aoc2020/15.rkt

#lang curly-fn racket

(require "scripts/aoc.rkt")

;; solution for day 15

(define (naive-solution input limit)
  (define seen-numbers (make-hash))
  (define prev
    (for/last ([num (in-list input)] [i (in-naturals)])
      (begin0 num (hash-set! seen-numbers num i))))
  ;; delay "seeing" numbers by one iteration
  (hash-remove! seen-numbers prev)

  (for/fold ([prev prev]) ([i (in-range (length input) limit)])
    (begin0
      (match (hash-ref seen-numbers prev #f)
        [#f 0]
        [idx (- i 1 idx)])
      ;; _now_ we've "seen" it, but it happened on the last turn
      (hash-set! seen-numbers prev (sub1 i)))))

(define part1 #{naive-solution % 2020})
(define part2 #{naive-solution % 30000000})

;; parse input file
(define (parse fname)
  (define input (string-trim (file->string fname)))
  (map string->number (string-split input ",")))

(module+ test
  (require rackunit)
  ;; tests here
  (displayln "no tests :("))

(module+ main
  (define input (parse "inputs/15"))
  (answer 15 1 (time (part1 input)))
  (answer 15 2 (time (part2 input)))
  (displayln "meow"))