aoc2020/21.rkt

#lang curly-fn racket

(require "scripts/aoc.rkt")

;; solution for day 21

(struct line [is as] #:transparent)
(struct data [lines all-is all-as tainted] #:transparent)

(define (part1 input)
  (match-define (data lines all-is all-as tainted) input)
  (define untainted (set-subtract all-is (apply set-union (hash-values tainted))))
  (for*/sum ([l (in-list lines)] [i (in-set (line-is l))] #:when (set-member? untainted i))
    1))

(define (part2 input)
  (match-define (data lines all-is all-as tainted) input)
  (define edges
    (for*/list ([(k v*) (in-hash tainted)] [v (in-set v*)])
      (list k v)))
  (define G (undirected-graph edges))
  (define matching (maximum-bipartite-matching G))

  (string-join
    (for/list ([val (in-list (sort matching #{string<? (first %1) (first %2)}))])
      (second val))
    ","))

;; parse input file
(define (parse fname)
  (define-values [lines all-is all-as]
    (for/fold ([lines '()] [all-is (set)] [all-as (set)]) ([l (in-list (file->lines fname))])
      (match-define (list _ p1 p2) (regexp-match #px"^([^\\(]+) \\(contains ([^\\)]+)\\)+" l))
      (define is (list->set (string-split p1 " ")))
      (define as (list->set (string-split p2 ", ")))
      (values (cons (line is as) lines) (set-union all-is is) (set-union all-as as))))

  (define tainted
    (for/hash ([a (in-set all-as)])
      (values a (for/fold ([necessary all-is]) ([l (in-list lines)]
                                                #:when (set-member? (line-as l) a))
                  (set-intersect (line-is l) necessary)))))
  (data lines all-is all-as tainted))

(module+ main
  (define input (parse "inputs/21"))
  (answer 21 1 (time (part1 input)))
  (answer 21 2 (time (part2 input)))
  (displayln "meow"))