1 files changed, 46 insertions, 0 deletions
diff --git a/racket/aoc2022/day-12/day-12.rkt b/racket/aoc2022/day-12/day-12.rkt
new file mode 100644
index 0000000..c3f01ac
--- /dev/null
+++ b/racket/aoc2022/day-12/day-12.rkt
@@ -0,0 +1,46 @@
+#lang racket
+
+(require advent-of-code
+         graph)
+
+(define raw-terrain (fetch-aoc-input (find-session) 2022 12 #:cache #true))
+(define special-points (make-hash))
+
+(define terrain-mesh
+  (for*/hash ([(row x) (in-indexed (string-split raw-terrain))] [(col y) (in-indexed row)])
+    (define p (cons x y))
+    (case col
+      [(#\S)
+       (hash-set! special-points 'start p)
+       (values p 0)]
+      [(#\E)
+       (hash-set! special-points 'end p)
+       (values p 25)]
+      [else (values p (- (char->integer col) (char->integer #\a)))])))
+
+(define (neighbors p)
+  (match-define (cons x y) p)
+  (for*/list ([dx (in-list '(-1 0 1))]
+              [dy (in-list '(-1 0 1))]
+              #:when (= 1 (abs (+ dx dy)))
+              #:do [(define p* (cons (+ x dx) (+ y dy)))]
+              #:when (hash-has-key? terrain-mesh p*))
+    p*))
+
+(define paths
+  (directed-graph (for*/list ([p (in-list (hash-keys terrain-mesh))]
+                              [p* (in-list (neighbors p))]
+                              #:unless (> (sub1 (hash-ref terrain-mesh p*))
+                                          (hash-ref terrain-mesh p)))
+                    (list p p*))))
+
+;; part 1
+(time (match-define-values (distances _) (bfs paths (hash-ref special-points 'start)))
+      (hash-ref distances (hash-ref special-points 'end)))
+
+;; part 2
+(time (for/lists
+       (lengths #:result (apply min lengths))
+       ([start (in-list (hash-keys terrain-mesh))] #:when (= 0 (hash-ref terrain-mesh start)))
+       (match-define-values (distances _) (bfs paths start))
+       (hash-ref distances (hash-ref special-points 'end))))