Add clear, reserve and capacity methods to QPainterPath

This allows anticipating and reusing internal allocations of
QPainterPathElements instead of using the common `m_myPath = QPainterPath{}` pattern.

[ChangeLog][QtGui][QPainterPath] Added clear(), reserve(), capacity().
clear() removes allocated QPainterPath elements but preserves allocated memory, which can be
useful for application with complex paths that are often recreated. reserve() and capacity()
follow QVector semantics.

Change-Id: I763461e2a421feda9053d3eb512af2fcf07ade2b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

3 files changed, 86 insertions, 6 deletions

diff --git a/src/gui/painting/qpainterpath.cpp b/src/gui/painting/qpainterpath.cpp
index c5ccf0003d..3687bcf7d0 100644
--- a/src/gui/painting/qpainterpath.cpp
+++ b/src/gui/painting/qpainterpath.cpp
@@ -629,6 +629,55 @@ QPainterPath::~QPainterPath()
 }
 
 /*!
+    Clears the path elements stored.
+
+    This allows the path to reuse previous memory allocations.
+
+    \sa reserve(), capacity()
+    \since 5.13
+*/
+void QPainterPath::clear()
+{
+    if (!d_ptr)
+        return;
+
+    detach();
+    d_func()->clear();
+}
+
+/*!
+    Reserves a given amount of elements in QPainterPath's internal memory.
+
+    Attempts to allocate memory for at least \a size elements.
+
+    \sa clear(), capacity(), QVector::reserve()
+    \since 5.13
+*/
+void QPainterPath::reserve(int size)
+{
+    Q_D(QPainterPath);
+    if ((!d && size > 0) || (d && d->elements.capacity() < size)) {
+        detach();
+        d->elements.reserve(size);
+    }
+}
+
+/*!
+    Returns the number of elements allocated by the QPainterPath.
+
+    \sa clear(), reserve()
+    \since 5.13
+*/
+int QPainterPath::capacity() const
+{
+    Q_D(QPainterPath);
+    if (d)
+        return d->elements.capacity();
+
+    return 0;
+}
+
+/*!
     Closes the current subpath by drawing a line to the beginning of
     the subpath, automatically starting a new path. The current point
     of the new path is (0, 0).
@@ -2271,13 +2320,19 @@ static inline bool epsilonCompare(const QPointF &a, const QPointF &b, const QSiz
 bool QPainterPath::operator==(const QPainterPath &path) const
 {
     QPainterPathData *d = reinterpret_cast<QPainterPathData *>(d_func());
-    if (path.d_func() == d)
+    QPainterPathData *other_d = path.d_func();
+    if (other_d == d)
         return true;
-    else if (!d || !path.d_func())
+    else if (!d || !other_d) {
+        if (!d && other_d->elements.empty() && other_d->fillRule == Qt::OddEvenFill)
+            return true;
+        if (!other_d && d && d->elements.empty() && d->fillRule == Qt::OddEvenFill)
+            return true;
         return false;
-    else if (d->fillRule != path.d_func()->fillRule)
+    }
+    else if (d->fillRule != other_d->fillRule)
         return false;
-    else if (d->elements.size() != path.d_func()->elements.size())
+    else if (d->elements.size() != other_d->elements.size())
         return false;
 
     const qreal qt_epsilon = sizeof(qreal) == sizeof(double) ? 1e-12 : qreal(1e-5);
@@ -2287,8 +2342,8 @@ bool QPainterPath::operator==(const QPainterPath &path) const
     epsilon.rheight() *= qt_epsilon;
 
     for (int i = 0; i < d->elements.size(); ++i)
-        if (d->elements.at(i).type != path.d_func()->elements.at(i).type
-            || !epsilonCompare(d->elements.at(i), path.d_func()->elements.at(i), epsilon))
+        if (d->elements.at(i).type != other_d->elements.at(i).type
+            || !epsilonCompare(d->elements.at(i), other_d->elements.at(i), epsilon))
             return false;
 
     return true;
diff --git a/src/gui/painting/qpainterpath.h b/src/gui/painting/qpainterpath.h
index db39c1c5a0..770b8f48d0 100644
--- a/src/gui/painting/qpainterpath.h
+++ b/src/gui/painting/qpainterpath.h
@@ -97,8 +97,13 @@ public:
     { qSwap(d_ptr, other.d_ptr); return *this; }
 #endif
     ~QPainterPath();
+
     inline void swap(QPainterPath &other) Q_DECL_NOEXCEPT { d_ptr.swap(other.d_ptr); }
 
+    void clear();
+    void reserve(int size);
+    int capacity() const;
+
     void closeSubpath();
 
     void moveTo(const QPointF &p);
diff --git a/src/gui/painting/qpainterpath_p.h b/src/gui/painting/qpainterpath_p.h
index aab318bcab..a36c8005bc 100644
--- a/src/gui/painting/qpainterpath_p.h
+++ b/src/gui/painting/qpainterpath_p.h
@@ -194,6 +194,7 @@ public:
     inline bool isClosed() const;
     inline void close();
     inline void maybeMoveTo();
+    inline void clear();
 
     const QVectorPath &vectorPath() {
         if (!pathConverter)
@@ -290,6 +291,25 @@ inline void QPainterPathData::maybeMoveTo()
     }
 }
 
+inline void QPainterPathData::clear()
+{
+    Q_ASSERT(ref.load() == 1);
+
+    elements.clear();
+
+    cStart = 0;
+
+    bounds = {};
+    controlBounds = {};
+
+    require_moveTo = false;
+    dirtyBounds = false;
+    dirtyControlBounds = false;
+    convex = false;
+
+    delete pathConverter;
+    pathConverter = nullptr;
+}
 #define KAPPA qreal(0.5522847498)