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#lang racket
(require "../../jj-aoc.rkt"
threading)
(define path-pairs
(for/list ([l (in-lines (open-day 12 2021))])
(match (string-split l "-")
[(list start end) (cons start end)])))
(define edges-hash (make-hash))
(for ([pair (in-list path-pairs)])
(match-define (cons start end) pair)
(hash-update! edges-hash
start
(curry cons end)
'())
(hash-update! edges-hash
end
(curry cons start)
'()))
;; part 1
(define (backtracking-allowed? next prevs)
(and (equal? (string-downcase next) next)
(member next prevs)))
(define (look-for-next-cave
[path-list '("start")]
#:two-visits? [visit-used-up? #t])
(define current-cave (car path-list))
(cond
[(equal? current-cave "end") (list path-list)]
[else
(~>> (for/list ([next-path (in-list (hash-ref edges-hash current-cave null))]
#:unless (equal? next-path "start")
#:when (not (and (backtracking-allowed? next-path path-list)
visit-used-up?)))
(look-for-next-cave
(cons next-path path-list)
#:two-visits? (or (backtracking-allowed? next-path path-list)
visit-used-up?)))
(apply append))]))
(~> (look-for-next-cave)
length
time)
;; part 2
(~> (look-for-next-cave #:two-visits? #f)
length
time)
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