Advent of Code, Day 3, Part II

advent-03.rkt

#lang racket
(define in "...")

(define (string-ref-mod xs i)
  (string-ref xs (modulo i (string-length xs))))

(define lines (call-with-input-string in port->lines))

(define (c δx δy)
  (for/sum ([x (in-range 0 +inf.0 δx)]
            [y (in-range 0 +inf.0 δy)]
            #:break (>= y (length lines)))
    (if (char=? #\# (string-ref-mod (list-ref lines y) x))
        1
        0)))

(* (c 1 1)
   (c 3 1)
   (c 5 1)
   (c 7 1)
   (c 1 2))