day 25: make it nicer

25.rkt

@@ -1,51 +1,28 @@
 #lang curly-fn racket
 
-(require "scripts/aoc.rkt")
+(require "scripts/aoc.rkt" math/number-theory)
 
 ;; solution for day 25
 
 (define *mod* 20201227)
-
-(define (loop subject times)
-  (for/fold ([val 1]) ([_ (in-range (+ 0 times))])
-    (remainder (* val subject) *mod*)))
+(define *subject* 7)
 
 (define (part1 input)
   (match-define (list you door) input)
-  (define you-loops #f)
-  (define door-loops #f)
-  (for/fold ([val 1]) ([loops (in-range 0 +inf.0)] #:break (and you-loops door-loops))
-    (when (= val you)
-      (set! you-loops loops))
-    (when (= val door)
-      (set! door-loops loops))
-    (remainder (* val 7) *mod*))
-  ; (displayln you-loops)
-  ; (displayln door-loops)
-  (displayln you-loops)
-  (displayln (loop 7 you-loops))
-  (displayln you)
-  (displayln door-loops)
-  (displayln (loop 7 door-loops))
-  (displayln door)
-  ; (displayln (loop door-loops you-loops))
-  ; (displayln (loop you-loops door-loops))
-  ; (displayln (loop 17807724 8))
-  ; (displayln (loop 5764801 11))
-  (loop you door-loops))
-; (part1 (list 5764801 17807724))
-; (error)
+  (with-modulus *mod*
+    (for/fold ([val 1] [you-loops #f] [door-loops #f] #:result (modexpt you door-loops))
+              ([loops (in-range 0 +inf.0)] #:break (and you-loops door-loops))
+      (define new-val (mod* val *subject*))
+      (cond
+        [(= val you) (values new-val loops door-loops)]
+        [(= val door) (values new-val you-loops loops)]
+        [else (values new-val you-loops door-loops)]))))
 
 ;; parse input file
 (define (parse fname)
   (define input (file->lines fname))
   (map string->number input))
 
-(module+ test
-  (require rackunit)
-  ;; tests here
-  (displayln "no tests :("))
-
 (module+ main
   (define input (parse "inputs/25"))
   (answer 25 1 (time (part1 input)))