Avoid real-rounding glitches near ends of a slider's range.

Floating point doesn't quite reliably satisfy certain standard
identities of arithmetic ...

Task-number: QTBUG-42358
Change-Id: I7417feedbfc1fd6f59510e0ee6d07d98ff7ddc46
Reviewed-by: Mitch Curtis <mitch.curtis@theqtcompany.com>

3 files changed, 107 insertions, 1 deletions

diff --git a/src/controls/Private/qquickrangemodel_p_p.h b/src/controls/Private/qquickrangemodel_p_p.h
index 67f0745c..235ba069 100644
--- a/src/controls/Private/qquickrangemodel_p_p.h
+++ b/src/controls/Private/qquickrangemodel_p_p.h
@@ -93,7 +93,11 @@ public:
             return minimum;
 
         const qreal scale = (maximum - minimum) / posRange;
-        return (pos - effectivePosAtMin()) * scale + minimum;
+        // Avoid perverse rounding glitches when at an end:
+        const qreal mid = (effectivePosAtMax() + effectivePosAtMin()) * 0.5;
+        if (pos < mid)
+            return (pos - effectivePosAtMin()) * scale + minimum;
+        return maximum - scale * (effectivePosAtMax() - pos);
     }
 
     qreal publicPosition(qreal position) const;
diff --git a/tests/auto/controls/data/slider/rounder.qml b/tests/auto/controls/data/slider/rounder.qml
new file mode 100644
index 00000000..b7191892
--- /dev/null
+++ b/tests/auto/controls/data/slider/rounder.qml
@@ -0,0 +1,75 @@
+/****************************************************************************
+**
+** Copyright (C) 2015 The Qt Company Ltd.
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** You may use this file under the terms of the BSD license as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.2
+import QtQuick.Controls 1.2
+import QtQuick.Controls.Styles 1.2
+
+/* Rounding errors in the value at end of range are elusive.
+ * They're sensitive to fine details in hard-to-anticipate ways.
+ */
+Item {
+    visible: true
+    /* Sensitive to slider width */
+    width: 800
+    height: 400
+
+    /* Let test know value and where to click */
+    property int waist: slider.y + slider.height / 2
+    property alias value: slider.value
+
+    Slider {
+        id: slider
+        width: parent.width
+
+        /* Sensitive to value range. */
+        minimumValue: 0
+        maximumValue: 100
+
+        style: SliderStyle {
+            /* Sensitive to handle size. */
+            handle: Rectangle {
+                implicitHeight: 26
+                implicitWidth: 26
+                color: "salmon"
+            }
+        }
+    }
+}
diff --git a/tests/auto/controls/data/tst_slider.qml b/tests/auto/controls/data/tst_slider.qml
index 305605f6..1589f2ab 100644
--- a/tests/auto/controls/data/tst_slider.qml
+++ b/tests/auto/controls/data/tst_slider.qml
@@ -330,5 +330,32 @@ Item {
 
             control.destroy()
         }
+
+        function test_dragRounding() {
+            // Regression test: ends of range should be exact, not 99.99999999 &c.
+            var component = Qt.createComponent("slider/rounder.qml")
+            compare(component.status, Component.Ready)
+            var control = component.createObject(container)
+
+            // Does moving to maximum (100) actually reach it ?
+            mousePress(control, 0, control.waist)
+            mouseMove(control, control.width, control.waist)
+            mouseRelease(control, control.width, control.waist)
+            // Equality checks are dodgy with floats, but this should still be exact:
+            verify(control.value == 100)
+            // Neither of the following caught the bug, with value 100 -1.421e-14
+            // compare(control.value, 100)
+            // fuzzyCompare(control.value, 100, 1e-16)
+
+            // Now check it all works going the other way, too:
+            mousePress(control, control.width, control.waist)
+            mouseMove(control, 0, control.waist)
+            mouseRelease(control, 0, control.waist)
+            verify(control.value == 0)
+
+            // Tidy up.
+            control.destroy()
+            component.destroy()
+        }
     }
 }