Function groupBy

src/util/itertools.ts:8–26

(list: T[], func: (element: T) => U)

 * @returns Map from group keys to list of elements mapped to given key
 */
export function groupBy<T, U>(list: T[], func: (element: T) => U): Map<U, T[]> {
  const map = new Map<U, T[]>();

  list.forEach((element) => {
    const key = func(element);
    var group: T[];

    if (map.has(key)) {
      group = map.get(key)!;
    } else {
      group = [];
      map.set(key, group);
    }

    group.push(element);
  });

  return map;
}