diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index 263cf6f..2f3f387 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -14,4 +14,5 @@
                (:file "day3")
                (:file "day4")
                (:file "day5")
-               (:file "day6")))
+               (:file "day6")
+               (:file "day7")))
diff --git a/day7.lisp b/day7.lisp
new file mode 100644
index 0000000..761e649
--- /dev/null
+++ b/day7.lisp
@@ -0,0 +1,63 @@
+;;;; day9.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day9-parse-line (line)
+  (list (char line 5) (char line 36)))
+
+(defun day9-build ()
+  (let ((counts (make-hash-table :test 'equal))
+        (links (make-hash-table :test 'equal)))
+    (loop-line-by-line (puzzlepath "input7.txt")
+      :for (before later) := (day9-parse-line line)
+      :do (push later (gethash before links nil))
+      :do (incf (gethash later counts 0))
+      :do (setf (gethash before counts) (gethash before counts 0))
+      :finally (return (list counts links)))))
+
+(defun day9-next (counts)
+  (loop
+    :with next-key := nil
+    :for key :being :the :hash-keys :of counts
+    :when (and (or (null next-key)
+                   (char< key next-key))
+               (= 0 (gethash key counts)))
+    :do (setf next-key key)
+    :finally (return next-key)))
+
+(defun day9-task-time (letter)
+  (- (char-code (char-upcase letter)) 4)); #\A -> 65 - 4 -> 61
+
+(defun day9-shortest-worker (w1 w2)
+  (if (< (first w1) (first w2))
+      w1
+      w2))
+
+(defun day9-solve (&optional (worker-count 5))
+  (destructuring-bind (counts links) (day9-build)
+    (loop
+      :with seq := nil
+      :with workers := nil
+      :with time := 0
+      :for next := (day9-next counts)
+      :while (or workers next)
+      :when (or (= (length workers) worker-count)
+                (null next))
+      :do (let ((earliest (reduce #'day9-shortest-worker workers)))
+            (setf workers (remove earliest workers))
+            (dolist (child (gethash (cdr earliest) links))
+              (decf (gethash child counts)))
+            (push (cdr earliest) seq)
+            (setf time (car earliest)))
+      :when next
+      :do (push (cons (+ time (day9-task-time next)) next) workers)
+      :and :do (remhash next counts)
+      :finally (return (list (coerce (nreverse seq) 'string) time)))))
+
+(defun day9 ()
+  (destructuring-bind (seq time) (day9-solve 1)
+    (format t "With 1 worker the tasks are completed in this order: ~a and take ~a seconds.~%"
+            seq time))
+  (destructuring-bind (seq time) (day9-solve 5)
+      (format t "With 5 workers the tasks are completed in this order: ~a and take ~a seconds.~%"
+              seq time)))