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Racket

#lang racket
(define (map-map f lst) ; map . map
(map (λ (x) (map (λ (y) (f y)) x)) lst))
(define/match (add-dx-dy points)
[((list (list x1 y1) (list x2 y2)))
(let* ([dx (- x2 x1)]
[dy (- y2 y1)]
[len (max (abs dx) (abs dy))])
(list (list x1 y1) (list x2 y2) dx dy len))])
(define (vectorize line)
(let* ([parts (string-split line " -> ")] ; '("0,9 -> 5,9") -> '("0,9" "5,9")
[strings (map (λ (part) (string-split part ",")) parts)] ; '("0,9" "5,9") -> '(("0" "9") ("5" "9"))
[nums (map-map string->number strings)] ; '(("0" "9") ("5" "9")) -> '((0 9) (5 9))
[vectors (add-dx-dy nums)]) ; '((x1 y2) (x2 y2) dx dy length)
vectors))
(define/match (non-diagonal? vect)
[((list _ _ dx dy _)) (or (= dx 0) (= dy 0))])
(define/match (get-points-from-vect vect) ; takes vect '((x1 y1) (x2 y2) dx dy len) and returns a list of points it hits '((x1 y1) (x2 y2) ..)
[((list (list x1 y1) (list x2 y2) dx dy len))
(let loop ([i len])
(if (= i 0) ; at the end
(list (list x1 y1)) ; return this point
(append (list (list (+ x1 (/ (* dx i) len)) (+ y1 (/ (* dy i) len)))) ; return the next point..
(loop (sub1 i)))))]) ; appended to the result of recursing with i - 1
(define (add-point ht point)
(let ([dups (hash-ref ht point 0)])
(hash-set! ht point (+ 1 dups))))
(define (travel-and-add ht vects)
(for ([i (in-list vects)])
(let ([points (get-points-from-vect i)])
(map (λ (p) (add-point ht p)) points))))
(define part1
(let* ([lines (file->lines "day5.txt")]
[vectors (map vectorize lines)]
[non-diagonals (filter non-diagonal? vectors)]
[ht (make-hash)]
[points-hit (travel-and-add ht non-diagonals)]
[counts (hash-values ht)])
(length (filter (λ (x) (> x 1)) counts))))
part1
(define part2
(let* ([lines (file->lines "day5.txt")]
[vectors (map vectorize lines)]
[ht (make-hash)]
[points-hit (travel-and-add ht vectors)]
[counts (hash-values ht)])
(length (filter (λ (x) (> x 1)) counts))))
part2