diff --git a/2018/07.rkt b/2018/07.rkt
index 4748f7c..17bea3c 100644
--- a/2018/07.rkt
+++ b/2018/07.rkt
@@ -8,40 +8,68 @@ but track the state of the prerequisites with another dag (in the reverse direct
 
 (define dag (directed-graph null))
 (define prereqs (weighted-graph/directed null))
-(for ([ln (in-lines (open-input-file "07.txt"))])
-  (match-define (list left right) (regexp-match* #rx"(?<=[Ss]tep )." ln))
-  (add-directed-edge! dag left right)
-  (add-directed-edge! prereqs right left +inf.0))
 
-(define (activate-prereq! v1 v2)
+(define (init-graphs!)
+  (for ([ln (in-lines (open-input-file "07.txt"))])
+    (match-define (list (list left) (list right))
+      (map string->list (regexp-match* #rx"(?<=[Ss]tep )." ln)))
+    (add-directed-edge! dag left right)
+    (add-directed-edge! prereqs right left +inf.0)))
+
+(define (meet-prereq! v1 v2)
   ;; a prereq is "met" if the edge weight is zero
   (add-directed-edge! prereqs v2 v1 0))
 
 (define (prereqs-met? v)
   ;; check if all v's edges in the prereq graph are zero
-  (andmap zero? (map (λ (n) (edge-weight prereqs v n)) (get-neighbors prereqs v))))
+  (zero? (for/sum ([n (in-list (get-neighbors prereqs v))])
+           (edge-weight prereqs v n))))
 
 (define (find-available g)
   (filter prereqs-met? (get-vertices g)))
 
 (define (★)
-  (let loop ([vs-available (find-available prereqs)] [visited null])
+  (init-graphs!)
+  (let loop ([vs-available (find-available prereqs)] [done null])
     (cond
-      [(= (length visited) (length (get-vertices dag)))
-       (apply string-append (reverse visited))]
+      [(= (length done) (length (get-vertices dag))) (list->string (reverse done))]
       [else
-       (match (sort vs-available string<?)
-         [(list this-v others ...)
-          (for-each (λ (n) (activate-prereq! this-v n)) (get-neighbors dag this-v))
-          (define this-visited (cons this-v visited))
-          (loop (filter-not (λ (v) (member v this-visited)) (find-available prereqs))
-                this-visited)])])))
+       (match-define (list this-v others ...) (sort vs-available char<?))
+       (for ([n (in-list (get-neighbors dag this-v))])
+         (meet-prereq! this-v n))
+       (define this-visited (cons this-v done))
+       (loop (filter-not (λ (v) (memv v this-visited)) (find-available prereqs))
+             this-visited)])))
+
+(define (worker-done? worker)
+  (match-define (cons char time) worker)
+  (= time (+ 60 (- (char->integer char) 65))))
 
-#;(define (★★)
-    )
-#;(★★)
+(define (★★)
+  (init-graphs!)
+  (define worker-count 5)
+  (let loop ([workers null][done null][steps 0])
+    (cond
+      [(= (length done) (length (get-vertices dag))) (sub1 steps)]
+      [else
+       (define-values (done-ws working-ws) (partition worker-done? workers))
+       (define done-vs (map car done-ws))
+       (for* ([v (in-list done-vs)]
+              [n (in-list (get-neighbors dag v))])
+         (meet-prereq! v n))
+       (define next-done (append done-vs done))
+       (define updated-ws (for/list ([w (in-list working-ws)])
+                            (match-define (cons v time) w)
+                            (cons v (add1 time))))
+       (define vs-available (for/list ([v (in-list (find-available prereqs))]
+                                       #:unless (memv v (append next-done (map car updated-ws))))
+                              v))
+       (define new-vs (take (sort vs-available char<?)
+                            (min (length vs-available) (- worker-count (length working-ws)))))
+       (define new-workers (map (λ (v) (cons v 0)) new-vs))
+       (loop (append new-workers updated-ws) next-done (add1 steps))])))
 
 (module+ test
   (require rackunit)
   (check-equal? (time (★)) "GNJOCHKSWTFMXLYDZABIREPVUQ")
-  #;(check-equal? (time (★★)) 566))
\ No newline at end of file
+  (check-equal? (time (★★)) 886))
\ No newline at end of file