master
Matthew Butterick 2 years ago
parent 7e4c615c74
commit 2e0676ac47

@ -0,0 +1,18 @@
#lang br
(require racket/file rackunit)
(define (solve str max-turns)
(define nums (map string->number (string-split str ",")))
(define previously-seen (make-hasheq))
(for ([(num turn) (in-indexed (drop-right nums 1))])
(hash-set! previously-seen num (add1 turn)))
(for/fold ([last-seen (last nums)])
([turn (in-range (add1 (length nums)) (add1 max-turns))])
(begin0
(match (hash-ref previously-seen last-seen #false)
[#false 0]
[where-previously-seen (- (sub1 turn) where-previously-seen)])
(hash-set! previously-seen last-seen (sub1 turn)))))
(check-equal? (solve "11,0,1,10,5,19" 2020) 870)
(check-equal? (solve "11,0,1,10,5,19" 30000000) 9136)