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44#lang racket/base
(struct worldmap (w h m)
#:transparent)
(define m
(call-with-input-file "input03.txt"
(lambda (in)
(define m
(for/vector ([line (in-lines in)])
(for/vector ([c (in-string line)])
(case c
[(#\.) #f]
[(#\#) #t]))))
(worldmap (vector-length (vector-ref m 0))
(vector-length m)
m))))
(define (step m x y r d)
(define new-x (remainder (+ x r) (worldmap-w m)))
(define new-y (remainder (+ y d) (worldmap-h m)))
(define row (vector-ref (worldmap-m m) new-y))
(define col (vector-ref row new-x))
(values new-x new-y col))
(define (check m r d)
(let loop ([x 0]
[y 0]
[hits 0])
(cond
[(= y (sub1 (worldmap-h m))) hits]
[else
(define-values (new-x new-y tree?)
(step m x y r d))
(loop new-x new-y (if tree? (add1 hits) hits))])))
(check m 3 1)
(* (check m 1 1)
(check m 3 1)
(check m 5 1)
(check m 7 1)
(check m 1 2))