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#lang racket
(require advent-of-code
threading
algorithms
memoize)
;; part 1
(define adapters
(~> (fetch-aoc-input (find-session) 2020 10)
(string-split "\n")
(map string->number _)
(sort <)
((λ (xs) (flatten (list 0 xs (+ 3 (last xs))))))))
(~>> adapters
(sliding _ 2)
(map (match-lambda
[(list b a) (- a b)]))
(group-by identity)
(map length)
(apply *))
;; part 2
(define subpaths
(for*/hash ([adapter (in-list adapters)])
(define predecessor-candidates (inclusive-range (+ 1 adapter) (+ 3 adapter)))
(values adapter (filter (λ (p) (member p adapters)) predecessor-candidates))))
(define/memo (find-paths from to)
(define paths (hash-ref subpaths from 'failed))
(match paths
['failed 0]
[(list-no-order (== to) _ ...) 1]
[ts (for/sum ([t (in-list ts)]) (find-paths t to))]))
(find-paths (first adapters) (last adapters))
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