Make minification dependent on file modification date

Here's a stab at the drush function to base minification on modification dates. Note that there are a few tweaks here:
* I created a hacked version of file_scan_directory (speedy_file_scan_directory()) which includes a date stamp with the returned $file. Core's file_scan_directory has a callback function option but only passes the $uri to it rather than the $file object so you can't actually modify the $file that is being returned.
* I've tried to abstract this code a bit so it's easier to duplicate- or even just extend
* I haven't tested this code yet. Just working on a proof of concept here

function drush_uglifyjs_uglify_files($uglifyjs = '') {
  // Path to the minified JS library
  $minified_directory = variable_get('speedy_js_path', DRUPAL_ROOT . '/' . drupal_get_path('module', 'speedy') . '/js/');
  // Ensure that we can write to this directory
  // @todo what is the error condition here?
  $minified_directory = file_prepare_directory($minified_directory, FILE_CREATE_DIRECTORY);

  // Find all JS files
  $all_js_files = speedy_file_scan_directory(DRUPAL_ROOT, '/.js/');

  // Current minfied files
  $current_minfied_files = variable_get('speedy_min_js_files', array());

  // Start minifying files
  foreach ($all_js_files as $file) {
    // By default we do not modify this file
    $minify = FALSE;

    // Make sure the file is not blacklisted.
    if (speedy_file_blacklisted($file->uri)) {
      $minify = FALSE;
    }
    // Has this file ever been minified?
    elseif (empty($current_minified_files[$file->uri])) {
      $minify = TRUE;
    }
    // Is the original modified date more recent than the current minified file?
    elseif ($current_minified_files[$file->uri] > $file->modified) {
      $minify = TRUE;
    }

    // Should we minify this file?
    if ($minify) {
      // Create a file path to the store directory
      $filepath = $minified_directory . preg_replace("/.js/", ".min.js", basename($file->uri));

      // Now we can attempt to generate the file
      if (uglify_minify_file($file->uri, $filepath)) {
        // Add the minified filepath to the list of saved files
        $file->minified_file = $filepath;
        $current_minfied_files[$file->uri] = $file;
        drush_log(dt('!filename was minified.', array('!filename' => $file->uri)), 'success');
      }
      else {
        // @todo Note that a file was created with the error information in it.
        drush_log(dt('There was an error when trying to minify !filename.', array('!filename' => $file->uri)), 'error');
        unset($current_minified_files[$file->uri]);
      }
    }

  }

  // Save the list of minified files
  variable_get('speedy_min_js_files', $current_minfied_files);
}

First sorry for this ridiculous patch. It's got changes all over the place. Basically this was a "add modification dates" patch and then suddenly snowballed. I've moved all the core functionality into speedy_minify() This allows you to call this function with your own callback to implement minification. I don't like this so much but it allows the module to handle all the file finding, blacklisting, date checking, etc. Personally I think this is preferable to having developers implement their own mechanisms for doing this. Anyway, I'm happy to discuss some of the changes or untangle things, it just happens to be what is working in my dev environment right now.

Seems like we could just store an md5 hash of the file rather than the modified date. In speedy_file_scan_directory() we could replace the modified date code with $files[$key]->md5 = md5_file($files[$key]->uri) and in speedy_minify() replace
elseif ($current_minified_files[$file->drupal_filepath]->modified > $file->modified)
with
elseif ($current_minified_files[$file->drupal_filepath]->md5 == md5_file($current_minified_files[$file->drupal_filepath]->uri))

That work for you?

True, but these would only be done on calls to speedy_minify() during generation- which is also likely to be called from drush- not ideal I agree with you. Is that a reasonable performance compromise?