diff --git a/2018/07-test.txt b/2018/07-test.txt
new file mode 100644
index 0000000..1dfd2ea
--- /dev/null
+++ b/2018/07-test.txt
@@ -0,0 +1,7 @@
+Step C must be finished before step A can begin.
+Step C must be finished before step F can begin.
+Step A must be finished before step B can begin.
+Step A must be finished before step D can begin.
+Step B must be finished before step E can begin.
+Step D must be finished before step E can begin.
+Step F must be finished before step E can begin.
\ No newline at end of file
diff --git a/2018/07.rkt b/2018/07.rkt
new file mode 100644
index 0000000..4748f7c
--- /dev/null
+++ b/2018/07.rkt
@@ -0,0 +1,47 @@
+#lang debug br
+(require graph)
+
+#|
+Track the progress through the steps with a dag,
+but track the state of the prerequisites with another dag (in the reverse direction)
+|#
+
+(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)
+  ;; 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))))
+
+(define (find-available g)
+  (filter prereqs-met? (get-vertices g)))
+
+(define (★)
+  (let loop ([vs-available (find-available prereqs)] [visited null])
+    (cond
+      [(= (length visited) (length (get-vertices dag)))
+       (apply string-append (reverse visited))]
+      [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)])])))
+
+#;(define (★★)
+    )
+#;(★★)
+
+(module+ test
+  (require rackunit)
+  (check-equal? (time (★)) "GNJOCHKSWTFMXLYDZABIREPVUQ")
+  #;(check-equal? (time (★★)) 566))
\ No newline at end of file
diff --git a/2018/07.txt b/2018/07.txt
new file mode 100644
index 0000000..86ba215
--- /dev/null
+++ b/2018/07.txt
@@ -0,0 +1,101 @@
+Step O must be finished before step C can begin.
+Step Y must be finished before step D can begin.
+Step N must be finished before step D can begin.
+Step G must be finished before step F can begin.
+Step C must be finished before step Z can begin.
+Step H must be finished before step K can begin.
+Step W must be finished before step T can begin.
+Step T must be finished before step F can begin.
+Step S must be finished before step I can begin.
+Step X must be finished before step B can begin.
+Step J must be finished before step A can begin.
+Step K must be finished before step D can begin.
+Step Z must be finished before step A can begin.
+Step A must be finished before step B can begin.
+Step L must be finished before step V can begin.
+Step F must be finished before step M can begin.
+Step B must be finished before step V can begin.
+Step M must be finished before step Q can begin.
+Step D must be finished before step E can begin.
+Step I must be finished before step U can begin.
+Step R must be finished before step V can begin.
+Step E must be finished before step U can begin.
+Step P must be finished before step V can begin.
+Step V must be finished before step Q can begin.
+Step U must be finished before step Q can begin.
+Step P must be finished before step U can begin.
+Step O must be finished before step F can begin.
+Step T must be finished before step M can begin.
+Step I must be finished before step Q can begin.
+Step M must be finished before step U can begin.
+Step R must be finished before step E can begin.
+Step T must be finished before step R can begin.
+Step H must be finished before step S can begin.
+Step L must be finished before step B can begin.
+Step S must be finished before step Q can begin.
+Step E must be finished before step Q can begin.
+Step B must be finished before step Q can begin.
+Step S must be finished before step M can begin.
+Step C must be finished before step D can begin.
+Step S must be finished before step R can begin.
+Step G must be finished before step D can begin.
+Step T must be finished before step E can begin.
+Step T must be finished before step Q can begin.
+Step N must be finished before step I can begin.
+Step S must be finished before step P can begin.
+Step N must be finished before step J can begin.
+Step X must be finished before step L can begin.
+Step G must be finished before step K can begin.
+Step N must be finished before step E can begin.
+Step H must be finished before step D can begin.
+Step H must be finished before step P can begin.
+Step O must be finished before step A can begin.
+Step V must be finished before step U can begin.
+Step F must be finished before step D can begin.
+Step B must be finished before step P can begin.
+Step T must be finished before step L can begin.
+Step I must be finished before step P can begin.
+Step K must be finished before step Z can begin.
+Step G must be finished before step M can begin.
+Step F must be finished before step Q can begin.
+Step J must be finished before step L can begin.
+Step H must be finished before step Q can begin.
+Step W must be finished before step R can begin.
+Step R must be finished before step U can begin.
+Step P must be finished before step Q can begin.
+Step D must be finished before step V can begin.
+Step G must be finished before step C can begin.
+Step Z must be finished before step B can begin.
+Step O must be finished before step H can begin.
+Step S must be finished before step A can begin.
+Step J must be finished before step Q can begin.
+Step N must be finished before step F can begin.
+Step L must be finished before step R can begin.
+Step O must be finished before step R can begin.
+Step W must be finished before step M can begin.
+Step J must be finished before step F can begin.
+Step G must be finished before step W can begin.
+Step K must be finished before step U can begin.
+Step D must be finished before step U can begin.
+Step W must be finished before step I can begin.
+Step E must be finished before step V can begin.
+Step Y must be finished before step Q can begin.
+Step L must be finished before step E can begin.
+Step S must be finished before step B can begin.
+Step T must be finished before step V can begin.
+Step C must be finished before step U can begin.
+Step M must be finished before step P can begin.
+Step G must be finished before step S can begin.
+Step B must be finished before step R can begin.
+Step K must be finished before step M can begin.
+Step X must be finished before step A can begin.
+Step R must be finished before step P can begin.
+Step B must be finished before step I can begin.
+Step C must be finished before step X can begin.
+Step O must be finished before step P can begin.
+Step D must be finished before step Q can begin.
+Step F must be finished before step B can begin.
+Step I must be finished before step R can begin.
+Step Y must be finished before step I can begin.
+Step M must be finished before step D can begin.
+Step F must be finished before step U can begin.
\ No newline at end of file