Correctly count the number of torrents in subcategories

PR #18261.
Closes #18137.

src/gui/categoryfiltermodel.cpp

@@ -56,7 +56,7 @@ public:
         clear();
         if (m_parent)
         {
-            m_parent->m_torrentsCount -= m_torrentsCount;
+            m_parent->decreaseTorrentsCount(m_torrentsCount);
             const QString uid = m_parent->m_children.key(this);
             m_parent->m_children.remove(uid);
             m_parent->m_childUids.removeOne(uid);
@@ -86,18 +86,18 @@ public:
         return m_torrentsCount;
     }
 
-    void increaseTorrentsCount()
+    void increaseTorrentsCount(const int delta = 1)
     {
-        ++m_torrentsCount;
+        m_torrentsCount += delta;
         if (m_parent)
-            m_parent->increaseTorrentsCount();
+            m_parent->increaseTorrentsCount(delta);
     }
 
-    void decreaseTorrentsCount()
+    void decreaseTorrentsCount(const int delta = 1)
     {
-        --m_torrentsCount;
+        m_torrentsCount -= delta;
         if (m_parent)
-            m_parent->decreaseTorrentsCount();
+            m_parent->decreaseTorrentsCount(delta);
     }
 
     int pos() const
@@ -139,7 +139,7 @@ public:
         item->m_parent = this;
         m_children[uid] = item;
         m_childUids.append(uid);
-        m_torrentsCount += item->torrentsCount();
+        increaseTorrentsCount(item->torrentsCount());
     }
 
     void clear()
@@ -408,9 +408,9 @@ void CategoryFilterModel::populate()
     m_rootItem->addChild(UID_UNCATEGORIZED, new CategoryModelItem(nullptr, tr("Uncategorized"), torrentsCount));
 
     using BitTorrent::Torrent;
-    for (const QString &categoryName : asConst(session->categories()))
-    {
     if (m_isSubcategoriesEnabled)
+    {
+        for (const QString &categoryName : asConst(session->categories()))
         {
             CategoryModelItem *parent = m_rootItem;
             for (const QString &subcat : asConst(session->expandCategory(categoryName)))
@@ -425,7 +425,10 @@ void CategoryFilterModel::populate()
                 parent = parent->child(subcatName);
             }
         }
+    }
     else
+    {
+        for (const QString &categoryName : asConst(session->categories()))
         {
             const int torrentsCount = std::count_if(torrents.begin(), torrents.end()
                     , [categoryName](Torrent *torrent) { return torrent->belongsToCategory(categoryName); });