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#lang racket
(require advent-of-code
threading
memo)
(struct condition (template spring-set))
(define conditions
(for/list ([line (in-lines (open-aoc-input (find-session) 2023 12 #:cache #true))])
(match (string-split line #px"[ ,]")
[(list* template spring-set)
(condition (string->list template) (map string->number spring-set))])))
(define/memoize
(do-count template spring-group left-in-group need-gap?)
;; template: list of spring positions
;; spring-group: list of remaining contiguous groups of damaged springs
;; left-in-group: springs remaining in current bad spring group being placed
;; need-gap?: did we just finish placing a bad spring group
;; and need at least one undamaged spring before starting the next one?
(match (list template spring-group left-in-group need-gap?)
;; no springs left to place and no places left to place springs
;; this is an OK arrangement, count it
[(list '() '() 0 _) 1]
;; ambiguous wildcard, try both skipping this spot and starting a damaged spring group here
[(list (list* #\? t-rest) (list* g g-rest) 0 #f)
(+ (do-count t-rest g-rest (sub1 g) (= g 1)) (do-count t-rest spring-group 0 #f))]
;; definitely a place for a good spring (.), move on without consuming any spring groups
[(or (list (list* #\? t-rest) '() 0 #f) ; good spring, no more damaged springs to place
(list (list* #\? t-rest) _ 0 #t) ; good spring right after finishing a group of bad springs
(list (list* #\. t-rest) _ 0 _)) ; known good spring
(do-count t-rest spring-group 0 #f)]
;; start of bad spring (#) group, use the next spring group and remove 1 from it
[(list (list* #\# t-rest) (list* g g-rest) 0 #f) (do-count t-rest g-rest (sub1 g) (= g 1))]
;; continuation of bad spring group, same
[(list (list* (or #\? #\#) t-rest) g left #f) (do-count t-rest g (sub1 left) (= left 1))]
;; if nothing above works, this arrangement's invalid
[_ 0]))
(define (count-solutions c)
(match-define (condition template spring-set) c)
(do-count template spring-set 0 #f))
;; part 1
(for/sum ([c (in-list conditions)]) (count-solutions c))
;; part 2
(define expanded-conditions
(for/list ([c (in-list conditions)])
(condition (~> c condition-template (make-list 5 _) (add-between #\?) flatten)
(~> c condition-spring-set (make-list 5 _) flatten))))
(for/sum ([c* (in-list expanded-conditions)]) (count-solutions c*))
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