day 13 part 1

2020/13.rkt

@@ -0,0 +1,20 @@
+#lang br
+(require racket/file rackunit)
+
+(match-define (cons target buses-all)
+  (for/list ([tok
+              (in-port read
+                       (open-input-string
+                        (string-replace (file->string "13.rktd") "," " ")))])
+    tok))
+
+(define buses (filter integer? buses-all))
+
+(define (overshoot bus)
+  (for/last ([i (in-naturals)]
+             #:final (> (* i bus) target))
+    (- (* i bus) target)))
+
+(define winner (argmin overshoot buses))
+
+(check-equal? (* winner (overshoot winner)) 246)

2020/13.rktd

@@ -0,0 +1,2 @@
+1000066
+13,x,x,41,x,x,x,37,x,x,x,x,x,659,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19,x,x,x,23,x,x,x,x,x,29,x,409,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17