Simplify QBezier::split()

Now that result objects can't alias source ones anymore, the order of
writes no longer matters, and we can clean this function up. The
pattern of the algorithm now becomes visible. Before, it was just
drowing in noise.

Change-Id: I36c55ce09d6e13a994c7eda17d96cfe960e7fb95
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 13 insertions, 23 deletions

diff --git a/src/gui/painting/qbezier_p.h b/src/gui/painting/qbezier_p.h
index 02b29c5ba7..1c49f82416 100644
--- a/src/gui/painting/qbezier_p.h
+++ b/src/gui/painting/qbezier_p.h
@@ -225,29 +225,19 @@ inline QPointF QBezier::secondDerivedAt(qreal t) const
 
 std::pair<QBezier, QBezier> QBezier::split() const
 {
-    std::pair<QBezier, QBezier> r;
-    auto firstHalf = &r.first;
-    auto secondHalf = &r.second;
-
-    qreal c = (x2 + x3)*.5;
-    firstHalf->x2 = (x1 + x2)*.5;
-    secondHalf->x3 = (x3 + x4)*.5;
-    firstHalf->x1 = x1;
-    secondHalf->x4 = x4;
-    firstHalf->x3 = (firstHalf->x2 + c)*.5;
-    secondHalf->x2 = (secondHalf->x3 + c)*.5;
-    firstHalf->x4 = secondHalf->x1 = (firstHalf->x3 + secondHalf->x2)*.5;
-
-    c = (y2 + y3)/2;
-    firstHalf->y2 = (y1 + y2)*.5;
-    secondHalf->y3 = (y3 + y4)*.5;
-    firstHalf->y1 = y1;
-    secondHalf->y4 = y4;
-    firstHalf->y3 = (firstHalf->y2 + c)*.5;
-    secondHalf->y2 = (secondHalf->y3 + c)*.5;
-    firstHalf->y4 = secondHalf->y1 = (firstHalf->y3 + secondHalf->y2)*.5;
-
-    return r;
+    const auto mid = [](QPointF lhs, QPointF rhs) { return (lhs + rhs) * .5; };
+
+    const QPointF mid_12 = mid(pt1(), pt2());
+    const QPointF mid_23 = mid(pt2(), pt3());
+    const QPointF mid_34 = mid(pt3(), pt4());
+    const QPointF mid_12_23 = mid(mid_12, mid_23);
+    const QPointF mid_23_34 = mid(mid_23, mid_34);
+    const QPointF mid_12_23__23_34 = mid(mid_12_23, mid_23_34);
+
+    return {
+        fromPoints(pt1(), mid_12, mid_12_23, mid_12_23__23_34),
+        fromPoints(mid_12_23__23_34, mid_23_34, mid_34, pt4()),
+    };
 }
 
 inline void QBezier::parameterSplitLeft(qreal t, QBezier *left)