Speed up lookup time

By adding another variable we can get O(1) lookup time instead of O(n).
Fix up 5f415c292d.

src/gui/properties/peerlistwidget.cpp

@@ -328,6 +328,7 @@ void PeerListWidget::copySelectedPeers()
 void PeerListWidget::clear()
 {
     m_peerItems.clear();
+    m_itemsByIP.clear();
     const int nbrows = m_listModel->rowCount();
     if (nbrows > 0)
         m_listModel->removeRows(0, nbrows);
@@ -365,7 +366,13 @@ void PeerListWidget::loadPeers(const BitTorrent::TorrentHandle *torrent)
 
     // Remove peers that are gone
     for (const PeerEndpoint &peerEndpoint : asConst(existingPeers)) {
-        const QStandardItem *item = m_peerItems.take(peerEndpoint);
+        QStandardItem *item = m_peerItems.take(peerEndpoint);
+
+        QSet<QStandardItem *> &items = m_itemsByIP[peerEndpoint.address.ip];
+        items.remove(item);
+        if (items.isEmpty())
+            m_itemsByIP.remove(peerEndpoint.address.ip);
+
         m_listModel->removeRow(item->row());
     }
 }
@@ -387,6 +394,7 @@ void PeerListWidget::updatePeer(const BitTorrent::TorrentHandle *torrent, const
         m_listModel->setData(m_listModel->index(row, PeerListDelegate::IP_HIDDEN), peerIp);
 
         itemIter = m_peerItems.insert(peerEndpoint, m_listModel->item(row, PeerListDelegate::IP));
+        m_itemsByIP[peerEndpoint.address.ip].insert(itemIter.value());
     }
 
     const int row = (*itemIter)->row();
@@ -420,13 +428,9 @@ void PeerListWidget::updatePeer(const BitTorrent::TorrentHandle *torrent, const
 
 void PeerListWidget::handleResolved(const QHostAddress &ip, const QString &hostname) const
 {
-    for (auto iter = m_peerItems.cbegin(); iter != m_peerItems.cend(); ++iter) {
+    const QSet<QStandardItem *> items = m_itemsByIP.value(ip);
-        if (iter.key().address.ip == ip) {
+    for (QStandardItem *item : items)
-            QStandardItem *item {iter.value()};
         item->setData(hostname, Qt::DisplayRole);
-            break;
-        }
-    }
 }
 
 void PeerListWidget::handleSortColumnChanged(const int col)

src/gui/properties/peerlistwidget.h

@@ -30,6 +30,7 @@
 #define PEERLISTWIDGET_H
 
 #include <QHash>
+#include <QSet>
 #include <QTreeView>
 
 class QHostAddress;
@@ -85,6 +86,7 @@ private:
     PropertiesWidget *m_properties = nullptr;
     Net::ReverseResolution *m_resolver = nullptr;
     QHash<PeerEndpoint, QStandardItem *> m_peerItems;
+    QHash<QHostAddress, QSet<QStandardItem *>> m_itemsByIP;  // must be kept in sync with `m_peerItems`
     bool m_resolveCountries;
 };