Just out of curiosity, speed was not the objective when I wrote my solver, I wanted to avoid guesswork (as much as possible), but in comparison with Cale Gibbard's and Alson Kemp's solvers (which are much more beautifully coded), it turned out that mine is blazingly fast, so are there faster solvers around (in Haskell, in other languages)?
if I modify your solver to produce similar output to mine (input/first propagation, solved puzzle, number and list of guesses made), your's takes about a third of the time of mine (solving 36628 17hint puzzles in 6m vs 17m, 2GHz Pentium M), and I wasn't exactly unhappy with my solver before I did this comparison!-)
like you, I've been trying to remove guesses, and the speed came as a welcome bonus (I'm still using lists all over the place, with lots of not nice adhoc code still remaining; not all propagators are iterated fully yet because I only recently removed a logic bug that slowed down thesearch instead of speading it up; ..). so the more interesting bit is that our solvers disagree on which are the most difficult puzzles (requiring the largest number of guesses):
df puzzles involving guesses: 5319largest number of guesses: 10 (#36084), 11 (#22495)
cr puzzles involving guesses: 8165largest number of guesses: 10 (#9175), 10 (#17200), 10 (#29823), 10 (#30811)
df's solver needs 0/0/3/0 for cr's trouble spots, while cr's solver needs 5/9 guesses for df's. lots of potential for interesting investigations, though
mostly for me!-) cheers, claus
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