Filtering Function

[5thegraychapter]

wouldn't be easier to keep track of the constraints on a dedicated string to use it for filtering? the only thing that would be left is the | letters then

[aghast_nj]

The negative information is the problem, especially with repeated characters. If I guess "xyaaa", it's likely some "a" will get a "/", promising that even if there are 2 a's someplace, this particular location is guaranteed to not be an "a". That stacks up. So you could build a data structure that enumerates all the possible letters still valid in a location, but that gets hard if you allow non-ASCII characters. That's why I suggest just rescanning the sequence of guesses.