d17

2017/d17/main.rkt

@@ -0,0 +1,24 @@
+#lang reader "../aoc-lang.rkt"
+(provide (rename-out [#%mb #%module-begin]) ★ ★★)
+
+(define-macro (#%mb (STARS) (STEP-SIZE))
+  #`(#%module-begin
+     (time (STARS STEP-SIZE))))
+
+(define (★ step-size)
+  (define iterations 2017)
+  (for/fold ([buf '(0)]
+             [current-pos 0]
+             #:result (second (member iterations buf)))
+            ([i (in-range iterations)])
+    (define pos (add1 (modulo (+ current-pos step-size) (add1 i))))
+    (define-values (head tail) (split-at buf pos))
+    (values (append head (list (add1 i)) tail) pos))) 
+
+(define (★★ step-size)
+  (for/fold ([in-first-pos #f]
+             [current-pos 0]
+             #:result in-first-pos)
+            ([i (in-range (* 50 1e6))])
+    (define pos (add1 (modulo (+ current-pos step-size) (add1 i))))
+    (values (if (= 1 pos) (add1 i) in-first-pos) pos))) 

2017/d17/star1.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★ ; 417
+348

2017/d17/star2.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★★ ; 34334221
+348

2017/d17/test1.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★ ; 638
+3