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++ // Copyright (C) 2016 The Qt Company Ltd.
++ // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
++
++ #include "qquickfriction_p.h"
++ #include <qmath.h>
++
++ QT_BEGIN_NAMESPACE
++ /*!
++ \qmltype Friction
++ \nativetype QQuickFrictionAffector
++ \inqmlmodule QtQuick.Particles
++ \ingroup qtquick-particles
++ \inherits ParticleAffector
++ \brief For applying friction proportional to the particle's current velocity.
++
++ */
++
++ /*!
++ \qmlproperty real QtQuick.Particles::Friction::factor
++
++ A drag will be applied to moving objects which is this factor of their current velocity.
++ */
++ /*!
++ \qmlproperty real QtQuick.Particles::Friction::threshold
++
++ The drag will only be applied to objects with a velocity above the threshold velocity. The
++ drag applied will bring objects down to the threshold velocity, but no further.
++
++ The default threshold is 0
++ */
++ static qreal sign(qreal a)
++ {
++ return a >= 0 ? 1 : -1;
++ }
++
++ static const qreal epsilon = 0.00001;
++
++ QQuickFrictionAffector::QQuickFrictionAffector(QQuickItem *parent) :
++ QQuickParticleAffector(parent), m_factor(0.0), m_threshold(0.0)
++ {
++ }
++
++ bool QQuickFrictionAffector::affectParticle(QQuickParticleData *d, qreal dt)
++ {
++ if (!m_factor)
++ return false;
++ qreal curVX = d->curVX(m_system);
++ qreal curVY = d->curVY(m_system);
++ if (!curVX && !curVY)
++ return false;
++ qreal newVX = curVX + (curVX * m_factor * -1 * dt);
++ qreal newVY = curVY + (curVY * m_factor * -1 * dt);
++
++ if (!m_threshold) {
++ if (sign(curVX) != sign(newVX))
++ newVX = 0;
++ if (sign(curVY) != sign(newVY))
++ newVY = 0;
++ } else {
++ qreal curMag = qSqrt(curVX*curVX + curVY*curVY);
++ if (curMag <= m_threshold + epsilon)
++ return false;
++ qreal newMag = qSqrt(newVX*newVX + newVY*newVY);
++ if (newMag <= m_threshold + epsilon || //went past the threshold, stop there instead
++ sign(curVX) != sign(newVX) || //went so far past maybe it came out the other side!
++ sign(curVY) != sign(newVY)) {
++ qreal theta = qAtan2(curVY, curVX);
++ newVX = m_threshold * qCos(theta);
++ newVY = m_threshold * qSin(theta);
++ }
++ }
++
++ d->setInstantaneousVX(newVX, m_system);
++ d->setInstantaneousVY(newVY, m_system);
++ return true;
++ }
++ QT_END_NAMESPACE
++
++ #include "moc_qquickfriction_p.cpp"