Day 10: Part 1.

input/10.txt

@@ -0,0 +1,33 @@
+.#..#..##.#...###.#............#.
+.....#..........##..#..#####.#..#
+#....#...#..#.......#...........#
+.#....#....#....#.#...#.#.#.#....
+..#..#.....#.......###.#.#.##....
+...#.##.###..#....#........#..#.#
+..#.##..#.#.#...##..........#...#
+..#..#.......................#..#
+...#..#.#...##.#...#.#..#.#......
+......#......#.....#.............
+.###..#.#..#...#..#.#.......##..#
+.#...#.................###......#
+#.#.......#..####.#..##.###.....#
+.#.#..#.#...##.#.#..#..##.#.#.#..
+##...#....#...#....##....#.#....#
+......#..#......#.#.....##..#.#..
+##.###.....#.#.###.#..#..#..###..
+#...........#.#..#..#..#....#....
+..........#.#.#..#.###...#.....#.
+...#.###........##..#..##........
+.###.....#.#.###...##.........#..
+#.#...##.....#.#.........#..#.###
+..##..##........#........#......#
+..####......#...#..........#.#...
+......##...##.#........#...##.##.
+.#..###...#.......#........#....#
+...##...#..#...#..#..#.#.#...#...
+....#......#.#............##.....
+#......####...#.....#...#......#.
+...#............#...#..#.#.#..#.#
+.#...#....###.####....#.#........
+#.#...##...#.##...#....#.#..##.#.
+.#....#.###..#..##.#.##...#.#..##

lib.rkt

@@ -12,6 +12,7 @@
          sum
          neq?
          nzero?
+         negate
          number->digits-reverse
          number->digits
          rac
@@ -50,6 +51,10 @@
 (define (nzero? n)
   (not (zero? n)))
 
+;; negate : number -> number
+(define (negate n)
+  (- 0 n))
+
 ;; number->digits-reverse : number -> (listof number)
 ;; Return the digits of the given number in reverse order (i.e. RTL)
 (define (number->digits-reverse n)

src/10.rkt

@@ -0,0 +1,60 @@
+#lang racket
+
+(require "../lib.rkt")
+
+(define input
+  (problem-input 10))
+
+(define width (string-length (car input)))
+(define height (length input))
+
+(define (coprime? ij)
+  (= 1 (gcd (first  ij)
+            (second ij))))
+
+(define (offsets x y)
+  (define is
+    (range (negate x) (- width x)))
+  (define js
+    (range (negate y) (- height y)))
+  (filter coprime? (cartesian-product is js)))
+
+(define (asteroid? xy)
+  (define row (list-ref input (second xy)))
+  (eq? #\# (string-ref row (first xy))))
+
+(define (asteroid-offset? x y ij)
+  (define i (first  ij))
+  (define j (second ij))
+  (define max-multiple
+    (let ([max-x
+           (cond
+             [(zero? i) width]
+             [(positive? i) (truncate (/ (- (sub1 width) x) i))]
+             [else (abs (truncate (/ x i)))])]
+          [max-y
+           (cond
+             [(zero? j) height]
+             [(positive? j) (truncate (/ (- (sub1 height) y) j))]
+             [else (abs (truncate (/ y j)))])])
+      (min max-x max-y)))
+  (ormap (λ (m) (asteroid? `(,(+ x (* m i))
+                             ,(+ y (* m j)))))
+         (range 1 (add1 max-multiple))))
+
+(define (asteroids xy)
+  (define x (first xy))
+  (define y (second xy))
+  (count (curry asteroid-offset? x y) (offsets x y)))
+
+(define-values (part1 location)
+  (let* ([cols (range width)]
+         [rows (range height)]
+         [locations (filter asteroid? (cartesian-product cols rows))]
+         [asteroid-counts (map asteroids locations)]
+         [maximum (apply max asteroid-counts)]
+         [index (index-of asteroid-counts maximum)]
+         [location (list-ref locations index)])
+    (values maximum location)))
+
+(show-solution part1 #f)