Bravo! Very neat solution. And, a beautiful demonstration of the fact that the right data structure isn't just important, it's everything.
Also, I like the heavy usage of map and grep. A little harder to read at first (as a programmer who's functional at play, not at work), but very concise.
Question - how does the regex engine not choose a free square in d4 when looking for free squares in d3? I'm guessing it has to do with [\x00-\xFF]*, but I'm not positive as to how that's working ...
We are the carpenters and bricklayers of the Information Age.
The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6
Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.
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