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#lang rackjure
(require advent-of-code
threading
(only-in relation ->symbol ->number))
(struct instruction (direction distance) #:transparent)
(define (parse-instruction str)
(match str
[(regexp #px"(\\w)(\\d+)" (list _ dir dist)) (instruction (->symbol dir) (->number dist))]))
(define instructions
(~>> (fetch-aoc-input (find-session) 2020 12) string-split (map parse-instruction)))
;; part 1
(struct boat (x y nav) #:transparent)
(define (angle->direction n)
(case n
[(0) 'E]
[(90) 'N]
[(180) 'W]
[(270) 'S]))
(define (move-via-direct-command inst b)
(match-define (boat x y facing) b)
(match inst
[(instruction 'N n) (boat x (+ y n) facing)]
[(instruction 'S n) (boat x (- y n) facing)]
[(instruction 'E n) (boat (+ x n) y facing)]
[(instruction 'W n) (boat (- x n) y facing)]
[(instruction 'L n) (boat x y (modulo (+ facing n) 360))]
[(instruction 'R n) (boat x y (modulo (- facing n) 360))]
[(instruction 'F n) (move-via-direct-command (instruction (angle->direction facing) n) b)]))
(define (find-boat-destination using start instructions)
(match-define (boat x y _) (foldl using start instructions))
(+ (abs x) (abs y)))
(find-boat-destination move-via-direct-command (boat 0 0 0) instructions)
;; part 2
(define (move-via-waypoint inst b)
(match-define (boat x y (cons wp-x wp-y)) b)
(match inst
[(instruction 'N n) (boat x y (cons wp-x (+ wp-y n)))]
[(instruction 'S n) (boat x y (cons wp-x (- wp-y n)))]
[(instruction 'E n) (boat x y (cons (+ wp-x n) wp-y))]
[(instruction 'W n) (boat x y (cons (- wp-x n) wp-y))]
[(instruction _ 180) (boat x y (cons (- wp-x) (- wp-y)))]
[(or (instruction 'L 90) (instruction 'R 270)) (boat x y (cons (- wp-y) wp-x))]
[(or (instruction 'R 90) (instruction 'L 270)) (boat x y (cons wp-y (- wp-x)))]
[(instruction 'F n) (boat (+ x (* n wp-x)) (+ y (* n wp-y)) (cons wp-x wp-y))]))
(find-boat-destination move-via-waypoint (boat 0 0 '(10 . 1)) instructions)
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