master
Matthew Butterick 4 years ago
parent 04f5d9eb48
commit 8318605d86

@ -0,0 +1,61 @@
#lang br
(require racket/file rackunit racket/set)
(define ints (map string->number (regexp-match* #rx"." "389547612")))
(struct cup (val prev next) #:mutable)
(define (link-cups! left right)
(set-cup-next! left right)
(set-cup-prev! right left))
(define (find-cup cups dest-val)
(for/first ([c (in-list cups)]
#:when (eq? (cup-val c) dest-val))
c))
(define (next-cups start count)
(cdr (reverse (foldl (λ (val acc) (cons (cup-next (car acc)) acc))
(list start)
(range count)))))
(define (solve ints [max-int 9] [turns 100] [string-result? #true])
(define cups (let* ([ints (if (= max-int (apply max ints))
ints
(append ints (for/list ([i (in-range 1 (add1 max-int))]
#:unless (memq i ints))
i)))]
[cups (for/list ([int ints])
(cup int #f #f))])
(for-each link-cups! cups (append (cdr cups) (list (car cups))))
cups))
(define min-cup-val (apply min ints))
(define max-cup-val max-int)
(for/fold ([current (car cups)]
#:result (void))
([turn (in-range turns)])
(match-define (list cup1 cup2 cup3 cup4) (next-cups current 4))
(link-cups! current cup4)
(define dest-cup (let ([pickup-vals (map cup-val (list cup1 cup2 cup3))])
(let loop ([dest (sub1 (cup-val current))])
(cond
[(< dest min-cup-val) (loop max-cup-val)]
[(memq dest pickup-vals) (loop (sub1 dest))]
[else (find-cup cups dest)]))))
(define dest-cup-next (cup-next dest-cup))
(link-cups! dest-cup cup1)
(link-cups! cup3 dest-cup-next)
(cup-next current))
(if string-result?
(string-append*
(for/list ([cup (next-cups (find-cup cups 1) (sub1 (length cups)))])
(~a (cup-val cup))))
(apply * (map cup-val (next-cups (find-cup cups 1) 2)))))
(check-equal? (solve ints) "45286397")
;; brutality won't be fast enough, but it was worth trying
#;(solve ints 1000000 10000000 #f)