adventofcode/2018/internal/tree/findorder.go

package tree

// FindOrder determines the correct order according to these rules:
// 1. A node cannot come before all of its parents have been included.
// 2. At any given time, the lowest-lettered available step must be executed.
func FindOrder(root *Node) []*Node {
	seen := map[rune]bool{
		0: true,
	}
	order := []*Node{}

	for {
		// continuously loop over the tree, retrieving the next usable node.
		candidates := findCandidates(root, seen)

		if len(candidates) == 0 {
			return order
		}
		sortNodes(candidates)
		order = append(order, candidates[0])
		seen[candidates[0].Name] = true
	}
}

func findCandidates(node *Node, seen map[rune]bool) []*Node {
	candidates := []*Node{}

	// if we've been seen before, we're clear to recurse
	if seen[node.Name] == true {
		for _, child := range node.Children {
			candidates = append(candidates, findCandidates(child, seen)...)
		}
		return candidates
	}

	// add the node, but only if all its parents have been seen
	for _, parent := range node.Parents {
		if seen[parent.Name] == false {
			return candidates
		}
	}

	// We haven't seen this one before, and its parents are seen.
	// Return it as a candidate.
	return append(candidates, node)
}