diff --git a/2018/day07-2.go b/2018/day07-2.go
new file mode 100644
index 0000000..a282213
--- /dev/null
+++ b/2018/day07-2.go
@@ -0,0 +1,19 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/tree"
+	"internal/util"
+)
+
+func main() {
+	data := util.ReadInput()
+
+	// Build a tree of dependencies.
+	root := tree.BuildDependencyTree(data)
+
+	// Build
+	time := tree.SimulateConstruction(root)
+	fmt.Println(time)
+}
diff --git a/2018/internal/tree/construction.go b/2018/internal/tree/construction.go
new file mode 100644
index 0000000..e4355b1
--- /dev/null
+++ b/2018/internal/tree/construction.go
@@ -0,0 +1,54 @@
+package tree
+
+func SimulateConstruction(root *Node) int {
+	working := make(map[rune]*Node)
+	done := map[rune]bool{
+		0: true,
+	}
+	elapsed := 0
+
+	for {
+		// returns true when no work is left to do.
+		if tick(root, working, done) {
+			return elapsed
+		}
+
+		elapsed++
+	}
+}
+
+func tick(root *Node, working map[rune]*Node, done map[rune]bool) bool {
+	complete := true
+
+	// Find candidates, if we need new ones.
+	if len(working) < 5 {
+		candidates := findCandidates(root, done)
+		for _, candidate := range candidates {
+			if len(working) >= 5 {
+				break
+			}
+
+			// if we see a candidate that we can work on, put it in the map.
+			if working[candidate.Name] == nil {
+				working[candidate.Name] = candidate
+				complete = false
+			}
+		}
+	}
+
+	// work on each in-progress node.
+	for _, node := range working {
+		complete = false
+		node.Worked++
+
+		// if we're done, add it to done and delete from worker queue
+		// `-4` is exploiting the fact that the rune capital A is value
+		// 65 as an integer (and so on)
+		if node.Worked >= int(node.Name)-4 {
+			done[node.Name] = true
+			delete(working, node.Name)
+		}
+	}
+
+	return complete
+}
diff --git a/2018/internal/tree/tree.go b/2018/internal/tree/tree.go
index 9479098..65b8244 100644
--- a/2018/internal/tree/tree.go
+++ b/2018/internal/tree/tree.go
@@ -9,6 +9,7 @@ type Node struct {
 	Name     rune
 	Parents  []*Node
 	Children []*Node
+	Worked   int // the amount of time spent working on this node.
 }
 
 // BuildDependencyTree parses the input and returns the root node.