Method groupBy

data_structures/unstable_multimap.ts:775–790

* Groups items from an iterable by the result of `keyFn`, returning a new * multimap. Mirrors the shape of * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/groupBy | `Map.groupBy`, * but returns a code MultiMap so further mutation is supp

(
    items: Iterable<T>,
    keyFn: (item: T, index: number) => K,
  )

   * ```
   */
  static groupBy<K, T>(
    items: Iterable<T>,
    keyFn: (item: T, index: number) => K,
  ): MultiMap<K, T> {
    if (typeof keyFn !== "function") {
      throw new TypeError(
        `Cannot call MultiMap.groupBy: "keyFn" is not a function: received ${typeof keyFn}`,
      );
    }
    const map = new MultiMap<K, T>();
    let index = 0;
    for (const item of items) {
      map.add(keyFn(item, index++), item);
    }
    return map;
  }
}