"use strict";

const isIterable = require("is-iterable");
const mapAsync = require("../map-async");

module.exports = async function treeMapAsync(tree, mapper, options = {}) {
	let key = options.key ?? "children";

	async function step(subtree) {
		let mapped = await mapper(subtree);
		let modifiedProperties = {};

		if (isIterable(mapped[key])) {
			modifiedProperties[key] = await mapAsync(mapped[key], step);
		} else if (mapped[key] != null && typeof mapped[key] === "object") {
			modifiedProperties[key] = await step(mapped[key]);
		}

		// We track modified properties separately and (immutably) merge them at the end, because it's the fastest way to ensure that we don't mutate the input object under any circumstances
		return {
			... mapped,
			... modifiedProperties
		};
	}

	return isIterable(tree)
		? mapAsync(tree, step)
		: step(tree);
};