aoc2020/25.rkt

#lang curly-fn racket

(require "scripts/aoc.rkt" math/number-theory)

;; solution for day 25

(define *mod* 20201227)
(define *subject* 7)

(define (part1 input)
  (match-define (list you door) input)
  (with-modulus *mod*
    (for/fold ([val 1] [you-loops #f] [door-loops #f] #:result (modexpt you door-loops))
              ([loops (in-range 0 +inf.0)] #:break (and you-loops door-loops))
      (define new-val (mod* val *subject*))
      (cond
        [(= val you) (values new-val loops door-loops)]
        [(= val door) (values new-val you-loops loops)]
        [else (values new-val you-loops door-loops)]))))

;; parse input file
(define (parse fname)
  (define input (file->lines fname))
  (map string->number input))

(module+ main
  (define input (parse "inputs/25"))
  (answer 25 1 (time (part1 input)))
  (aoc-finish)
  (displayln "meow"))