Headings into structured Node test

Test.kt

data class Heading(val weight: Int, val text: String)
data class Node(val heading: Heading, val children: MutableList<Node>)



/**
 * Convert [headings] into structured [Node]
 *
 * @throws IllegalArgumentException if [headings] does not have any items
 */
fun headingsIntoStructure(headings: Iterator<Heading>): Node {
    if (!headings.hasNext()){
        throw IllegalArgumentException("Iterator cannot be empty")
    }

    lateinit var rootNode: Node

    val stack = Stack<Node>()

    headings.forEach { currentHeading ->
        val node = currentHeading.intoNode()

        if (stack.empty()) {
            rootNode = node
            stack.push(rootNode)
        } else {
            while (!stack.empty() && stack.peek().heading.weight >= currentHeading.weight) {
                stack.pop()
            }

            val currentNode = stack.peek()

            currentNode.children.add(node)
            stack.push(node)
        }
    }

    return rootNode
}

fun Heading.intoNode() = Node(this, arrayListOf())