Day 9

src/09.rkt

@@ -0,0 +1,47 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input (map #{string-split %1 " "} (problem-input 9)))
+
+(define (sign H T)
+  (cond
+    [(> H T)  1]
+    [(= H T)  0]
+    [(< H T) -1]))
+
+(define (follow xH yH xT yT)
+  (if (or (and (= xH xT) (> (abs (- yH yT)) 1))
+          (and (= yH yT) (> (abs (- xH xT)) 1))
+          (> (+ (abs (- xH xT)) (abs (- yH yT))) 2))
+      (values (+ xT (sign xH xT)) (+ yT (sign yH yT)))
+      (values xT yT)))
+
+(define (model knots)
+  (define visited (mutable-set))
+  (for/fold ([xH 0] [yH 0]
+             [xTs (repeat knots '(0))]
+             [yTs (repeat knots '(0))])
+            ([motion input])
+    (match-let* ([`(,dir , n) motion]
+                 [n (string->number n)])
+      (for/fold ([xH  xH]  [yH  yH]
+                 [xTs xTs] [yTs yTs])
+                ([_ (in-range n)])
+        (define-values (xH* yH*)
+          (match dir
+            ["U" (values xH (add1 yH))]
+            ["D" (values xH (sub1 yH))]
+            ["R" (values (add1 xH) yH)]
+            ["L" (values (sub1 xH) yH)]))
+        (define-values (xTs* yTs*)
+          (for/fold ([xTs `(,xH)] [yTs `(,yH)]
+                     #:result (values (rest (reverse xTs)) (rest (reverse yTs))))
+                    ([xT xTs] [yT yTs])
+            (define-values (xT* yT*) (follow (first xTs) (first yTs) xT yT))
+            (values (cons xT* xTs) (cons yT* yTs))))
+        (set-add! visited (cons (last xTs*) (last yTs*)))
+        (values xH* yH* xTs* yTs*))))
+  (set-count visited))
+
+(show-solution (model 1) (model 9))