import concat from './internal/concat';
import doParallel from './internal/doParallel';

/**
 * Applies `iteratee` to each item in `coll`, concatenating the results. Returns
 * the concatenated list. The `iteratee`s are called in parallel, and the
 * results are concatenated as they return. There is no guarantee that the
 * results array will be returned in the original order of `coll` passed to the
 * `iteratee` function.
 *
 * @name concat
 * @static
 * @memberOf module:Collections
 * @method
 * @category Collection
 * @param {Array|Object} coll - A collection to iterate over.
 * @param {Function} iteratee - A function to apply to each item in `coll`.
 * The iteratee is passed a `callback(err, results)` which must be called once
 * it has completed with an error (which can be `null`) and an array of results.
 * Invoked with (item, callback).
 * @param {Function} [callback(err)] - A callback which is called after all the
 * `iteratee` functions have finished, or an error occurs. Results is an array
 * containing the concatenated results of the `iteratee` function. Invoked with
 * (err, results).
 * @example
 *
 * async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files) {
 *     // files is now a list of filenames that exist in the 3 directories
 * });
 */
export default doParallel(concat);