Improve performance of QColor::name, now more than 4 times faster

Before: HexRgb: 0.00230 ms per iteration, HexArgb: 0.00290 ms per iteration
After:  HexRgb: 0.00051 ms per iteration, HexArgb: 0.00061 ms per iteration

This showed up as a relevant optimization when profiling KIconLoader
which uses QColor::name() as part of the key -- thanks to Mark Gaiser for
the investigation and first suggestion of a solution. I have also seen
customer code writing a replacement for QColor::name() because it was
too slow to be used as a hash key.

Change-Id: I009ccdd712ea0d869d466e2c9894e0cea58f0e68
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/gui/painting/qcolor.cpp b/src/gui/painting/qcolor.cpp
index 3f30c061dc..d0a60f3704 100644
--- a/src/gui/painting/qcolor.cpp
+++ b/src/gui/painting/qcolor.cpp
@@ -521,9 +521,10 @@ QString QColor::name(NameFormat format) const
 {
     switch (format) {
     case HexRgb:
-        return QString::asprintf("#%02x%02x%02x", red(), green(), blue());
+        return QLatin1Char('#') + QString::number(rgba() | 0x1000000, 16).rightRef(6);
     case HexArgb:
-        return QString::asprintf("#%02x%02x%02x%02x", alpha(), red(), green(), blue());
+        // it's called rgba() but it does return AARRGGBB
+        return QLatin1Char('#') + QString::number(rgba() | 0x100000000, 16).rightRef(8);
     }
     return QString();
 }