[tag_history] add diff colour-coding to tag changes

ext/tag_history/main.php

@@ -301,6 +301,22 @@ class TagHistory extends Extension
 		", ["offset" => ($page_id - 1) * 100]);
     }
 
+    /**
+     * @return array<string, mixed>|null
+     */
+    public function get_previous_tags(int $image_id, int $id): ?array
+    {
+        global $database;
+        $row = $database->get_row("
+				SELECT tags
+				FROM tag_histories
+				WHERE image_id = :image_id AND id < :id
+				ORDER BY id DESC
+				LIMIT 1
+		", ["image_id" => $image_id, "id" => $id]);
+        return ($row ? $row : null);
+    }
+
     /**
      * This function attempts to revert all changes by a given IP within an (optional) timeframe.
      */

ext/tag_history/style.css

@@ -0,0 +1,2 @@
+.added-tag{background:lightgreen;}
+.deleted-tag{background:pink;text-decoration:line-through;}

ext/tag_history/theme.php

@@ -132,10 +132,30 @@ class TagHistoryTheme extends Themelet
             : null;
         $setter = A(["href" => make_link("user/" . url_escape($name))], $name);
 
+        $th = new TagHistory();
+        $pt = $th->get_previous_tags($image_id, $current_id);
+        if ($pt) {
+            $previous_tags = Tag::explode($pt["tags"]);
+        }
         $current_tags = Tag::explode($current_tags);
+        if ($pt) {
+            $tags = array_unique(array_merge($current_tags, $previous_tags));
+            sort($tags);
+        } else {
+            $tags = $current_tags;
+        }
         $taglinks = SPAN();
-        foreach ($current_tags as $tag) {
+        foreach ($tags as $tag) {
-            $taglinks->appendChild(A(["href" => search_link([$tag])], $tag));
+            $class = "";
+            if ($pt) {
+                if (!in_array($tag, $previous_tags)) {
+                    $class = "added-tag";
+                }
+                if (!in_array($tag, $current_tags)) {
+                    $class = "deleted-tag";
+                }
+            }
+            $taglinks->appendChild(A(["href" => search_link([$tag]), "class" => $class], $tag));
             $taglinks->appendChild(" ");
         }