1 files changed, 49 insertions, 0 deletions
diff --git a/aoc2021/day-15/day-15.rkt b/aoc2021/day-15/day-15.rkt
new file mode 100644
index 0000000..5e61c55
--- /dev/null
+++ b/aoc2021/day-15/day-15.rkt
@@ -0,0 +1,49 @@
+#lang racket
+(require "../../jj-aoc.rkt"
+         threading
+         graph)
+
+(struct Point (x y) #:transparent)
+
+(define data
+  (for/fold ([cells (hash)]) ([row (in-lines (open-day 15 2021))] [x (in-naturals)])
+    (for/fold ([cells cells]) ([n (in-string row)] [y (in-naturals)])
+      (hash-set cells (Point x y) (~> n string string->number)))))
+
+(define x-max (~>> data hash-keys (map Point-x) (apply max)))
+(define y-max (~>> data hash-keys (map Point-y) (apply max)))
+
+(define (neighbors pt d)
+  (match-define (Point x y) pt)
+  (~>> (list (Point (add1 x) y) (Point (sub1 x) y) (Point x (add1 y)) (Point x (sub1 y)))
+       (filter (curry hash-has-key? d))))
+
+(define (grid-graph d)
+  (weighted-graph/directed (for*/list ([coord (in-list (hash-keys d))]
+                                       [neighbor (in-list (neighbors coord d))]
+                                       [weight (in-value (hash-ref d neighbor))])
+                             (list weight coord neighbor))))
+
+;; part 1
+(define (find-path-weight d)
+  (define grid (grid-graph d))
+  (let-values ([(node-distances _) (dijkstra grid (Point 0 0))])
+    (define xm (~>> d hash-keys (map Point-x) (apply max)))
+    (define ym (~>> d hash-keys (map Point-y) (apply max)))
+    (hash-ref node-distances (Point xm ym))))
+
+(~> data find-path-weight time)
+
+;; part 2
+(define nine-cycle (in-cycle (inclusive-range 1 9)))
+
+(define (expand-data data)
+  (for/fold ([cells (hash)]) ([coord (in-list (hash-keys data))])
+    (match-define (Point x y) coord)
+    (for*/fold ([cells cells])
+               ([n (in-range 5)] [m (in-range 5)] [val (in-value (hash-ref data coord))])
+      (hash-set cells
+                (Point (+ x (* n (add1 x-max))) (+ y (* m (add1 y-max))))
+                (sequence-ref nine-cycle (+ val n m -1))))))
+
+(~> data expand-data find-path-weight time)