cartesian-product.js

// @flow
//
// flow-annotated variant of https://github.com/izaakschroeder/cartesian-product
//
function product<T>(...elements: $ReadOnlyArray<$ReadOnlyArray<T>>): $ReadOnlyArray<$ReadOnlyArray<T>> {
  const end = elements.length - 1;
  const result: Array<Array<T>> = [];

  function addTo(curr: Array<T>, start: number) {
		const first = elements[start];
    const last = (start === end);

		for (let i = 0; i < first.length; ++i) {
			var copy = curr.slice();
      copy.push(first[i]);

      // Handle the base case of the recursion, otherwise, make recursive call.
			if (last) {
				result.push(copy);
			} else {
				addTo(copy, start + 1);
			}
		}
  }

  // Split cases: empty inputs or start in at first element.
	if (elements.length) {
		addTo([], 0);
	} else {
		result.push([]);
  }
	return result;
}