From 3bd4194aa19fde6c671356e4a99532342573becb Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Wed, 19 Dec 2018 08:39:55 -0800
Subject: [PATCH] d09 p1 (faster)

---
 2018/09.rkt | 37 +++++++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/2018/09.rkt b/2018/09.rkt
index 5aeb6e7..b630af8 100644
--- a/2018/09.rkt
+++ b/2018/09.rkt
@@ -1,24 +1,41 @@
 #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)
     (map string->number (regexp-match* #px"\\d+" (file->string "09.txt"))))
   (define scores (make-hasheqv))
-  (let loop ([marble 1] [pos 0] [circle (list 0)])     
+  (define circle (mcons #f (mcons 0 null)))
+  (let loop ([marble 1] [pos 0])     
     (cond
       [(> marble marble-count) (argmax cdr (hash->list scores))]
-      [(zero? (modulo marble 23))
-       (define len (length circle))
-       (define deletion-pos (modulo (+ (- pos 7) len) len))
-       (define-values (head tail) (split-at circle deletion-pos))
+     [(zero? (modulo marble 23))
+       (define marble-count (sub1 (mlength circle)))
+       (define deletion-pos (modulo (+ (- pos 7) marble-count) marble-count))
+       (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) (+ marble (car tail) sc)) 0)
-       (loop (add1 marble) deletion-pos (append head (cdr tail)))]
+       (hash-update! scores player (λ (sc) (+ removed-marble marble sc)) 0)
+       (loop (add1 marble) deletion-pos)]
       [else
-       (define next-pos (add1 (modulo (add1 pos) (length circle))))
-       (define-values (head tail) (split-at circle next-pos))
-       (loop (add1 marble) next-pos (append head (list marble) tail))])))
+       (define marble-count (sub1 (mlength circle)))
+       (define next-pos (add1 (modulo (add1 pos) marble-count)))
+       (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 (★))
 
 #;(define (★★)
     )