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#lang racket
(require "lib/common.rkt"
(struct segment (p1 p2) #:transparent)
(struct posn (x y) #:transparent)
(define (parse lines)
(for/vector ([line (in-list lines)])
(match line
[(pregexp #px"([0-9]*),([0-9]*) -> ([0-9]*),([0-9]*)"
(list _
(app string->number x1) (app string->number y1)
(app string->number x2) (app string->number y2)))
(segment (posn x1 y1) (posn x2 y2))])))
(define (is-cardinal? seg)
(match seg
[(segment (posn x _) (posn x _)) #t]
[(segment (posn _ y) (posn _ y)) #t]
[_ #f]))
(define (segment->point-set seg)
(match seg
[(segment (posn x y1) (posn x y2))
(define y-init (min y1 y2))
(for/vector ([i (in-inclusive-range 0 (abs (- y1 y2)))])
(posn x (+ i y-init)))]
[(segment (posn x1 y) (posn x2 y))
(define x-init (min x1 x2))
(for/vector ([i (in-inclusive-range 0 (abs (- x1 x2)))])
(posn (+ i x-init) y))]
[(segment (posn x1 y1) (posn x2 y2))
(define (direction a b)
(if (<= a b) 1 -1))
(define x-direction (direction x1 x2))
(define y-direction (direction y1 y2))
(for/vector ([x (in-range x1 (+ x2 x-direction) x-direction)]
[y (in-range y1 (+ y2 y-direction) y-direction)])
(posn x y))]))
(define (day5a segments)
(day5b (vector-filter is-cardinal? segments)))
(define (day5b segments)
(define covered (vector-map segment->point-set segments))
(define counts (make-hash))
(for* ([point-set (in-vector covered)]
[point (in-vector point-set)])
(hash-update! counts point add1 0))
(length (filter (<= 2 _) (hash-values counts))))
(module+ main
(call-with-input-file "data/day5.txt"
(λ (prt)
(define lines (parse (port->lines prt)))
(answer 5 1 (day5a lines))
(answer 5 2 (day5b lines)))))
(module+ test
(require rackunit)
(call-with-input-file "data/day5.test.txt"
(λ (prt)
(define lines (parse (port->lines prt)))
(check-equal? (day5a lines) 5)
(check-equal? (day5b lines) 12))))