diff --git a/17.rkt b/17.rkt
index bdb9499..d2f2a19 100644
--- a/17.rkt
+++ b/17.rkt
@@ -5,85 +5,68 @@
 ;; solution for day 17
 
 (struct pt [x y z w] #:transparent)
+(define pt-getters (list pt-x pt-y pt-z pt-w))
 
-(define (part1 input)
-  (define world (hash-copy input))
-  (define (simulate world)
-    (define new-world (make-hash))
-    (define min-x (apply min (map pt-x (hash-keys world))))
-    (define max-x (apply max (map pt-x (hash-keys world))))
-    (define min-y (apply min (map pt-y (hash-keys world))))
-    (define max-y (apply max (map pt-y (hash-keys world))))
-    (define min-z (apply min (map pt-z (hash-keys world))))
-    (define max-z (apply max (map pt-z (hash-keys world))))
-    (for* ([x (in-range (- min-x 1) (+ max-x 2))]
-           [y (in-range (- min-y 1) (+ max-y 2))]
-           [z (in-range (- min-z 1) (+ max-z 2))])
-      (define ct (for*/sum ([dx (in-range -1 2)]
-                            [dy (in-range -1 2)]
-                            [dz (in-range -1 2)]
-                            #:unless (and (zero? dx) (zero? dy) (zero? dz))
-                            #:when (hash-ref world (pt (+ x dx) (+ y dy) (+ z dz) 0) #f))
-                           1))
-      (define this (pt x y z 0))
-      (cond
-        [(hash-ref world this #f)
-         (when (or (= 2 ct) (= 3 ct))
-           (hash-set! new-world this #t))]
-        [else
-          (when (= 3 ct)
-            (hash-set! new-world this #t))]))
-    new-world)
+(define (pt+ . args)
+  (for/fold ([ans (pt 0 0 0 0)]) ([x (in-list args)])
+    (apply pt (map #{+ (% ans) (% x)} pt-getters))))
 
-  (for ([i (in-range 6)])
-    (set! world (simulate world)))
-  (length (filter identity (hash-values world))))
+(define (in-pts lower upper use-w?)
+  (define (in-range+ getter)
+    (in-range (sub1 (getter lower)) (+ 2 (getter upper))))
+  (for*/stream ([x (in-range+ pt-x)]
+                [y (in-range+ pt-y)]
+                [z (in-range+ pt-z)]
+                [w (if use-w? (in-range+ pt-w) (in-range 1))])
+    (pt x y z w)))
 
-(define (part2 input)
-  (define world (hash-copy input))
-  (define (simulate world)
-    (define new-world (make-hash))
-    (define min-x (apply min (map pt-x (hash-keys world))))
-    (define max-x (apply max (map pt-x (hash-keys world))))
-    (define min-y (apply min (map pt-y (hash-keys world))))
-    (define max-y (apply max (map pt-y (hash-keys world))))
-    (define min-z (apply min (map pt-z (hash-keys world))))
-    (define max-z (apply max (map pt-z (hash-keys world))))
-    (define min-w (apply min (map pt-w (hash-keys world))))
-    (define max-w (apply max (map pt-w (hash-keys world))))
-    (for* ([x (in-range (- min-x 1) (+ max-x 2))]
-           [y (in-range (- min-y 1) (+ max-y 2))]
-           [z (in-range (- min-z 1) (+ max-z 2))]
-           [w (in-range (- min-w 1) (+ max-w 2))])
-      (define ct (for*/sum ([dx (in-range -1 2)]
-                            [dy (in-range -1 2)]
-                            [dz (in-range -1 2)]
-                            [dw (in-range -1 2)]
-                            #:unless (and (zero? dx) (zero? dy) (zero? dz) (zero? dw))
-                            #:when (hash-ref world (pt (+ x dx) (+ y dy) (+ z dz) (+ w dw)) #f))
-                           1))
-      (define this (pt x y z w))
-      (cond
-        [(hash-ref world this #f)
-         (when (or (= 2 ct) (= 3 ct))
-           (hash-set! new-world this #t))]
-        [else
-          (when (= 3 ct)
-            (hash-set! new-world this #t))]))
-    new-world)
+(define nearby/4d
+  (for*/list ([x (in-range -1 2)]
+              [y (in-range -1 2)]
+              [z (in-range -1 2)]
+              [w (in-range -1 2)]
+              #:unless (and (zero? x) (zero? y) (zero? z) (zero? w)))
+    (pt x y z w)))
 
-  (for ([i (in-range 6)])
-    (set! world (simulate world)))
-  (length (filter identity (hash-values world))))
+(define nearby/3d (filter #{zero? (pt-w %)} nearby/4d))
+
+(define (calc-bounds world)
+  (define (helper getter reducer)
+    (apply reducer (set-map world getter)))
+  (define (helper2 reducer)
+    (apply pt (map #{helper % reducer} pt-getters)))
+  (values (helper2 min) (helper2 max)))
+
+(define (simulate world use-w?)
+  (define new-world (mutable-set))
+  (define-values [lower upper] (calc-bounds world))
+  (define nearby (if use-w? nearby/4d nearby/3d))
+
+  (for ([pt (in-pts lower upper use-w?)])
+    (define ct
+      (for*/sum ([nb (in-list nearby)])
+        (if (set-member? world (pt+ pt nb)) 1 0)))
+    (when (if (set-member? world pt) (<= 2 ct 3) (= 3 ct))
+      (set-add! new-world pt)))
+
+  new-world)
+
+(define (simulate/times world times use-w?)
+  (set-count
+    (for/fold ([world world]) ([_ (in-range times)])
+      (simulate world use-w?))))
+
+(define part1 #{simulate/times % 6 #f})
+(define part2 #{simulate/times % 6 #t})
 
 ;; parse input file
 (define (parse fname)
   (define input (file->lines fname))
-  (define world (make-hash))
+  (define world (mutable-set))
   (for ([line (in-list input)] [y (in-naturals)])
     (for ([char (in-string line)] [x (in-naturals)])
       (when (char=? #\# char)
-        (hash-set! world (pt x y 0 0) #t))))
+        (set-add! world (pt x y 0 0)))))
   world)
 
 (module+ test