Day 20, Part 1.

src/20.rkt

@@ -0,0 +1,40 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input (map string-lines (problem-input-grouped 20)))
+
+(define (parse-tile tile-strings)
+  (match-let ([`(,name ,rows ...) tile-strings])
+    (list (string->number (second (regexp-match #px"Tile (\\d+):" name)))
+          (map string->list rows))))
+
+(define tiles
+  (map parse-tile input))
+
+(define (tile-edges tile)
+  (match-let ([(list name rows) tile])
+    (list name (list (first rows)
+                     (map last rows)
+                     (reverse (last rows))
+                     (reverse (map first rows))))))
+
+(define tiles-edges
+  (map tile-edges tiles))
+
+(define (corner? edges tiles)
+  (let ([tiles (map second tiles)])
+    (= 2 (count #{for/or ([target tiles])
+                   (or (member % target)
+                       (member (reverse %) target))}
+                edges))))
+
+(define part1
+  (for/product ([tile tiles-edges]
+                #:when (corner? (second tile)
+                                (remove tile tiles-edges)))
+    (first tile)))
+
+(define part2 #f)
+
+(show-solution part1 part2)