1 files changed, 32 insertions, 0 deletions
diff --git a/racket/aoc2020/day-13/day-13.rkt b/racket/aoc2020/day-13/day-13.rkt
new file mode 100644
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--- /dev/null
+++ b/racket/aoc2020/day-13/day-13.rkt
@@ -0,0 +1,32 @@
+#lang racket
+
+(require advent-of-code
+         (only-in relation ->number)
+         threading)
+
+(define (process-ids str)
+  (~> str (string-split ",") (filter-map (λ (s) (string->number s 10 'number-or-false)) _)))
+
+(match-define (regexp #px"(\\d+)\n(.+)" (list _ (app ->number timestamp) raw-bus-ids))
+  (fetch-aoc-input (find-session) 2020 13))
+
+(define bus-ids (process-ids raw-bus-ids))
+
+;; part 1
+(for/first ([minute (in-naturals timestamp)]
+            #:do [(define departing-bus
+                    (for/first ([b bus-ids] #:when (= 0 (remainder minute b)))
+                      b))]
+            #:when departing-bus)
+  (* departing-bus (- minute timestamp)))
+
+;; part 2
+(for/fold ([step 1] [current-timestamp 1] #:result current-timestamp)
+          ([b* (in-list (string-split (string-trim raw-bus-ids) ","))]
+           [offset (in-naturals)]
+           #:unless (equal? b* "x")
+           #:do [(define bus (->number b*))])
+  (values
+   (* step bus)
+   (for/first ([n (in-range current-timestamp +inf.0 step)] #:when (= 0 (remainder (+ n offset) bus)))
+     n)))