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#lang br/quicklang
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;; http://adventofcode.com/2016/day/13
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(require graph)
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(provide read-syntax
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(rename-out [mb #%module-begin]))
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(define (read-syntax path port)
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(strip-bindings
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#`(module mod "lang.rkt"
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#,(string->number (string-trim (port->string port))))))
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(define-macro (mb NUM)
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#'(#%module-begin
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(solve 50 NUM) ; 50 is arbitrarily large space to search
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(solve2 NUM)))
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(define starting-pt 1+1i)
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(define (solve dim num)
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(define open? (make-open-pred num))
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(define g (undirected-graph (list starting-pt)))
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(for* ([row (in-range dim)]
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[col (in-range dim)]
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[p (in-value (+ col (* +i row)))]
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#:when (open? p))
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(when (open? (+ p 1)) (add-edge! g p (+ p 1)))
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(when (open? (+ p +i)) (add-edge! g p (+ p +i))))
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(define path (fewest-vertices-path g 1+1i 31+39i))
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(displayln (and path (sub1 (length path)))))
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(define (solve2 num)
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(define open? (make-open-pred num))
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(define (nonnegative? pt) (and (not (negative? (real-part pt)))
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(not (negative? (imag-part pt)))
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pt))
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(let take-step ([all-visited-pts empty]
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[last-visited-pts (list starting-pt)]
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[count 0])
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(if (= count 50)
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(length (remove-duplicates (append all-visited-pts last-visited-pts)))
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(take-step
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(append last-visited-pts all-visited-pts)
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(flatten
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(for*/list ([p (in-list last-visited-pts)]
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[next-p (in-list (map (curry + p) '(1 -1 +i -i)))]
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#:when (and (nonnegative? next-p)
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(open? next-p)
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(not (member next-p all-visited-pts))))
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next-p))
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(add1 count)))))
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(define (make-open-pred num)
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(λ(pt)
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(define col (real-part pt))
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(define row (imag-part pt))
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(define sum
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(+ (* col col) (* 3 col) (* 2 col row) row (* row row) num))
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(even? (length (regexp-match* "1" (format "~b" sum))))))
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