Matthew Butterick 4 years ago
parent 5a2032e734
commit feee839834

@ -1 +1 @@
10 players; last marble is worth 1618 points
9 players; last marble is worth 25 points

@ -1,38 +1,59 @@
#lang debug br
(require racket/file)
(define (nth-mpr mprs n)
(for/fold ([mprs mprs])
([i (in-range n)])
(mcdr mprs)))
(define ()
(match-define (list player-count max-marbles)
(map string->number (regexp-match* #px"\\d+" (file->string "09.txt"))))
This puzzle seemed annoying at first but turned out to be educational.
An epic difference between the naive solution (= make a list and iterate from the start)
and using a double-linked list, which minimizes traversal.
(struct dll (val prev next) #:mutable)
(define (move-by dll n)
(define iterator
(match n
[(? positive?) dll-next]
[(? negative?) dll-prev]
[_ values]))
(for/fold ([dll dll])
([i (in-range (abs n))])
(iterator dll)))
(define (remove-marble! marble)
(set-dll-next! (dll-prev marble) (dll-next marble))
(set-dll-prev! (dll-next marble) (dll-prev marble)))
(define (find-winner player-count max-marbles)
(define scores (make-hasheqv))
(define circle (mcons #f (mcons 0 null)))
(let loop ([marble 1] [marbles-in-circle 1] [pos 0])
(define first-marble (dll 0 #f #f))
(set-dll-prev! first-marble first-marble)
(set-dll-next! first-marble first-marble)
(for/fold ([current-marble first-marble]
#:result (cdr (argmax cdr (hash->list scores))))
([marble (in-range 1 max-marbles)])
[(> marble max-marbles) (cdr (argmax cdr (hash->list scores)))]
[(zero? (modulo marble 23))
(define deletion-pos (modulo (+ (- pos 7) marbles-in-circle) marbles-in-circle))
(define last-left-mpr (nth-mpr circle deletion-pos))
(define removed-marble (mcar (mcdr last-left-mpr)))
(set-mcdr! last-left-mpr (mcdr (mcdr last-left-mpr)))
[(zero? (modulo marble 23))
(define marble-to-remove (move-by current-marble -7))
(remove-marble! marble-to-remove)
(define player (modulo marble player-count))
(hash-update! scores player (λ (sc) (+ removed-marble marble sc)) 0)
(loop (add1 marble) (sub1 marbles-in-circle) deletion-pos)]
(hash-update! scores player (λ (sc) (+ (dll-val marble-to-remove) marble sc)) 0)
(dll-next marble-to-remove)]
(define next-pos (add1 (modulo (add1 pos) marbles-in-circle)))
(define last-left-mpr (nth-mpr circle next-pos))
(set-mcdr! last-left-mpr (mcons marble (mcdr last-left-mpr)))
(loop (add1 marble) (add1 marbles-in-circle) next-pos)])))
(define left-marble (move-by current-marble 1))
(define right-marble (dll-next left-marble))
(define new-marble (dll marble left-marble right-marble))
(set-dll-next! left-marble new-marble)
(set-dll-prev! right-marble new-marble)
(match-define (list player-count max-marbles)
(map string->number (regexp-match* #px"\\d+" (file->string "09.txt"))))
(define () (find-winner player-count max-marbles))
#;(define (★★)
(define (★★) (find-winner player-count (* 100 max-marbles)))
(module+ test
(require rackunit)
(check-equal? (time ()) 437654)
#;(check-equal? (time (★★)) 566))
(require rackunit)
(check-equal? (time ()) 437654)
(check-equal? (time (★★)) 3689913905))