First (unsuccessful) attempt at day 7.1.

2018/internal/tree/breadthfirst.go

@@ -0,0 +1,44 @@
+// BreadthFirstOrder and its companion functions ended up not giving the
+// right solution, but I'm leaving them here as a useful generic tree
+// algorithm.
+package tree
+
+func BreadthFirstOrder(root *Node) []*Node {
+	seen := make(map[rune]bool)
+	return breadthFirstOrderR(root, seen)
+}
+
+func findTreeRoot(depMap map[rune]*Node) *Node {
+	for _, node := range depMap {
+		if len(node.Parents) == 0 {
+			return node
+		}
+	}
+
+	return nil
+}
+
+func breadthFirstOrderR(node *Node, seen map[rune]bool) []*Node {
+	order := []*Node{}
+
+	// handle root
+	if len(node.Parents) == 0 {
+		order = append(order, node)
+		seen[node.Name] = true
+	}
+
+	// add children we haven't seen before
+	for _, child := range node.Children {
+		if _, ok := seen[child.Name]; !ok {
+			order = append(order, child)
+			seen[child.Name] = true
+		}
+	}
+
+	// recurse
+	for _, child := range node.Children {
+		order = append(order, breadthFirstOrderR(child, seen)...)
+	}
+
+	return order
+}