WIP ibidem first steps

src/Plugin/Filter/ReferenceFootnotesFilter.php

@@ -18,6 +18,7 @@ use Drupal\footnotes\Plugin\Filter\FootnotesFilter;
  *   cache = FALSE,
  *   settings = {
  *     "footnotes_collapse" = FALSE,
+ *     "footnotes_ibid" = FALSE,
  *     "notes_section_label" = "Notes",
  *     "references_section_label" = "References"
  *   },
@@ -66,10 +67,15 @@ class ReferenceFootnotesFilter extends FootnotesFilter {
   public function settingsForm(array $form, FormStateInterface $form_state) {
     $settings['footnotes_collapse'] = [
       '#type' => 'checkbox',
-      '#title' => t('Collapse reference footnotes with identical content'),
+      '#title' => $this->t('Collapse reference footnotes with identical content'),
       '#default_value' => $this->settings['footnotes_collapse'],
       '#description' => t('If two reference footnotes have the exact same content, they will be collapsed into one as if using the same value="" attribute.'),
     ];
+    $settings['footnotes_ibid'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Display subsequent instances of multiple references with \'Ibid.\''),
+      '#default_value' => $this->settings['footnotes_ibid'],
+    ];
     $settings['notes_section_label'] = [
       '#type' => 'textfield',
       '#title' => t('Notes section label'),
@@ -133,6 +139,9 @@ class ReferenceFootnotesFilter extends FootnotesFilter {
     }
 
     if ($op == 'output footer') {
+      if ($this->settings['footnotes_ibid']) {
+        $this->ibidemify($store_matches);
+      }
       if (count($store_matches) > 0) {
         // Only if there are stored fn matches, pass the array of fns to be
         // themed as a list Drupal 7 requires we use "render element" which
@@ -304,4 +313,24 @@ class ReferenceFootnotesFilter extends FootnotesFilter {
     }
     return $value;
   }
+
+  protected function ibidemify(&$footnotes) {
+    $prev_reference_id = FALSE;
+    foreach ($footnotes as $index => $fn) {
+      if (!empty($fn['text'])) {
+        $prev_reference_id = FALSE;
+        continue;
+      }
+      if ($prev_reference_id) {
+        if ($fn['reference'] == $prev_reference_id) {
+          unset($footnotes[$index]['reference']);
+          $footnotes[$index]['text'] = $this->t('Ibid.');
+          continue;
+        }
+      }
+      $prev_reference_id = $fn['reference']; // Could be empty, that's OK.
+
+    }
+    ksm($footnotes);
+  }
 }