day 9: very late

9.rkt

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+#lang curly-fn racket
+
+(require "scripts/aoc.rkt")
+
+;; solution for day 9
+
+(define (cmp a b)
+  (cond
+    [(< a b) '<]
+    [(> a b) '>]
+    [else '=]))
+
+(define (move1 hx hy tx ty dir)
+  (match* (dir (cmp hx tx) (cmp hy ty))
+    [("U" _ (not '>)) (values hx (add1 hy) tx ty)]
+    [("D" _ (not '<)) (values hx (sub1 hy) tx ty)]
+    [("R" (not '>) _) (values (add1 hx) hy tx ty)]
+    [("L" (not '<) _) (values (sub1 hx) hy tx ty)]
+
+    [("U" '= _) (values hx (add1 hy) tx (add1 ty))]
+    [("U" '< _) (values hx (add1 hy) (sub1 tx) (add1 ty))]
+    [("U" '> _) (values hx (add1 hy) (add1 tx) (add1 ty))]
+
+    [("D" '= _) (values hx (sub1 hy) tx (sub1 ty))]
+    [("D" '< _) (values hx (sub1 hy) (sub1 tx) (sub1 ty))]
+    [("D" '> _) (values hx (sub1 hy) (add1 tx) (sub1 ty))]
+
+    [("R" _ '=) (values (add1 hx) hy (add1 tx) ty)]
+    [("R" _ '<) (values (add1 hx) hy (add1 tx) (sub1 ty))]
+    [("R" _ '>) (values (add1 hx) hy (add1 tx) (add1 ty))]
+
+    [("L" _ '=) (values (sub1 hx) hy (sub1 tx) ty)]
+    [("L" _ '<) (values (sub1 hx) hy (sub1 tx) (sub1 ty))]
+    [("L" _ '>) (values (sub1 hx) hy (sub1 tx) (add1 ty))]))
+
+(define (part1 input)
+  (for*/fold ([hx 0] [hy 0] [tx 0] [ty 0] [visited (set (cons 0 0))] #:result (set-count visited))
+             ([inst (in-list input)]
+              [dir (in-value (first inst))]
+              [_ (in-range (second inst))])
+    (define-values [hx* hy* tx* ty*] (move1 hx hy tx ty dir))
+    ; (printf "DIR: ~a | h: (~a, ~a) t: (~a ~a)\n" dir hx* hy* tx* ty*)
+    (values hx* hy* tx* ty* (set-add visited (cons tx* ty*)))))
+
+(define (apply-move dir p)
+  (match dir
+    ["U" (cons (car p) (add1 (cdr p)))]
+    ["D" (cons (car p) (sub1 (cdr p)))]
+    ["R" (cons (add1 (car p)) (cdr p))]
+    ["L" (cons (sub1 (car p)) (cdr p))]))
+
+(define (catch-up p1 p2)
+  (match* ((- (car p1) (car p2)) (- (cdr p1) (cdr p2)))
+    [(2 0) (cons (add1 (car p2)) (cdr p2))]
+    [(-2 0) (cons (sub1 (car p2)) (cdr p2))]
+    [(0 2) (cons (car p2) (add1 (cdr p2)))]
+    [(0 -2) (cons (car p2) (sub1 (cdr p2)))]
+    [(2 1) (cons (add1 (car p2)) (add1 (cdr p2)))]
+    [(2 -1) (cons (add1 (car p2)) (sub1 (cdr p2)))]
+    [(-2 1) (cons (sub1 (car p2)) (add1 (cdr p2)))]
+    [(-2 -1) (cons (sub1 (car p2)) (sub1 (cdr p2)))]
+    [(1 2) (cons (add1 (car p2)) (add1 (cdr p2)))]
+    [(-1 2) (cons (sub1 (car p2)) (add1 (cdr p2)))]
+    [(1 -2) (cons (add1 (car p2)) (sub1 (cdr p2)))]
+    [(-1 -2) (cons (sub1 (car p2)) (sub1 (cdr p2)))]
+    [(2 2) (cons (add1 (car p2)) (add1 (cdr p2)))]
+    [(2 -2) (cons (add1 (car p2)) (sub1 (cdr p2)))]
+    [(-2 2) (cons (sub1 (car p2)) (add1 (cdr p2)))]
+    [(-2 -2) (cons (sub1 (car p2)) (sub1 (cdr p2)))]
+    [(_ _) p2]))
+
+(define (draw state)
+  (define lines
+    (for/list ([y (in-range 10 -10 -1)])
+      (apply string-append
+        (for/list ([x (in-range -10 10)])
+          (match (index-of state (cons x y))
+            [#f "."]
+            [0 "H"]
+            [i (~a i)])))))
+  (string-join lines "\n"))
+
+(define (part2 input)
+  (define initial-state (build-list 10 (λ (_) (cons 0 0))))
+  (for*/fold ([rope initial-state] [visited (set (cons 0 0))] #:result (set-count visited))
+             ([inst (in-list input)]
+              [dir (in-value (first inst))]
+              [_ (in-range (second inst))])
+    (define new-list (list (apply-move dir (first rope))))
+
+    (for ([i (in-range 1 10)])
+      (define h (list-ref new-list (sub1 i)))
+      (define t (list-ref rope i))
+      (define t* (catch-up h t))
+      (set! new-list (append new-list (list t*))))
+
+    ; (printf "~a ~a\n" dir new-list)
+    ; (displayln (draw new-list))
+
+    (values new-list (set-add visited (last new-list)))))
+
+(define (parse fname)
+  (for/list ([line (in-list (file->lines fname))])
+    (match-define (list dir (app string->number num)) (string-split line " "))
+    (list dir num)))
+
+; (part2 (parse "inputs/9-test2"))
+; (error "x")
+
+(module+ main
+  (define input (parse "inputs/9"))
+  (answer 9 1 (time (part1 input)))
+  (answer 9 2 (time (part2 input)))
+  (displayln "meow"))

inputs/9-test1

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+R 4
+U 4
+L 3
+D 1
+R 4
+D 1
+L 5
+R 2

inputs/9-test2

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+R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20