QTimeZone: don't use QSet, use sorted QList

QSet, as a node-based container, requires one memory allocation per element
inserted. QList, as a contiguous-memory container (at least in the case of
a QByteArray payload), requires one memory allocation per container.

The higher lookup speed might still speak for using QSet, but there are only
two uses of the sets:

1. Checking for existence (or lack thereof) of timezone names.
   For this, first generating a container full of data just to check for
   existence of one item of data is extremely wasteful. The QTZPrivate
   API should be extended to allow said lookup to be performed on the
   native data store instead.

That leaves

2. Returning a sorted(!) list(!) from the public QTimeZone API.
   There is no reason why, during the construction of those sorted
   lists, the data should be held in a set. Instead, the well-known
   technique of first cramming everything into a result container,
   which is subsequently sorted and has its duplicates removed,
   can be used here.

Saves more than 8K of text size on AMD64 stripped release builds.

Change-Id: I71c2298e94e02d55b0c9fb6f7ebeaed79a1fe2db
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 22 insertions, 20 deletions

diff --git a/src/corelib/tools/qtimezone.cpp b/src/corelib/tools/qtimezone.cpp
index b6160a3fd8..c2ee99a2d8 100644
--- a/src/corelib/tools/qtimezone.cpp
+++ b/src/corelib/tools/qtimezone.cpp
@@ -378,12 +378,10 @@ QTimeZone::QTimeZone(int offsetSeconds)
 
 QTimeZone::QTimeZone(const QByteArray &ianaId, int offsetSeconds, const QString &name,
                      const QString &abbreviation, QLocale::Country country, const QString &comment)
+    : d()
 {
-    // ianaId must be a valid ID and must not clash with the standard system names
-    if (QTimeZonePrivate::isValidId(ianaId) && !availableTimeZoneIds().contains(ianaId))
+    if (!isTimeZoneIdAvailable(ianaId))
         d = new QUtcTimeZonePrivate(ianaId, offsetSeconds, name, abbreviation, country, comment);
-    else
-        d = 0;
 }
 
 /*!
@@ -821,7 +819,20 @@ bool QTimeZone::isTimeZoneIdAvailable(const QByteArray &ianaId)
 {
     // isValidId is not strictly required, but faster to weed out invalid
     // IDs as availableTimeZoneIds() may be slow
-    return (QTimeZonePrivate::isValidId(ianaId) && (availableTimeZoneIds().contains(ianaId)));
+    if (!QTimeZonePrivate::isValidId(ianaId))
+        return false;
+    const QList<QByteArray> tzIds = availableTimeZoneIds();
+    return std::binary_search(tzIds.begin(), tzIds.end(), ianaId);
+}
+
+static QList<QByteArray> set_union(const QList<QByteArray> &l1, const QList<QByteArray> &l2)
+{
+    QList<QByteArray> result;
+    result.reserve(l1.size() + l2.size());
+    std::set_union(l1.begin(), l1.end(),
+                   l2.begin(), l2.end(),
+                   std::back_inserter(result));
+    return result;
 }
 
 /*!
@@ -832,11 +843,8 @@ bool QTimeZone::isTimeZoneIdAvailable(const QByteArray &ianaId)
 
 QList<QByteArray> QTimeZone::availableTimeZoneIds()
 {
-    QSet<QByteArray> set = QUtcTimeZonePrivate().availableTimeZoneIds()
-                           + global_tz->backend->availableTimeZoneIds();
-    QList<QByteArray> list = set.toList();
-    std::sort(list.begin(), list.end());
-    return list;
+    return set_union(QUtcTimeZonePrivate().availableTimeZoneIds(),
+                     global_tz->backend->availableTimeZoneIds());
 }
 
 /*!
@@ -852,11 +860,8 @@ QList<QByteArray> QTimeZone::availableTimeZoneIds()
 
 QList<QByteArray> QTimeZone::availableTimeZoneIds(QLocale::Country country)
 {
-    QSet<QByteArray> set = QUtcTimeZonePrivate().availableTimeZoneIds(country)
-                           + global_tz->backend->availableTimeZoneIds(country);
-    QList<QByteArray> list = set.toList();
-    std::sort(list.begin(), list.end());
-    return list;
+    return set_union(QUtcTimeZonePrivate().availableTimeZoneIds(country),
+                     global_tz->backend->availableTimeZoneIds(country));
 }
 
 /*!
@@ -868,11 +873,8 @@ QList<QByteArray> QTimeZone::availableTimeZoneIds(QLocale::Country country)
 
 QList<QByteArray> QTimeZone::availableTimeZoneIds(int offsetSeconds)
 {
-    QSet<QByteArray> set = QUtcTimeZonePrivate().availableTimeZoneIds(offsetSeconds)
-                           + global_tz->backend->availableTimeZoneIds(offsetSeconds);
-    QList<QByteArray> list = set.toList();
-    std::sort(list.begin(), list.end());
-    return list;
+    return set_union(QUtcTimeZonePrivate().availableTimeZoneIds(offsetSeconds),
+                     global_tz->backend->availableTimeZoneIds(offsetSeconds));
 }
 
 /*!