class CssRewriteFilter

CssImportFilterTest.php in drupal/core/vendor/kriswallsmith/assetic/tests/Assetic/Test/Filter/CssImportFilterTest.php

CssRewriteFilterTest.php in drupal/core/vendor/kriswallsmith/assetic/tests/Assetic/Test/Filter/CssRewriteFilterTest.php

class CssRewriteFilter extends BaseCssFilter {
  public function filterLoad(AssetInterface $asset) {
  }
  public function filterDump(AssetInterface $asset) {
    $sourceBase = $asset
      ->getSourceRoot();
    $sourcePath = $asset
      ->getSourcePath();
    $targetPath = $asset
      ->getTargetPath();
    if (null === $sourcePath || null === $targetPath || $sourcePath == $targetPath) {
      return;
    }

    // learn how to get from the target back to the source
    if (false !== strpos($sourceBase, '://')) {
      list($scheme, $url) = explode('://', $sourceBase . '/' . $sourcePath, 2);
      list($host, $path) = explode('/', $url, 2);
      $host = $scheme . '://' . $host . '/';
      $path = false === strpos($path, '/') ? '' : dirname($path);
      $path .= '/';
    }
    else {

      // assume source and target are on the same host
      $host = '';

      // pop entries off the target until it fits in the source
      if ('.' == dirname($sourcePath)) {
        $path = str_repeat('../', substr_count($targetPath, '/'));
      }
      elseif ('.' == ($targetDir = dirname($targetPath))) {
        $path = dirname($sourcePath) . '/';
      }
      else {
        $path = '';
        while (0 !== strpos($sourcePath, $targetDir)) {
          if (false !== ($pos = strrpos($targetDir, '/'))) {
            $targetDir = substr($targetDir, 0, $pos);
            $path .= '../';
          }
          else {
            $targetDir = '';
            $path .= '../';
            break;
          }
        }
        $path .= ltrim(substr(dirname($sourcePath) . '/', strlen($targetDir)), '/');
      }
    }
    $content = $this
      ->filterReferences($asset
      ->getContent(), function ($matches) use ($host, $path) {
      if (false !== strpos($matches['url'], '://') || 0 === strpos($matches['url'], '//') || 0 === strpos($matches['url'], 'data:')) {

        // absolute or protocol-relative or data uri
        return $matches[0];
      }
      if ('/' == $matches['url'][0]) {

        // root relative
        return str_replace($matches['url'], $host . $matches['url'], $matches[0]);
      }

      // document relative
      $url = $matches['url'];
      while (0 === strpos($url, '../') && 2 <= substr_count($path, '/')) {
        $path = substr($path, 0, strrpos(rtrim($path, '/'), '/') + 1);
        $url = substr($url, 3);
      }
      $parts = array();
      foreach (explode('/', $host . $path . $url) as $part) {
        if ('..' === $part && count($parts) && '..' !== end($parts)) {
          array_pop($parts);
        }
        else {
          $parts[] = $part;
        }
      }
      return str_replace($matches['url'], implode('/', $parts), $matches[0]);
    });
    $asset
      ->setContent($content);
  }

}