Optimize QSet set operations

Replace the identity check with a check for the underlying QHash
objects being shared and replace backwards iteration, which is really
forwards iteration with wrapping at bucket boundaries, with forward
iteration. QSet cannot contain duplicates, so the order in which the
RHS elements are presented to the algorithms does not matter.

Change-Id: Iad8528e3a9501b14cb85601b221a848aad91480c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 9 insertions, 16 deletions

diff --git a/src/corelib/tools/qset.h b/src/corelib/tools/qset.h
index 83e574bf1c..98caabfb57 100644
--- a/src/corelib/tools/qset.h
+++ b/src/corelib/tools/qset.h
@@ -284,11 +284,9 @@ Q_INLINE_TEMPLATE void QSet<T>::reserve(int asize) { q_hash.reserve(asize); }
 template <class T>
 Q_INLINE_TEMPLATE QSet<T> &QSet<T>::unite(const QSet<T> &other)
 {
-    QSet<T> copy(other);
-    typename QSet<T>::const_iterator i = copy.constEnd();
-    while (i != copy.constBegin()) {
-        --i;
-        insert(*i);
+    if (!q_hash.isSharedWith(other.q_hash)) {
+        for (const T &e : other)
+            insert(e);
     }
     return *this;
 }
@@ -306,11 +304,9 @@ Q_INLINE_TEMPLATE QSet<T> &QSet<T>::intersect(const QSet<T> &other)
         copy2 = *this;
         *this = copy1;
     }
-    typename QSet<T>::const_iterator i = copy1.constEnd();
-    while (i != copy1.constBegin()) {
-        --i;
-        if (!copy2.contains(*i))
-            remove(*i);
+    for (const auto &e : qAsConst(copy1)) {
+        if (!copy2.contains(e))
+            remove(e);
     }
     return *this;
 }
@@ -346,14 +342,11 @@ Q_INLINE_TEMPLATE bool QSet<T>::intersects(const QSet<T> &other) const
 template <class T>
 Q_INLINE_TEMPLATE QSet<T> &QSet<T>::subtract(const QSet<T> &other)
 {
-    if (&other == this) {
+    if (q_hash.isSharedWith(other.q_hash)) {
         clear();
     } else {
-        auto i = other.constEnd();
-        while (i != other.constBegin()) {
-            --i;
-            remove(*i);
-        }
+        for (const auto &e : other)
+            remove(e);
     }
     return *this;
 }