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authorHunky Jimpjorps <thechairman@thechairman.info>2024-02-02 17:05:12 -0500
committerHunky Jimpjorps <thechairman@thechairman.info>2024-02-02 17:05:12 -0500
commit48e35ad3b0b0c62f936784e4aca70b17c3b0e3f9 (patch)
treef59a13e0b5e80ab925220b4488c6e36b1bec660a /aoc2023-racket/day-09/day-09.rkt
parent87e9ab25ff70e215b537939a4bc23ab101f41dbe (diff)
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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 +))