spread.js

export function getSpreadUpdate(levels, updates) {
	var update = {};

	var to_null_out = {};
	var accounted_for = {};

	var i = levels.length;
	while (i--) {
		var o = levels[i];
		var n = updates[i];

		if (n) {
			for (var key in o) {
				if (!(key in n)) to_null_out[key] = 1;
			}

			for (var key in n) {
				if (!accounted_for[key]) {
					update[key] = n[key];
					accounted_for[key] = 1;
				}
			}

			levels[i] = n;
		} else {
			for (var key in o) {
				accounted_for[key] = 1;
			}
		}
	}

	for (var key in to_null_out) {
		if (!(key in update)) update[key] = undefined;
	}

	return update;
}