1 files changed, 76 insertions, 0 deletions
diff --git a/racket/aoc2022/day-09/day-09.rkt b/racket/aoc2022/day-09/day-09.rkt
new file mode 100644
index 0000000..0390d2e
--- /dev/null
+++ b/racket/aoc2022/day-09/day-09.rkt
@@ -0,0 +1,76 @@
+#lang racket
+(require advent-of-code
+         threading)
+
+(struct cmd (dir amt))
+(struct posn (x y) #:transparent)
+
+(define moves
+  (~> (fetch-aoc-input (find-session) 2022 9)
+      (string-split "\n")
+      (map (λ~> (string-split _)
+                (match _
+                  [(list dir amt) (cmd (string->symbol dir) (string->number amt))]))
+           _)))
+
+(define (move-head p dir)
+  (match-define (posn x y) p)
+  (match dir
+    ['U (posn x (add1 y))]
+    ['D (posn x (sub1 y))]
+    ['R (posn (add1 x) y)]
+    ['L (posn (sub1 x) y)]))
+
+(define (avg n m)
+  (/ (+ n m) 2))
+
+(define (manhattan-distance p1 p2)
+  (match-define (posn x1 y1) p1)
+  (match-define (posn x2 y2) p2)
+  (+ (abs (- x2 x1)) (abs (- y2 y1))))
+
+(define (follow-head head tail)
+  (match-define (posn hx hy) head)
+  (match-define (posn tx ty) tail)
+
+  (case (manhattan-distance head tail)
+    [(0 1) tail]
+    [(2 4)
+     (cond
+       [(and (= 1 (abs (- hx tx)) (abs (- hy ty)))) tail]
+       [else (posn (avg hx tx) (avg hy ty))])]
+    [(3)
+     (cond
+       [(= 2 (abs (- hx tx))) (posn (avg hx tx) hy)]
+       [(= 2 (abs (- hy ty))) (posn hx (avg hy ty))])]))
+
+;; part 1
+(for*/fold ([head (posn 0 0)] [tail (posn 0 0)] [tail-posns (set)] #:result (set-count tail-posns))
+           ([move (in-list moves)] #:do [(match-define (cmd dir amt) move)] [_ (in-range amt)])
+  (define new-head (move-head head dir))
+  (define new-tail (follow-head new-head tail))
+  (values new-head new-tail (set-add tail-posns new-tail)))
+
+;; part 2
+(for*/fold ([knots (make-list 10 (posn 0 0))] [tail-posns (set)] #:result (set-count tail-posns))
+           ([move (in-list moves)] #:do [(match-define (cmd dir amt) move)] [_ (in-range amt)])
+  (define updated-knots
+    (for/fold ([knots-list (list (move-head (first knots) dir))])
+              ([following-knot (in-list (rest knots))])
+      (cons (follow-head (car knots-list) following-knot) knots-list)))
+  (values (reverse updated-knots) (set-add tail-posns (first updated-knots))))
+
+;; refactor: part 1 and 2 combined
+(define (follow-tail move-list rope-length)
+  (for*/fold ([knots (make-list rope-length (posn 0 0))]
+              [tail-posns (set)]
+              #:result (set-count tail-posns))
+             ([move (in-list move-list)] #:do [(match-define (cmd dir amt) move)] [_ (in-range amt)])
+    (define updated-knots
+      (for/fold ([knots-list (list (move-head (first knots) dir))])
+                ([following-knot (in-list (rest knots))])
+        (cons (follow-head (car knots-list) following-knot) knots-list)))
+    (values (reverse updated-knots) (set-add tail-posns (first updated-knots)))))
+
+(time (follow-tail moves 2))
+(time (follow-tail moves 10))