Lazy merge sort based on iterator.

mergesortirator.js

/* global Symbol */
'use strict';

/**
 * @param {?} iterable_a
 * @param {?} [iterable_b]
 */
export function mergesortirator(iterable_a, iterable_b) {

	// Grab the iterator from the provided iterable.
	const iterator_a = iterable_a[Symbol.iterator];
	
	if (iterable_b) {
		const iterator_b = iterable_b[Symbol.iterator];
		var next_a = null;
		var next_b = null;

		return {
			next: () => {

				if (!next_a) {
					next_a = iterator_a.next();
				}

				if (!next_b) {
					next_b = iterator_b.next();
				}

				if (next_a.done) {

					if (next_b.done) {
						return {
							done: true
						};
					}

					const returned = next_b;
					next_b = null;

					return {
						value: returned,
						done: false
					};

				}
				
				if (next_b.done) {
					
					const returned = next_a;
					next_a = null;
					
					return {
						value: returned,
						done: false
					};
					
				}

				if (greater(next_a, next_b)) {
					const returned = next_a;
					next_a = null;
					
					return {
						value: returned,
						done: false
					};

				}
				
				const returned = next_b;
				next_b = null;
				
				return {
					value: returned,
					done: false
				};

			}
		};
		
	}

	return {
		next: function() {

			const next = iterator_a.next();
			if (next.done) {
				return next;
			}

			return {
				value: null,
				done: false
			};

		}
	};

}