/**
 * Copyright 2015 CANAL+ Group
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// Ugly transitory type to make duck typing work
type ArrayWithFlatMap<T> = T[] & {
  flatMap<U, This = undefined>(
    callback: (this: This, value: T, index: number, array: T[]) => U | U[],
    thisArg?: This,
  ): U[];
};

/**
 * Map each element using a mapping function, then flat the result into
 * a new array.
 * @param {Array.<*>} originalArray
 * @param {Function} fn
 */
export default function flatMap<T, U>(originalArray: T[], fn: (arg: T) => U[] | U): U[] {
  if (typeof (Array.prototype as ArrayWithFlatMap<T>).flatMap === "function") {
    return (originalArray as ArrayWithFlatMap<T>).flatMap(fn);
  }

  return originalArray.reduce((acc: U[], arg: T): U[] => {
    const r = fn(arg);
    if (Array.isArray(r)) {
      acc.push(...r);
      return acc;
    }
    acc.push(r);
    return acc;
  }, []);
}