Day 17.

src/17.rkt

@@ -0,0 +1,34 @@
+#lang racket
+
+(require "../lib.rkt")
+
+(define x-min 88)
+(define x-max 125)
+(define y-min -157)
+(define y-max -103)
+
+(define part1
+  (let ([δy (sub1 (abs y-min))])
+    (/ (* δy (add1 δy)) 2)))
+
+(define δy-max (sub1 (abs y-min)))
+(define δy-min y-min)
+(define δx-max x-max)
+(define δx-min (floor (sqrt (* x-min 2))))
+
+(define (reaches-target? δx δy)
+  (let loop ([x   0] [y   0]
+             [δx δx] [δy δy])
+    (cond
+      [(or  (> x x-max) (< y y-min)) #f]
+      [(and (<= x-min x x-max) (<= y-min y y-max)) #t]
+      [else (loop (+ x δx) (+ y δy)
+                  (if (zero? δx) δx (sub1 δx))
+                  (sub1 δy))])))
+
+(define part2
+  (for*/sum ([δx (range* δx-min δx-max)]
+             [δy (range* δy-min δy-max)]
+             #:when (reaches-target? δx δy)) 1))
+
+(show-solution part1 part2)