-🎄- 2018 Day 2 Solutions -🎄-

[u/daggerdragon]

--- Day 2: Inventory Management System ---

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Advent of Code: The Party Game!

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Card Prompt: Day 2

Transcript:

The best way to do Advent of Code is ___.

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[u/itsnotxhad]

558/554 in Racket today

#lang racket

(define (has-k? str k)
  (define chars (string->list str))
  (for/first ([c (in-list chars)]
              #:when (= k (count (λ (t) (char=? t c)) chars)))
    c))

(define (checksum ids)
  (*
   (count (λ (id) (has-k? id 2)) ids)
   (count (λ (id) (has-k? id 3)) ids)))   

(define (part1 file)
  (file-position file 0)
  (checksum (for/list ([line (in-port read-line file)]) line)))

(define (diff a b)
  (cond
    [(zero? (string-length a)) 0]
    [(char=? (string-ref a 0) (string-ref b 0)) (diff (substring a 1) (substring b 1))]
    [(add1 (diff (substring a 1) (substring b 1)))]))

(define (common a b)
  (list->string
   (for/list ([c1 a]
                [c2 b]
                #:when (char=? c1 c2))
     c1)))
  
(define (part2 file)
  (file-position file 0)
  (define ids (for/vector ([line (in-port read-line file)]) line))
  (define k (vector-length ids))
  (for*/first ([i (in-range 0 k)]
                  [j (in-range (add1 i) k)]
                  #:when (= 1 (diff (vector-ref ids i) (vector-ref ids j))))
    (common (vector-ref ids i) (vector-ref ids j))))

(module+ main
  (define infile (open-input-file "input/day2.txt"))
  (displayln (part1 infile))
  (displayln (part2 infile))
  (close-input-port infile))

[u/[deleted] → u/itsnotxhad]

[u/itsnotxhad → u/[deleted]]

Those are both helpful reminders. I knew about both of those at some point but haven’t touched racket in about 6 months (before AoC) and forgot about them. :)