Replace code with simpler but equivalent code

The if-branching is unnecessary because
  ceil(a-b) == -floor(b-a)

Also add comments to clarify subtle differences in seemingly
identical code snippets.

Change-Id: Idfd72e4aecf53805183f5b2741336d805fbfbb6a
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

4 files changed, 27 insertions, 33 deletions

diff --git a/src/charts/axis/cartesianchartaxis.cpp b/src/charts/axis/cartesianchartaxis.cpp
index 4941bc23..62de5906 100644
--- a/src/charts/axis/cartesianchartaxis.cpp
+++ b/src/charts/axis/cartesianchartaxis.cpp
@@ -132,15 +132,16 @@ void CartesianChartAxis::updateMinorTickItems()
             expectedCount = qMax(expectedCount, 0);
         } else {
             const qreal interval = valueAxis->tickInterval();
-            qreal firstMajorTick = valueAxis->tickAnchor();
+            const qreal anchor = valueAxis->tickAnchor();
             const qreal max = valueAxis->max();
             const qreal min = valueAxis->min();
             const int _minorTickCount = valueAxis->minorTickCount();
 
-            if (min < firstMajorTick)
-                firstMajorTick = firstMajorTick - std::ceil((firstMajorTick - min) / interval) * interval;
-            else
-                firstMajorTick = firstMajorTick + std::floor((min - firstMajorTick) / interval) * interval;
+            // Find the closest major tick <= the min of the range, even if it's not drawn!
+            // This is where we'll start counting minor ticks from, because minor ticks
+            // might need to be drawn even before the first major tick.
+            const qreal ticksFromAnchor = (anchor - min) / interval;
+            const qreal firstMajorTick = anchor - std::ceil(ticksFromAnchor) * interval;
 
             const qreal deltaMinor = interval / qreal(_minorTickCount + 1);
             qreal minorTick = firstMajorTick + deltaMinor;
@@ -374,16 +375,16 @@ void CartesianChartAxis::updateLabelsValues(QValueAxis *axis)
             static_cast<ValueAxisLabel *>(labelItems().at(i))->setValue(value);
         }
     } else {
-        qreal value = axis->tickAnchor();
-        if (value > min())
-            value = value - std::floor((value - min()) / axis->tickInterval()) * axis->tickInterval();
-        else
-            value = value + std::ceil((min() - value) / axis->tickInterval()) * axis->tickInterval();
+        const qreal anchor = axis->tickAnchor();
+        const qreal interval = axis->tickInterval();
+        const qreal ticksFromAnchor = (anchor - min()) / interval;
+        const qreal firstMajorTick = anchor - std::floor(ticksFromAnchor) * interval;
 
         int i = axis->isReverse() ? labelItems().count()-1 : 0;
+        qreal value = firstMajorTick;
         while (value <= max()) {
             static_cast<ValueAxisLabel *>(labelItems().at(i))->setValue(value);
-            value += axis->tickInterval();
+            value += interval;
             i += axis->isReverse() ? -1 : 1;
         }
     }
diff --git a/src/charts/axis/chartaxiselement.cpp b/src/charts/axis/chartaxiselement.cpp
index b17f5cdb..d036041c 100644
--- a/src/charts/axis/chartaxiselement.cpp
+++ b/src/charts/axis/chartaxiselement.cpp
@@ -430,12 +430,10 @@ QStringList ChartAxisElement::createValueLabels(qreal min, qreal max, int ticks,
                 labels << presenter()->numberToString(value, 'f', n);
             }
         } else {
-            qreal value = tickAnchor;
-            if (value > min)
-                value = value - std::floor((value - min) / tickInterval) * tickInterval;
-            else
-                value = value + std::ceil((min - value) / tickInterval) * tickInterval;
+            const qreal ticksFromAnchor = (tickAnchor - min) / tickInterval;
+            const qreal firstMajorTick = tickAnchor - std::floor(ticksFromAnchor) * tickInterval;
 
+            qreal value = firstMajorTick;
             while (value <= max) {
                 labels << presenter()->numberToString(value, 'f', n);
                 value += tickInterval;
@@ -472,12 +470,10 @@ QStringList ChartAxisElement::createValueLabels(qreal min, qreal max, int ticks,
                 labels << formatLabel(formatSpec, array, value, precision, preStr, postStr);
             }
         } else {
-            qreal value = tickAnchor;
-            if (value > min)
-                value = value - std::floor((value - min) / tickInterval) * tickInterval;
-            else
-                value = value + std::ceil((min - value) / tickInterval) * tickInterval;
+            const qreal ticksFromAnchor = (tickAnchor - min) / tickInterval;
+            const qreal firstMajorTick = tickAnchor - std::floor(ticksFromAnchor) * tickInterval;
 
+            qreal value = firstMajorTick;
             while (value <= max) {
                 labels << formatLabel(formatSpec, array, value, precision, preStr, postStr);
                 value += tickInterval;
diff --git a/src/charts/axis/valueaxis/chartvalueaxisx.cpp b/src/charts/axis/valueaxis/chartvalueaxisx.cpp
index 8c937bff..bd9be1aa 100644
--- a/src/charts/axis/valueaxis/chartvalueaxisx.cpp
+++ b/src/charts/axis/valueaxis/chartvalueaxisx.cpp
@@ -78,19 +78,17 @@ QList<qreal> ChartValueAxisX::calculateLayout() const
         const qreal anchor = m_axis->tickAnchor();
         const qreal maxValue = max();
         const qreal minValue = min();
-        qreal value;
 
-        // Find the first major tick right after the min of range
-        if (anchor > minValue)
-            value = anchor - std::floor((anchor - minValue) / interval) * interval;
-        else
-            value = anchor + std::ceil((minValue - anchor) / interval) * interval;
+        // Find the first major tick right after the min of the range
+        const qreal ticksFromAnchor = (anchor - minValue) / interval;
+        const qreal firstMajorTick = anchor - std::floor(ticksFromAnchor) * interval;
 
         const QRectF &gridRect = gridGeometry();
         const qreal deltaX = gridRect.width() / (maxValue - minValue);
 
         QList<qreal> points;
         const qreal leftPos = gridRect.left();
+        qreal value = firstMajorTick;
         while (value <= maxValue) {
             points << (value - minValue) * deltaX + leftPos;
             value += interval;
diff --git a/src/charts/axis/valueaxis/chartvalueaxisy.cpp b/src/charts/axis/valueaxis/chartvalueaxisy.cpp
index 50ecf322..1b48e446 100644
--- a/src/charts/axis/valueaxis/chartvalueaxisy.cpp
+++ b/src/charts/axis/valueaxis/chartvalueaxisy.cpp
@@ -74,21 +74,20 @@ QList<qreal> ChartValueAxisY::calculateLayout() const
         return points;
     } else {
         const qreal interval = m_axis->tickInterval();
-        qreal value = m_axis->tickAnchor();
+        const qreal anchor = m_axis->tickAnchor();
         const qreal maxValue = max();
         const qreal minValue = min();
 
-        // Find the first major tick right after the min of range
-        if (value > minValue)
-            value = value - std::floor((value - minValue) / interval) * interval;
-        else
-            value = value + std::ceil((minValue - value) / interval) * interval;
+        // Find the first major tick right after the min of the range
+        const qreal ticksFromAnchor = (anchor - minValue) / interval;
+        const qreal firstMajorTick = anchor - std::floor(ticksFromAnchor) * interval;
 
         const QRectF &gridRect = gridGeometry();
         const qreal deltaY = gridRect.height() / (maxValue - minValue);
 
         QList<qreal> points;
         const qreal bottomPos = gridRect.bottom();
+        qreal value = firstMajorTick;
         while (value <= maxValue) {
             points << (value - minValue) * -deltaY + bottomPos;
             value += interval;