Examples: use std::atan2 for simper angle calculations

Using std::atan2 gets the right answer directly from dy and dx,
without having to fix up quadrant as we needed to with acos (albeit we
have to negate dy in some cases, to match prior sense of angles).  In
the process, it avoids explicit division, which would be an error when
the line's length is zero.

Change-Id: Ia2923159d38834e08e6f15cbff6766ed419fa804
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

3 files changed, 7 insertions, 16 deletions

diff --git a/examples/widgets/graphicsview/collidingmice/mouse.cpp b/examples/widgets/graphicsview/collidingmice/mouse.cpp
index b94c650c18..cc1a15cd82 100644
--- a/examples/widgets/graphicsview/collidingmice/mouse.cpp
+++ b/examples/widgets/graphicsview/collidingmice/mouse.cpp
@@ -54,7 +54,7 @@
 #include <QPainter>
 #include <QStyleOption>
 
-#include <math.h>
+#include <cmath>
 
 static const double Pi = 3.14159265358979323846264338327950288419717;
 static double TwoPi = 2.0 * Pi;
@@ -140,9 +140,7 @@ void Mouse::advance(int step)
 //! [5]
     QLineF lineToCenter(QPointF(0, 0), mapFromScene(0, 0));
     if (lineToCenter.length() > 150) {
-        qreal angleToCenter = ::acos(lineToCenter.dx() / lineToCenter.length());
-        if (lineToCenter.dy() < 0)
-            angleToCenter = TwoPi - angleToCenter;
+        qreal angleToCenter = std::atan2(lineToCenter.dy(), lineToCenter.dx());
         angleToCenter = normalizeAngle((Pi - angleToCenter) + Pi / 2);
 
         if (angleToCenter < Pi && angleToCenter > Pi / 4) {
@@ -171,9 +169,7 @@ void Mouse::advance(int step)
             continue;
 
         QLineF lineToMouse(QPointF(0, 0), mapFromItem(item, 0, 0));
-        qreal angleToMouse = ::acos(lineToMouse.dx() / lineToMouse.length());
-        if (lineToMouse.dy() < 0)
-            angleToMouse = TwoPi - angleToMouse;
+        qreal angleToMouse = std::atan2(lineToMouse.dy(), lineToMouse.dx());
         angleToMouse = normalizeAngle((Pi - angleToMouse) + Pi / 2);
 
         if (angleToMouse >= 0 && angleToMouse < Pi / 2) {
diff --git a/examples/widgets/graphicsview/diagramscene/arrow.cpp b/examples/widgets/graphicsview/diagramscene/arrow.cpp
index 012b9ea2ed..ae7fa7af90 100644
--- a/examples/widgets/graphicsview/diagramscene/arrow.cpp
+++ b/examples/widgets/graphicsview/diagramscene/arrow.cpp
@@ -51,7 +51,7 @@
 
 #include "arrow.h"
 
-#include <math.h>
+#include <cmath>
 
 #include <QPen>
 #include <QPainter>
@@ -132,9 +132,7 @@ void Arrow::paint(QPainter *painter, const QStyleOptionGraphicsItem *,
     setLine(QLineF(intersectPoint, myStartItem->pos()));
 //! [5] //! [6]
 
-    double angle = ::acos(line().dx() / line().length());
-    if (line().dy() >= 0)
-        angle = (Pi * 2) - angle;
+    double angle = std::atan2(-line().dy(), line().dx());
 
     QPointF arrowP1 = line().p1() + QPointF(sin(angle + Pi / 3) * arrowSize,
                                     cos(angle + Pi / 3) * arrowSize);
diff --git a/examples/widgets/graphicsview/elasticnodes/edge.cpp b/examples/widgets/graphicsview/elasticnodes/edge.cpp
index e794e803cf..d9e4c62e34 100644
--- a/examples/widgets/graphicsview/elasticnodes/edge.cpp
+++ b/examples/widgets/graphicsview/elasticnodes/edge.cpp
@@ -51,12 +51,11 @@
 #include "edge.h"
 #include "node.h"
 
-#include <math.h>
+#include <cmath>
 
 #include <QPainter>
 
 static const double Pi = 3.14159265358979323846264338327950288419717;
-static double TwoPi = 2.0 * Pi;
 
 //! [0]
 Edge::Edge(Node *sourceNode, Node *destNode)
@@ -139,9 +138,7 @@ void Edge::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
 
 //! [6]
     // Draw the arrows
-    double angle = ::acos(line.dx() / line.length());
-    if (line.dy() >= 0)
-        angle = TwoPi - angle;
+    double angle = std::atan2(-line.dy(), line.dx());
 
     QPointF sourceArrowP1 = sourcePoint + QPointF(sin(angle + Pi / 3) * arrowSize,
                                                   cos(angle + Pi / 3) * arrowSize);