Optimize QInotifyFileSystemWatcherEngine::getPathFromID()

The code basically wants to get the last element of equal_range(id).

The problem is that backwards iteration on QHash is very expensive,
because it's implemented as forward search with wrap-around at bucket
end.

So it was implementing its own equal_range with look-ahead. The
problem is that it compared each key in the equal_range twice: once in
the if, and once more in the following while iteration.

I expect to see this kind of algorithm more as we move away from the
fake bidirectionalism of QHash, so I decided to implement it in a
generic way. We can copy it somewhere else when we find more users.

Change-Id: I7951652107ab897f6a456035f02e0339835e078d
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/io/qfilesystemwatcher_inotify.cpp

@@ -422,16 +422,27 @@ void QInotifyFileSystemWatcherEngine::readFromInotify()
     }
 }
 
+template <typename Hash, typename Key>
+typename Hash::const_iterator
+find_last_in_equal_range(const Hash &c, const Key &key)
+{
+    // find c.equal_range(key).second - 1 without backwards iteration:
+    auto i = c.find(key);
+    const auto end = c.cend();
+    if (i == end)
+        return end;
+    decltype(i) prev;
+    do {
+        prev = i;
+        ++i;
+    } while (i != end && i.key() == key);
+    return prev;
+}
+
 QString QInotifyFileSystemWatcherEngine::getPathFromID(int id) const
 {
-    QHash<int, QString>::const_iterator i = idToPath.find(id);
+    auto i = find_last_in_equal_range(idToPath, id);
-    while (i != idToPath.constEnd() && i.key() == id) {
+    return i == idToPath.cend() ? QString() : i.value() ;
-        if ((i + 1) == idToPath.constEnd() || (i + 1).key() != id) {
-            return i.value();
-        }
-        ++i;
-    }
-    return QString();
 }
 
 QT_END_NAMESPACE