Add Advent 2021 - day 17.

Slightly hacky solution, enabled by small input size:

- max number of recursions is arbitrary, found via testing
- bounds for x and minimal bound for y are analytic; max bound for y is
  from testing

2021/day17/p.rkt

@@ -0,0 +1,41 @@
+#lang racket/base
+(require racket/string)
+
+(define (make-line t)
+  (let* ([t (string-trim t "target area: ")]
+         [sl (string-split t ", ")]
+         [x (string-trim (car sl) "x=")]
+         [y (string-trim (cadr sl) "y=")])
+    (values (map string->number (string-split x "..")) (map string->number (string-split y "..")))))
+
+(define-values (xp yp) (make-line (read-line)))
+
+(define (run-steps x y)
+  (define (recu x y px py max-y number-of-recursions)
+    (define (lower-abs v)
+      (if (> v 0)
+          (sub1 v)
+          (if (< v 0)
+              (add1 v)
+              0)))
+
+    (let* ([px (+ x px)]
+           [py (+ y py)]
+           [max-y (if (> y 0) py max-y)]
+           [x (lower-abs x)]
+           [y (sub1 y)])
+      (if (and (<= (car xp) px (cadr xp)) (<= (car yp) py (cadr yp)))
+          (cons 'hit max-y)
+          (if (or (> number-of-recursions 500) (and (= x 0) (< px (car xp))))
+              (cons 'fail 0)
+              (recu x y px py max-y (add1 number-of-recursions))))))
+
+  (recu x y 0 0 0 0))
+
+(define result
+  (for*/list ([x (in-range (add1 (cadr xp)))] [y (in-range (car yp) (* -4 (cadr yp)))])
+    (run-steps x y)))
+; part 1
+(apply max (map cdr result))
+; part 2
+(length (filter (lambda (x) (equal? (car x) 'hit)) result))

2021/day17/test1.txt

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+target area: x=20..30, y=-10..-5

2021/day17/test2.txt

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+target area: x=155..215, y=-132..-72