1 files changed, 47 insertions, 0 deletions
diff --git a/racket/aoc2023/day-13/day-13.rkt b/racket/aoc2023/day-13/day-13.rkt
new file mode 100644
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--- /dev/null
+++ b/racket/aoc2023/day-13/day-13.rkt
@@ -0,0 +1,47 @@
+#lang racket
+
+(require advent-of-code
+         threading)
+
+(define input
+  (~>(fetch-aoc-input (find-session) 2023 13 #:cache #true)
+     (string-split "\n\n")
+     (map (λ~> string-split) _)))
+
+(define (do-symmetric? lefts rights errs)
+  (cond
+    [(empty? rights) #f]
+    [else
+     (define found-errs
+       (for/sum ([l (in-string (string-join lefts ""))]
+                 [r (in-string (string-join rights ""))]
+                 #:unless (char=? l r))
+         1))
+     (if (= errs found-errs)
+         (length lefts)
+         (do-symmetric? (cons (first rights) lefts)
+                        (rest rights)
+                        errs))]))
+
+(define (symmetric? xss errs)
+  (do-symmetric? (list (first xss)) (rest xss) errs))
+
+(define (transpose strs)
+  (~> strs
+      (map string->list _)
+      (apply map list _)
+      (map list->string _)))
+
+(define (find-symmetry-score xss errs)
+  (cond
+    [(symmetric? xss errs) => (curry * 100)]
+    [else (symmetric? (transpose xss) errs)]))
+
+;; part 1
+(for/sum ([note (in-list input)])
+  (find-symmetry-score note 0))
+
+;; part 2
+(for/sum ([note (in-list input)])
+  (find-symmetry-score note 1))
+