aoc2020/22.rkt

#lang curly-fn racket

(require "scripts/aoc.rkt")

;; solution for day 22
(define (score cards)
  (for/sum ([card (in-list cards)] [mult (in-range (length cards) 0 -1)])
    (* card mult)))

(define (play p1 p2)
  (match* (p1 p2)
    [('() _) (score p2)]
    [(_ '()) (score p1)]
    [((cons p1-card p1-rest) (cons p2-card p2-rest))
     (if (> p1-card p2-card)
       (play (append p1-rest (list p1-card p2-card)) p2-rest)
       (play p1-rest (append p2-rest (list p2-card p1-card))))]))

(define (part1 input)
  (match-define (list p1 p2) input)
  (play p1 p2))

(define (part2 input)
  (match-define (list p1 p2) input)

  (define/memoized (play/recur p1 p2)
    (define local-memo (mutable-set))
    (let loop ([p1 p1] [p2 p2])
      (cond
        [(set-member? local-memo (list p1 p2)) (cons 'p1 p1)]
        [(empty? p1) (cons 'p2 p2)]
        [(empty? p2) (cons 'p1 p1)]
        [else
         (set-add! local-memo (list p1 p2))
         (match-define (cons p1-card p1-rest) p1)
         (match-define (cons p2-card p2-rest) p2)
         (cond
          [(and (>= (length p1-rest) p1-card)
                (>= (length p2-rest) p2-card))
           (match-define (cons winner score)
             (play/recur (take p1-rest p1-card) (take p2-rest p2-card)))
           (if (symbol=? winner 'p1)
             (loop (append p1-rest (list p1-card p2-card)) p2-rest)
             (loop p1-rest (append p2-rest (list p2-card p1-card))))]
          [(> p1-card p2-card)
           (loop (append p1-rest (list p1-card p2-card)) p2-rest)]
          [else
           (loop p1-rest (append p2-rest (list p2-card p1-card)))])])))

  (score (cdr (play/recur p1 p2))))

;; parse input file
(define (parse fname)
  (match-define (list p1 p2) (string-split (file->string fname) "\n\n"))
  (list
    (map string->number (rest (string-split p1 "\n")))
    (map string->number (rest (string-split p2 "\n")))))

(time (part2 (parse "/tmp/adventofcode-2020/input/22.txt")))

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

(module+ main
  (define input (parse "inputs/22"))
  (answer 22 1 (time (part1 input)))
  (answer 22 2 (time (part2 input)))
  (displayln "meow"))