<?php
/**
 * Get correct renderer for the given request.
 *
 * @return
 *  String, a renderer.
 */
function drupal_requested_renderer() {
  $renderers = drupal_get_renderers();

  // First search if 'format' argument is set.
  if ($_GET['format'] && array_key_exists($_GET['format'], $renderers)) {
    return $_GET['format'];
  }
  else {
    $accept = drupal_parse_accept_header();

    // Search for a renderer that best match the request.
    foreach($renderers as $renderer => $mime_types) {
      foreach ($mime_types as $mime_type) {
        if (in_array($mime_type, $accept)) {
          // Renderer found, return it.
          return $renderer;
        }
      }
    }

    // No renderer found. Default to html.
    return 'html';
  }
}

/**
 * Parse HTTP Accept header and return an ordered list of mime types for
 * the current request.
 *
 * @return
 *   Ordered array of mime types.
 */
function drupal_parse_accept_header() {
  $accept = array();

  $accept_parts = explode(',', $_SERVER['HTTP_ACCEPT']);

  foreach ($accept_parts as $order => $part) {
    list($type, $quality) = preg_split('/;\s*q=/', $part);

    $accept[$type] = array(
      'order' => $order,
      'quality' => isset($quality) ? $quality : 1,
    );
  }
 
  uasort($accept, '_drupal_parse_accept_header_sort');

  return array_keys($accept);
}

/**
 * Helper function for sorting Accept header based on order
 * and quality values.
 * See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1
 */
function _drupal_parse_accept_header_sort($a, $b) {
  if ($a['quality'] == $b['quality']) {
    // Lower first.
    return ($a['order'] < $b['order']) ? -1 : 1;
  }
  else {
    // Higher first.
    return ($a['quality'] > $b['quality']) ? -1 : 1;
  }
}

/**
 * Returns a hardcoded list of renderers along with their supported mime types.
 */
function drupal_get_renderers() {
  // A hook should allow modules to add/alter types.
  // Specialized xml types should take precedence over 'xml'.
  return array(
    'html' => array('text/html', 'application/xhtml+xml'),
    'xml' => array('application/xml', 'text/xml', 'application/x-xml'),
    'json' => array('application/json', 'text/x-json'),
    'javascript' => array('application/javascript', 'text/javascript', 'application/x-javascript'),
    'text' => array('text/plain'),
  );
}

?>