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#lang racket
(require advent-of-code
threading)
(define histories
(for/list ([raw-history (in-lines (open-aoc-input (find-session) 2023 9 #:cache #true))])
(~>> raw-history string-split (map string->number))))
(define (constant? xs) (= 1 (length (remove-duplicates xs))))
(define/match (derivative xs)
[((list a b)) (list (- b a))]
[((list* a b _)) (cons (- b a) (derivative (rest xs)))])
(define (extrapolate xs) (if (constant? xs) (car xs) (+ (last xs) (extrapolate (derivative xs)))))
;; part 1
(~>> histories (map extrapolate) (apply +))
;; part 2
(~>> histories (map (λ~> reverse extrapolate)) (apply +))
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