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#lang br/quicklang ;; http://adventofcode.com/2016/day/19
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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-trim (port->string port)))))
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(define-macro (mb NUM-STR)
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#'(#%module-begin
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#;(displayln (solve (string->number NUM-STR) #f))
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(displayln (solve (string->number NUM-STR) #t))))
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(define (idx-after vec x)
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(or
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(for/first ([idx (in-range (modulo (add1 x) (vector-length vec)) (vector-length vec))]
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#:when (vector-ref vec idx))
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idx)
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(idx-after vec -1)))
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(define (solve num [circle? #f])
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(define elves (make-vector num #t))
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(let loop ([taker 0][elves-left num])
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(cond
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[(= elves-left 2) (add1 taker)]
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[else
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(define giver (for/fold ([elf taker])
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([i (in-range (if circle? (floor (/ elves-left 2)) 1))])
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(idx-after elves elf)))
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(vector-set! elves giver #f)
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(loop (idx-after elves taker) (sub1 elves-left))])))
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