import { type ExtractNestedArrayType, flattenDeep } from './flattenDeep.ts';

/**
 * Recursively maps each element in an array using a provided iteratee function and then deeply flattens the resulting array.
 *
 * @template T - The type of elements within the array.
 * @template U - The type of elements within the returned array from the iteratee function.
 * @param arr - The array to flatten.
 * @param iteratee - The function that produces the new array elements. It receives the element, its index, and the array.
 * @returns A new array that has been flattened.
 *
 * @example
 * const result = flatMapDeep([1, 2, 3], n => [[n, n]]);
 * // [1, 1, 2, 2, 3, 3]
 */
export function flatMapDeep<T, U>(
  arr: readonly T[],
  iteratee: (item: T, index: number, array: readonly T[]) => U
): Array<ExtractNestedArrayType<U>> {
  return flattenDeep(arr.map((item: T, index: number) => iteratee(item, index, arr)));
}
