Day 12

input/12.txt

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src/12.rkt

@@ -0,0 +1,53 @@
+#lang curly-fn racket
+
+(require "../lib.rkt"
+         data/queue)
+
+(define input (list->vector (problem-input 12)))
+(define width (string-length (vector-first input)))
+(define height (vector-length input))
+
+;; coord? = (number? . number?)
+
+;; get : coord? -> (or/c char? #f)
+(define (get xy)
+  (match-define `(,x . ,y) xy)
+  (if (and (<= 0 x (sub1 width)) (<= 0 y (sub1 height)))
+      (string-ref (vector-ref input y) x)
+      #f))
+
+;; accessible? : coord? coord? -> boolean?
+(define (accessible? from-xy to-xy)
+  (define from (get from-xy))
+  (define to   (get to-xy))
+  (and to (or (and (char=? #\S from) (char=? #\a to))
+              (and (char=? #\S from) (char=? #\b to))
+              (and (char=? #\y from) (char=? #\E to))
+              (and (char=? #\z from) (char=? #\E to))
+              (<= (char->integer to) (add1 (char->integer from))))))
+
+;; neighbours : coord? -> (listof coord?)
+(define (neighbours xy)
+  (match-define `(,x . ,y) xy)
+  (filter #{accessible? xy %}
+          `(,(cons (add1 x) y)
+            ,(cons (sub1 x) y)
+            ,(cons x (add1 y))
+            ,(cons x (sub1 y)))))
+
+;; shortest : char? -> number?
+(define (shortest c)
+  (define seen (mutable-set))
+  (define Q (make-queue))
+  (for*/list ([x (in-range width)]
+              [y (in-range height)]
+              #:when (char=? c (get (cons x y))))
+    (enqueue! Q (cons (cons x y) 0)))
+  (let loop ()
+    (match-define `(,xy . ,steps) (dequeue! Q))
+    (for ([to (filter #{not (set-member? seen %)} (neighbours xy))])
+      (enqueue! Q (cons to (add1 steps)))
+      (set-add! seen to))
+    (if (char=? #\E (get xy)) steps (loop))))
+
+(show-solution (shortest #\S) (shortest #\a))