/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @format
 * @flow strict
 */

// Extract platform extension: index.ios.js -> ios
export default function getPlatformExtension(
  file: string,
  platforms: $ReadOnlySet<string>,
): ?string {
  const last = file.lastIndexOf('.');
  const secondToLast = file.lastIndexOf('.', last - 1);
  if (secondToLast === -1) {
    return null;
  }
  const platform = file.substring(secondToLast + 1, last);
  return platforms.has(platform) ? platform : null;
}