It's not lossy if you can always recover the correct data. Rather than use a general algorithm, your compressor would have to reason: I'd have to spend a bit t

It's not lossy if you can always recover the correct data. Rather than use a general algorithm, your compressor would have to reason: I'd have to spend a bit to disambiguate x & y, but only x is a valid board state, so I won't bother and the decompressor also knows I won't bother so it will all work out. This sort of implicit coding can work, but it is fragile.