bibcite_footnotes/bibcite_footnotes.module

<?php

/**
 * @file
 * Contains bibcite_footnotes.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function bibcite_footnotes_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the bibcite_footnotes module.
    case 'help.page.bibcite_footnotes':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Inline footnote links for BibCite References') . '</p>';
      return $output;

    default:
  }
}

/**
 * Implements hook_theme().
 */
function bibcite_footnotes_theme() {
  return [
    'bibcite_footnotes' => [
      'render element' => 'children',
    ],
  ];
}

function bibcite_footnotes_theme_registry_alter(&$theme_registry) {
  $theme_registry['footnote_list']['function'] = 'bibcite_footnotes_theme_footnote_list';
}

function bibcite_footnotes_theme_footnote_list($footnotes) {
  // @todo: change this ugly array for arguments in the function.
  $footnotes = $footnotes['footnotes']['#footnotes'];
  $str = '<ul class="footnotes">';
  // Drupal 7 requires we use "render element" which just introduces a wrapper
  // around the old array.
  // $footnotes = $footnotes['footnotes'];
  // loop through the footnotes.
  $view_builder = \Drupal::entityTypeManager()->getViewBuilder('bibcite_reference');
  foreach ($footnotes as $fn) {
    if (!is_array($fn['ref_id'])) {
      // Output normal footnote.
      $str .= '<li class="footnote" id="' . $fn['fn_id'] . '"><a class="footnote-label" href="#' . $fn['ref_id'] . '">' . $fn['value'] . '.</a> ';
      preg_match('/\[bibcite_reference:(\d*)\]/', $fn['text'], $matches);
      $reference_entity_id = $matches[1];
      if (!empty($reference_entity_id)) {
        $reference_storage = \Drupal::entityTypeManager()
          ->getStorage('bibcite_reference')
          ->load($reference_entity_id);

        $build = $view_builder->view($reference_storage, 'citation');
        $render = render($build);

        $str .= $render . "</li>\n";
      }
      else {
        $str .= $fn['text'] . "</li>\n";
      }
    }
    else {
      // Output footnote that has more than one reference to it in the body.
      // The only difference is to insert backlinks to all references.
      // Helper: we need to enumerate a, b, c...
      $abc = str_split("abcdefghijklmnopqrstuvwxyz");
      $i = 0;

      $str .= '<li class="footnote" id="' . $fn['fn_id'] . '"><a href="#' . $fn['ref_id'][0] . '" class="footnote-label">' . $fn['value'] . '.</a> ';
      foreach ($fn['ref_id'] as $ref) {
        $str .= '<a class="footnote-multi" href="#' . $ref . '">' . $abc[$i] . '.</a> ';
        $i++;
      }
      $str .= $fn['text'] . "</li>\n";
    }
  }
  $str .= "</ul>\nThat's all, folks";
  return $str;

}