. If you are interested in hearing more, let me know, and I will move it up my to-do list.