QTZP: use binary search more on timezone ID tables

The zone and Windows data tables are sorted on Windows ID and its key
(which are monotonic functions of one another) so we can use binary
chop to find the first matching entry in each, when searching on
these. Furthermore, the search for ID from key can be shortcut as the
keys should normally be consecutive integers starting with 1.

Change-Id: I53f7ff8c93fd6d3d9e48c7bb86060746b68fab3d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qtimezoneprivate.cpp

@@ -37,32 +37,63 @@ static bool earlierWinData(const QWindowsData &less, const QWindowsData &more) n
         || less.windowsId().compare(more.windowsId(), Qt::CaseInsensitive) < 0;
 }
 
-/*
-    Static utilities for looking up Windows ID tables
-*/
+// For use with std::lower_bound():
+constexpr bool atLowerUtcOffset(const QUtcData &entry, qint32 offsetSeconds) noexcept
+{
+    return entry.offsetFromUtc < offsetSeconds;
+}
+
+constexpr bool atLowerWindowsKey(const QWindowsData &entry, qint16 winIdKey) noexcept
+{
+    return entry.windowsIdKey < winIdKey;
+}
 
+static bool earlierWindowsId(const QWindowsData &entry, QByteArrayView winId) noexcept
+{
+    return entry.windowsId().compare(winId, Qt::CaseInsensitive) < 0;
+}
+
+constexpr bool zoneAtLowerWindowsKey(const QZoneData &entry, qint16 winIdKey) noexcept
+{
+    return entry.windowsIdKey < winIdKey;
+}
+
+// Static table-lookup helpers
 static quint16 toWindowsIdKey(const QByteArray &winId)
 {
-    for (const QWindowsData &data : windowsDataTable) {
-        if (data.windowsId() == winId)
-            return data.windowsIdKey;
-    }
+    // Key and winId are monotonic, table is sorted on them.
+    const auto data = std::lower_bound(std::begin(windowsDataTable), std::end(windowsDataTable),
+                                       winId, earlierWindowsId);
+    if (data != std::end(windowsDataTable) && data->windowsId() == winId)
+        return data->windowsIdKey;
     return 0;
 }
 
 static QByteArray toWindowsIdLiteral(quint16 windowsIdKey)
 {
-    for (const QWindowsData &data : windowsDataTable) {
-        if (data.windowsIdKey == windowsIdKey)
+    // Caller should be passing a valid (in range) key; and table is sorted in
+    // increasing order, with no gaps in numbering, starting with key = 1 at
+    // index [0]. So this should normally work:
+    if (Q_LIKELY(windowsIdKey > 0 && windowsIdKey <= std::size(windowsDataTable))) {
+        const auto &data = windowsDataTable[windowsIdKey - 1];
+        if (Q_LIKELY(data.windowsIdKey == windowsIdKey))
             return data.windowsId().toByteArray();
     }
+    // Fall back on binary chop - key and winId are monotonic, table is sorted on them:
+    const auto data = std::lower_bound(std::begin(windowsDataTable), std::end(windowsDataTable),
+                                       windowsIdKey, atLowerWindowsKey);
+    if (data != std::end(windowsDataTable) && data->windowsIdKey == windowsIdKey)
+        return data->windowsId().toByteArray();
+
     return QByteArray();
 }
 
-// For use with std::lower_bound():
-static bool atLowerUtcOffset(const QUtcData &entry, qint32 offsetSeconds)
+static auto zoneStartForWindowsId(quint16 windowsIdKey) noexcept
 {
-    return entry.offsetFromUtc < offsetSeconds;
+    // Caller must check the resulting iterator isn't std::end(zoneDataTable)
+    // and does match windowsIdKey, since this is just the lower bound.
+    return std::lower_bound(std::begin(zoneDataTable), std::end(zoneDataTable),
+                            windowsIdKey, zoneAtLowerWindowsKey);
 }
 
 /*
@@ -530,11 +561,11 @@ QList<QByteArray> QTimeZonePrivate::availableTimeZoneIds(int offsetFromUtc) cons
     // First get all Zones in the table using the Offset
     for (const QWindowsData &winData : windowsDataTable) {
         if (winData.offsetFromUtc == offsetFromUtc) {
-            for (const QZoneData &data : zoneDataTable) {
-                if (data.windowsIdKey == winData.windowsIdKey) {
-                    for (auto l1 : data.ids())
-                        offsets << QByteArray(l1.data(), l1.size());
-                }
+            for (auto data = zoneStartForWindowsId(winData.windowsIdKey);
+                 data != std::end(zoneDataTable) && data->windowsIdKey == winData.windowsIdKey;
+                 ++data) {
+                for (auto l1 : data->ids())
+                    offsets << QByteArray(l1.data(), l1.size());
             }
         }
     }
@@ -708,13 +739,13 @@ QByteArray QTimeZonePrivate::ianaIdToWindowsId(const QByteArray &id)
 
 QByteArray QTimeZonePrivate::windowsIdToDefaultIanaId(const QByteArray &windowsId)
 {
-    for (const QWindowsData &data : windowsDataTable) {
-        if (data.windowsId() == windowsId) {
-            QByteArrayView id = data.ianaId();
-            if (qsizetype cut = id.indexOf(' '); cut >= 0)
-                id = id.first(cut);
-            return id.toByteArray();
-        }
+    const auto data = std::lower_bound(std::begin(windowsDataTable), std::end(windowsDataTable),
+                                       windowsId, earlierWindowsId);
+    if (data != std::end(windowsDataTable) && data->windowsId() == windowsId) {
+        QByteArrayView id = data->ianaId();
+        if (qsizetype cut = id.indexOf(' '); cut >= 0)
+            id = id.first(cut);
+        return id.toByteArray();
     }
     return QByteArray();
 }
@@ -731,11 +762,11 @@ QList<QByteArray> QTimeZonePrivate::windowsIdToIanaIds(const QByteArray &windows
     const quint16 windowsIdKey = toWindowsIdKey(windowsId);
     QList<QByteArray> list;
 
-    for (const QZoneData &data : zoneDataTable) {
-        if (data.windowsIdKey == windowsIdKey) {
-            for (auto l1 : data.ids())
-                list << QByteArray(l1.data(), l1.size());
-        }
+    for (auto data = zoneStartForWindowsId(windowsIdKey);
+         data != std::end(zoneDataTable) && data->windowsIdKey == windowsIdKey;
+         ++data) {
+        for (auto l1 : data->ids())
+            list << QByteArray(l1.data(), l1.size());
     }
 
     // Return the full list in alpha order
@@ -749,10 +780,12 @@ QList<QByteArray> QTimeZonePrivate::windowsIdToIanaIds(const QByteArray &windows
     QList<QByteArray> list;
     const quint16 windowsIdKey = toWindowsIdKey(windowsId);
     const qint16 land = static_cast<quint16>(territory);
-    for (const QZoneData &data : zoneDataTable) {
+    for (auto data = zoneStartForWindowsId(windowsIdKey);
+         data != std::end(zoneDataTable) && data->windowsIdKey == windowsIdKey;
+         ++data) {
         // Return the region matches in preference order
-        if (data.windowsIdKey == windowsIdKey && data.territory == land) {
-            for (auto l1 : data.ids())
+        if (data->territory == land) {
+            for (auto l1 : data->ids())
                 list << QByteArray(l1.data(), l1.size());
             break;
         }