From 5a2032e7341b777074e6395955cef28b74616711 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Wed, 19 Dec 2018 11:03:12 -0800
Subject: [PATCH] d09 p1 (fasterer)

---
 2018/09.rkt | 27 +++++++++------------------
 1 file changed, 9 insertions(+), 18 deletions(-)

diff --git a/2018/09.rkt b/2018/09.rkt
index b630af8..6499555 100644
--- a/2018/09.rkt
+++ b/2018/09.rkt
@@ -1,47 +1,38 @@
 #lang debug br
 (require racket/file)
 
-(define (mlength mprs [acc 0])
-  (if (null? mprs)
-      acc
-      (mlength (mcdr mprs) (add1 acc))))
-
 (define (nth-mpr mprs n)
   (for/fold ([mprs mprs])
             ([i (in-range n)])
     (mcdr mprs)))
 
 (define (★)
-  (match-define (list player-count marble-count)
+  (match-define (list player-count max-marbles)
     (map string->number (regexp-match* #px"\\d+" (file->string "09.txt"))))
   (define scores (make-hasheqv))
   (define circle (mcons #f (mcons 0 null)))
-  (let loop ([marble 1] [pos 0])     
+  (let loop ([marble 1] [marbles-in-circle 1] [pos 0])
     (cond
-      [(> marble marble-count) (argmax cdr (hash->list scores))]
+      [(> marble max-marbles) (cdr (argmax cdr (hash->list scores)))]
      [(zero? (modulo marble 23))
-       (define marble-count (sub1 (mlength circle)))
-       (define deletion-pos (modulo (+ (- pos 7) marble-count) marble-count))
+       (define deletion-pos (modulo (+ (- pos 7) marbles-in-circle) marbles-in-circle))
        (define last-left-mpr (nth-mpr circle deletion-pos))
        (define removed-marble (mcar (mcdr last-left-mpr)))
        (set-mcdr! last-left-mpr (mcdr (mcdr last-left-mpr)))
        (define player (modulo marble player-count))
        (hash-update! scores player (λ (sc) (+ removed-marble marble sc)) 0)
-       (loop (add1 marble) deletion-pos)]
+       (loop (add1 marble) (sub1 marbles-in-circle) deletion-pos)]
       [else
-       (define marble-count (sub1 (mlength circle)))
-       (define next-pos (add1 (modulo (add1 pos) marble-count)))
+       (define next-pos (add1 (modulo (add1 pos) marbles-in-circle)))
        (define last-left-mpr (nth-mpr circle next-pos))
        (set-mcdr! last-left-mpr (mcons marble (mcdr last-left-mpr)))
-       (loop (add1 marble) next-pos)])))
-
-(time (★))
+       (loop (add1 marble) (add1 marbles-in-circle) next-pos)])))
 
 #;(define (★★)
     )
 #;(★★)
 
-#;(module+ test
+(module+ test
     (require rackunit)
     (check-equal? (time (★)) 437654)
-    (check-equal? (time (★★)) 566))
\ No newline at end of file
+    #;(check-equal? (time (★★)) 566))
\ No newline at end of file