QWaylandDisplay: replace a QMap<pair<QString,int>, pointer*> with a std::vector<{QString, int, unique_ptr}>

The vector is sorted, so lookup performance is the same, or
better. Insertion performance is worse, of course, at least in big-O
terms, but probably not in practice.

But now the good part: saves a stunning 7.2KiB (0.6%) in text size on
optimized AMD 64 Linux GCC 9.1 builds.

Change-Id: Ie604f83d7d51111430db1d0bb89d8cc3a52301bf
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/plugins/platforms/wayland/qwaylandcursor.cpp

@@ -54,7 +54,7 @@ QT_BEGIN_NAMESPACE
 
 namespace QtWaylandClient {
 
-QWaylandCursorTheme *QWaylandCursorTheme::create(QWaylandShm *shm, int size, const QString &themeName)
+std::unique_ptr<QWaylandCursorTheme> QWaylandCursorTheme::create(QWaylandShm *shm, int size, const QString &themeName)
 {
     QByteArray nameBytes = themeName.toLocal8Bit();
     struct ::wl_cursor_theme *theme = wl_cursor_theme_load(nameBytes.constData(), size, shm->object());
@@ -64,7 +64,7 @@ QWaylandCursorTheme *QWaylandCursorTheme::create(QWaylandShm *shm, int size, con
         return nullptr;
     }
 
-    return new QWaylandCursorTheme(theme);
+    return std::unique_ptr<QWaylandCursorTheme>{new QWaylandCursorTheme(theme)};
 }
 
 QWaylandCursorTheme::~QWaylandCursorTheme()

src/plugins/platforms/wayland/qwaylandcursor_p.h

@@ -57,6 +57,8 @@
 
 #if QT_CONFIG(cursor)
 
+#include <memory>
+
 struct wl_cursor;
 struct wl_cursor_image;
 struct wl_cursor_theme;
@@ -73,7 +75,7 @@ class QWaylandShm;
 class Q_WAYLAND_CLIENT_EXPORT QWaylandCursorTheme
 {
 public:
-    static QWaylandCursorTheme *create(QWaylandShm *shm, int size, const QString &themeName);
+    static std::unique_ptr<QWaylandCursorTheme> create(QWaylandShm *shm, int size, const QString &themeName);
     ~QWaylandCursorTheme();
     ::wl_cursor *cursor(Qt::CursorShape shape);
 

src/plugins/platforms/wayland/qwaylanddisplay.cpp

@@ -85,6 +85,8 @@
 
 #include <errno.h>
 
+#include <tuple> // for std::tie
+
 QT_BEGIN_NAMESPACE
 
 namespace QtWaylandClient {
@@ -183,7 +185,7 @@ QWaylandDisplay::~QWaylandDisplay(void)
     delete mDndSelectionHandler.take();
 #endif
 #if QT_CONFIG(cursor)
-    qDeleteAll(mCursorThemes);
+    mCursorThemes.clear();
 #endif
     if (mDisplay)
         wl_display_disconnect(mDisplay);
@@ -602,15 +604,30 @@ QWaylandCursor *QWaylandDisplay::waylandCursor()
     return mCursor.data();
 }
 
+auto QWaylandDisplay::findExistingCursorTheme(const QString &name, int pixelSize) const noexcept
+    -> FindExistingCursorThemeResult
+{
+    const auto byNameAndSize = [](const WaylandCursorTheme &lhs, const WaylandCursorTheme &rhs) {
+        return std::tie(lhs.pixelSize, lhs.name) < std::tie(rhs.pixelSize, rhs.name);
+    };
+
+    const WaylandCursorTheme prototype = {name, pixelSize, nullptr};
+
+    const auto it = std::lower_bound(mCursorThemes.cbegin(), mCursorThemes.cend(), prototype, byNameAndSize);
+    if (it != mCursorThemes.cend() && it->name == name && it->pixelSize == pixelSize)
+        return {it, true};
+    else
+        return {it, false};
+}
+
 QWaylandCursorTheme *QWaylandDisplay::loadCursorTheme(const QString &name, int pixelSize)
 {
-    if (auto *theme = mCursorThemes.value({name, pixelSize}, nullptr))
+    const auto result = findExistingCursorTheme(name, pixelSize);
-        return theme;
+    if (result.found)
+        return result.theme();
 
-    if (auto *theme = QWaylandCursorTheme::create(shm(), pixelSize, name)) {
+    if (auto theme = QWaylandCursorTheme::create(shm(), pixelSize, name))
-        mCursorThemes[{name, pixelSize}] = theme;
+        return mCursorThemes.insert(result.position, {name, pixelSize, std::move(theme)})->theme.get();
-        return theme;
-    }
 
     return nullptr;
 }

src/plugins/platforms/wayland/qwaylanddisplay_p.h

@@ -238,7 +238,22 @@ private:
     QList<Listener> mRegistryListeners;
     QWaylandIntegration *mWaylandIntegration = nullptr;
 #if QT_CONFIG(cursor)
-    QMap<std::pair<QString, int>, QWaylandCursorTheme *> mCursorThemes; // theme name and size
+    struct WaylandCursorTheme {
+        QString name;
+        int pixelSize;
+        std::unique_ptr<QWaylandCursorTheme> theme;
+    };
+    std::vector<WaylandCursorTheme> mCursorThemes;
+
+    struct FindExistingCursorThemeResult {
+        std::vector<WaylandCursorTheme>::const_iterator position;
+        bool found;
+
+        QWaylandCursorTheme *theme() const noexcept
+        { return found ? position->theme.get() : nullptr; }
+    };
+    FindExistingCursorThemeResult findExistingCursorTheme(const QString &name, int pixelSize) const;
+
     QScopedPointer<QWaylandCursor> mCursor;
 #endif
 #if QT_CONFIG(wayland_datadevice)