day 13: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

13.rkt

@@ -0,0 +1,42 @@
+#lang curly-fn racket
+
+(require "scripts/aoc.rkt")
+(require math/number-theory)
+
+;; solution for day 13
+
+(define (part1 input)
+  (match-define (cons time buses) input)
+  (define (find-bus x)
+    (for/first ([bus (in-list buses)] #:unless (equal? 'x bus) #:when (zero? (modulo x bus)))
+      bus))
+  (for*/first ([x (in-range time +inf.0)] [bus (in-value (find-bus x))] #:when bus)
+    (* bus (- x time))))
+
+(define (part2 input)
+  (match-define (cons _ buses) input)
+  (define eqns
+    (for/fold ([eqns '()]) ([bus (in-list buses)] [i (in-naturals)] #:unless (equal? bus 'x))
+      (cons (cons (- bus i) bus) eqns)))
+  (solve-chinese (map car eqns) (map cdr eqns)))
+
+;; parse input file
+(define (parse fname)
+  (define input (file->lines fname))
+  (define time (string->number (first input)))
+  (define buses (for/list ([x (in-list (string-split (second input) ","))])
+                  (match x
+                    ["x" 'x]
+                    [_ (string->number x)])))
+  (cons time buses))
+
+(module+ test
+  (require rackunit)
+  ;; tests here
+  (displayln "no tests :("))
+
+(module+ main
+  (define input (parse "inputs/13"))
+  (answer 13 1 (time (part1 input)))
+  (answer 13 2 (time (part2 input)))
+  (displayln "meow"))