day 25

2020/25.rkt

@@ -0,0 +1,17 @@
+#lang br
+(require racket/file rackunit)
+
+(match-define (list card-pubkey door-pubkey) '(18356117 5909654))
+
+(define (transform subject-number [break-proc void])
+  (for/fold ([val 1]
+             [count 0]
+             #:result (cons val count))
+            ([i (in-naturals)]
+             #:break (break-proc val i))
+    (values (remainder (* val subject-number) 20201227) (add1 count))))
+
+(define card-loop-size (cdr (transform 7 (λ (val i) (eq? val card-pubkey)))))
+(define door-loop-size (cdr (transform 7 (λ (val i) (eq? val door-pubkey)))))
+
+(check-equal? (car (transform card-pubkey (λ (val i) (eq? i door-loop-size)))) 16902792)