/**
  * index files for given configuration from file system and create a testable collection of files
  *
  * @param \PhpUnitTestGenerator\Configuration\ConfigurationInterface $configuration
  * @return \PhpUnitTestGenerator\Testable\Collection
  */
 public function indexFilesFromConfiguration(\PhpUnitTestGenerator\Configuration\ConfigurationInterface $configuration)
 {
     return new \PhpUnitTestGenerator\Testable\Collection($configuration, $this->indexFilesFromSource($configuration->getSourceDirectory()));
 }