<?php
// $Id: comment_subject.module,v 1.7.2.3.2.2 2010-01-30 17:17:44 sinasalek Exp $

/**
 * Implementation of hook_form_alter().
 */
function comment_subject_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'comment_form' && empty($form['subject']['#default_value'])) {
    switch (arg(0)) {
      case 'node':
        // comment form bellow (e.g. node/NID)
        $node = node_load(arg(1));
        $subject = $node->title;
        break;
      case 'comment':
        if (arg(1) == 'reply') {
          if (is_numeric(arg(3))) {
            // replying to a comment (e.g. comment/reply/NID/CID)
            $comment = comment_load(arg(3));
            $subject = $comment->subject;
          }
          else {
            // replying to a node on a separated page (e.g. node/reply/NID)
            $node = node_load(arg(2));
            $subject = $node->title;
          }
        }
        elseif (arg(2) == 'edit' && is_numeric(arg(1))) {
          // editing a comment (e.g. comment/CID/edit)
          // subject shouldn't be empty (how could it be?)
          // therefore this point wouldn't be reached
          //
          // but, if for some reason (e.g. DB edit)
          // the subject of the comment becomes empty
          // it would require fetching its parent's title
          // nevertheless, testing the comment->pid to decide
          // whether to load node's title or parent's subject
          // is too complicated logic for a case that shouldn't happen
          // therefore, only avoid:
          // - PHP "Notice: Undefined variable: subject"
          // - #default_value = "Re: "
          return; // take no action at all
        }
        break;
    }
    if (!preg_match('/^' . preg_quote(t('Re:')) . '/i', $subject)) {
      $subject = t('Re:') . ' ' . $subject;
    }
    // comment subjects can not be longer than 64 characters
    $subject = truncate_utf8($subject, 64, TRUE, TRUE);

    // when comment subject is disabled $form['subject']['#access'] = FALSE
    $form['subject']['#default_value'] = $subject;
    // use 'hidden' when testing for quick inspect DOM (next line must be commented)
    //if (!$form['subject']['#access']) $form['subject'] = array('#type' => 'hidden', '#access' => TRUE) + $form['subject'];
  }
}