1 files changed, 38 insertions, 0 deletions
diff --git a/day09.rkt b/day09.rkt
new file mode 100644
index 0000000..1f3ca37
--- /dev/null
+++ b/day09.rkt
@@ -0,0 +1,38 @@
+#lang racket
+(require racket/set)
+
+(define (scan f i l)
+  (if (null? l)
+    l
+    (let ((v (f i (car l))))
+      (cons v (scan f v (cdr l))))))
+
+(define/match (move-after head tail)
+  [((list hx hy) (list tx ty))
+   (if (and (<= (abs (- hx tx)) 1)
+            (<= (abs (- hy ty)) 1))
+     tail
+     (list (+ tx (sgn (- hx tx)))
+           (+ ty (sgn (- hy ty)))))])
+
+(for/list ([rope-length '(1 9)])
+  (let ([visited (mutable-set)]
+        [head '(0 0)]
+        [tail (make-list rope-length '(0 0))])
+
+    (set-add! visited (last tail))
+    (for ([line (file->lines "day09")])
+      (let* ([parts (string-split line " ")]
+             [dir (string->symbol (first parts))]
+             [n (string->number (second parts))])
+        (for ([_ (in-range n)])
+          (set! head (map + head (case dir
+                                   [(L) '(-1 0)]
+                                   [(R) '(1 0)]
+                                   [(U) '(0 1)]
+                                   [(D) '(0 -1)])))
+          (set! tail (scan move-after head tail))
+          (set-add! visited (last tail)))))
+    (set-count visited)))
+; 6494
+; 2691