1 files changed, 46 insertions, 0 deletions
diff --git a/day13.rkt b/day13.rkt
new file mode 100644
index 0000000..74da917
--- /dev/null
+++ b/day13.rkt
@@ -0,0 +1,46 @@
+#lang racket
+
+(require json)
+
+(define lists (file->list "day13" read-json))
+
+(define data
+  (let loop ([lists lists]
+             [pairs '()])
+    (if (null? lists)
+      (reverse pairs)
+      (let-values ([(pair rest) (split-at lists 2)])
+        (loop rest (cons pair pairs))))))
+
+(define (compare left right)
+  (cond [(and (number? left) (number? right)) (- left right)]
+        [(and (list? left) (list? right))
+         (let loop ([leaning (compare (length left) (length right))]
+                    [left left]
+                    [right right])
+           (cond [(null? left) leaning]
+                 [(null? right) 1]
+                 [(compare (car left) (car right)) =>
+                  (lambda (c)
+                    (if (zero? c)
+                      (loop leaning (cdr left) (cdr right))
+                      c))]))]
+        [(and (number? left) (list? right))
+         (compare (list left) right)]
+        [(and (list? left) (number? right))
+         (compare left (list right))]
+        [else (error "can't happen")]))
+
+(for/sum ([(pair i) (in-indexed data)])
+  (if (negative? (apply compare pair))
+    (+ i 1)
+    0))
+; 6420
+
+(define part2
+  (sort (list* '((2)) '((6)) lists)
+        (compose negative? compare)))
+
+(* (+ 1 (index-of part2 '((2))))
+   (+ 1 (index-of part2 '((6)))))
+;; 22000