Add support for calendars beside Gregorian

Add QCalendarBackend as a base class for calendar implementations and
QCalendar as a facade via which to access it.

QDate's implicit implementation of the Gregorian calendar becomes
QGregorianCalendar and QDate methods now support choice of calendar.

Convert QLocale's CLDR data for month names to a locale-data component
of each supported calendar and relevant QLocale methods now support
choice of calendar. Adapt Python scripts for locale data generation to
extract month name data from CLDR (keeping on version v35.1) into the
new calendar-locale files. The locale data for the Gregorian calendar
is held in a Roman calendar base, for sharing with other calendars.

Add tests for basic uses of the new API.

[ChangeLog][QtCore][QCalendar] Added QCalendar to support diverse
calendars, supported by implementing QCalendarBackend.

[ChangeLog][QtCore][QDate] Allow choice of calendar in various
operations, with Gregorian remaining the default.

Done-with: Lars Knoll <lars.knoll@qt.io>
Done-with: Edward Welbourne <edward.welbourne@qt.io>
Fixes: QTBUG-17110
Fixes: QTBUG-950
Change-Id: I9d6278f394269a183aee8156e990cec4d5198ab8
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 1060 insertions, 0 deletions

diff --git a/src/corelib/time/qcalendar.cpp b/src/corelib/time/qcalendar.cpp
new file mode 100644
index 0000000000..06dd1c671f
--- /dev/null
+++ b/src/corelib/time/qcalendar.cpp
@@ -0,0 +1,1060 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 or (at your option) any later version
+** approved by the KDE Free Qt Foundation. The licenses are as published by
+** the Free Software Foundation and appearing in the file LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#include "qcalendar.h"
+#include "qcalendarbackend_p.h"
+#include "qgregoriancalendar_p.h"
+
+#include "qdatetime.h"
+#include "qcalendarmath_p.h"
+#include <qhash.h>
+#include <qdebug.h>
+
+#include <unordered_map>
+
+QT_BEGIN_NAMESPACE
+
+namespace {
+
+struct CalendarName : public QString
+{
+    CalendarName(const QString &name) : QString(name) {}
+};
+
+inline bool operator==(const CalendarName &u, const CalendarName &v)
+{
+    return u.compare(v, Qt::CaseInsensitive) == 0;
+}
+
+inline uint qHash(const CalendarName &key, uint seed = 0) noexcept
+{
+    return qHash(key.toLower(), seed);
+}
+
+struct Registry {
+    std::vector<QCalendarBackend *> byId;
+    QHash<CalendarName, QCalendarBackend *> byName;
+    QCalendarBackend *gregorianCalendar = nullptr;
+    bool populated = false;
+
+    Registry()
+    {
+        byId.resize(int(QCalendar::System::Last) + 1);
+    }
+
+    ~Registry()
+    {
+        qDeleteAll(byId);
+    }
+
+    bool registerName(QCalendarBackend *calendar, const QString &name)
+    {
+        if (byName.find(name) != byName.end()) {
+            qWarning() << "Calendar name" << name
+                       << "is already taken, new calendar will not be registered.";
+            return false;
+        }
+        byName.insert(name, calendar);
+        return true;
+    }
+    void addCalendar(QCalendarBackend *calendar, const QString &name, QCalendar::System id)
+    {
+        if (!registerName(calendar, name))
+            return;
+        Q_ASSERT(byId.size() >= size_t(id));
+        if (id == QCalendar::System::User) {
+            byId.push_back(calendar);
+        } else {
+            Q_ASSERT(byId.at(size_t(id)) == nullptr);
+            byId[size_t(id)] = calendar;
+        }
+        if (id == QCalendar::System::Gregorian) {
+            Q_ASSERT(!gregorianCalendar);
+            gregorianCalendar = calendar;
+        }
+    }
+    /*
+        \internal
+        Ensures each enum-available calendar has been instantiated.
+
+        This arranges for each to register itself by name; it only does anything on
+        its first call, which ensures that name-based lookups can always find all
+        the calendars available via the enum.
+    */
+    void populate()
+    {
+        if (populated)
+            return;
+
+        for (int i = 0; i <= int(QCalendar::System::Last); ++i)
+            (void)QCalendar(QCalendar::System(i));
+    }
+};
+
+}
+
+Q_GLOBAL_STATIC(Registry, calendarRegistry);
+
+
+/*!
+    \since 5.14
+
+    \class QCalendarBackend
+    \inmodule QtCore
+    \reentrant
+    \brief The QCalendarBackend class provides basic calendaring functions.
+
+    QCalendarBackend provides the base class on which all calendar types are
+    implemented. On construction, the backend is registered with its primary
+    name.
+
+    A backend may also be registered with aliases, where the calendar is known
+    by several names. Registering with the name used by CLDR (the Unicode
+    consortium's Common Locale Data Repository) is recommended, particularly
+    when interacting with third-party software. Once a backend is registered for
+    a name, QCalendar can be constructed using that name to select the backend.
+
+    Each calendar backend must inherit from QCalendarBackend and implement its
+    pure virtual methods. It may also override some other virtual methods, as
+    needed.
+
+    Most backends are pure code, with no data elements. Such backends should
+    normally be implemented as singletons. For a backend to be added to the
+    QCalendar::System enum, it should be such a singleton, with a case in
+    QCalendar::fromEnum()'s switch statement to instantiate it.
+
+    Non-singleton calendar backends should ensure that each instance is created
+    with a distinct primary name. Later instances attempting to register with a
+    name already in use shall fail to register and be unavailable to QCalendar,
+    hence unusable.
+
+    \sa registerAlias(), QDate, QDateTime, QDateEdit, QDateTimeEdit, QCalendarWidget
+*/
+
+/*!
+    Constructs the calendar and registers it.
+*/
+QCalendarBackend::QCalendarBackend(const QString &name, QCalendar::System id)
+{
+    calendarRegistry->addCalendar(this, name, id);
+}
+
+/*!
+    Destroys the calendar.
+
+    Never call this from user code. Each calendar backend, once instantiated,
+    shall exist for the lifetime of the program. Its destruction is taken care
+    of by destruction of the registry of calendar backends and their names.
+*/
+QCalendarBackend::~QCalendarBackend()
+{
+}
+
+/*!
+    The calendar system of this calendar.
+
+    Each calendar backend constructible from the QCalendar::System enum should
+    return the member of that enum that produces it. Other calendars should
+    return User.
+
+    \sa QCalendar::fromEnum()
+*/
+QCalendar::System QCalendarBackend::calendarSystem() const
+{
+    return QCalendar::System::User;
+}
+
+/*!
+  The primary name of this calendar.
+ */
+QString QCalendar::name() const
+{
+    return d ? d->name() : QString();
+}
+
+// date queries
+/*!
+   \fn int QCalendarBackend::daysInMonth(int month, int year) const
+
+   Returns number of days in the month number \a month, in year \a year.
+
+   An implementation should return 0 if the given year had no such month. If
+   year is QCalendar::Unspecified, return the usual number of days for the
+   month, in those years that include it.
+
+   Calendars with intercallary days may represent these as extra days of the
+   preceding month, or as short months separate from the usual ones. In the
+   former case, daysInMonth(month, year) should be the number of ordinary days
+   in the month, although \c{isDateValid(year, month, day)} might return \c true for
+   some larger values of \c day.
+
+   \sa daysInYear(), monthsInYear(), minDaysInMonth(), maxDaysInMonth()
+*/
+
+// properties of the calendar
+
+/*!
+    \fn bool QCalendarBackend::isLeapYear(int year) const
+
+    Returns \c true if the specified \a year is a leap year for this calendar.
+
+    \sa daysInYear(), isDateValid()
+*/
+
+/*!
+    \fn bool QCalendarBackend::isLunar() const
+
+    Returns \c true if this calendar is a lunar calendar. Otherwise returns \c
+    false.
+
+    A lunar calendar is a calendar based upon the monthly cycles of the Moon's
+    phases (synodic months). This contrasts with solar calendars, whose annual
+    cycles are based only upon the solar year.
+
+    \sa isLuniSolar(), isSolar(), isProleptic()
+*/
+
+/*!
+    \fn bool QCalendarBackend::isLuniSolar() const
+
+    Returns \c true if this calendar is a lunisolar calendar. Otherwise returns
+    \c false.
+
+    A lunisolar calendar is a calendar whose date indicates both the moon phase
+    and the time of the solar year.
+
+    \sa isLunar(), isSolar(), isProleptic()
+*/
+
+/*!
+    \fn bool QCalendarBackend::isSolar() const
+
+    Returns \c true if this calendar is a solar calendar. Otherwise returns
+    \c false.
+
+    A solar calendar is a calendar whose dates indicate the season or almost
+    equivalently the apparent position of the sun relative to the fixed stars.
+    The Gregorian calendar, widely accepted as standard in the world,
+    is an example of solar calendar.
+
+    \sa isLuniSolar(), isLunar(), isProleptic()
+*/
+
+/*!
+    Returns the total number of days in the year number \a year.
+    Returns zero if there is no such year in this calendar.
+
+    This base implementation returns 366 for leap years and 365 for ordinary
+    years.
+
+    \sa monthsInYear(), daysInMonth(), isLeapYear()
+*/
+int QCalendarBackend::daysInYear(int year) const
+{
+    return monthsInYear(year) ? isLeapYear(year) ? 366 : 365 : 0;
+}
+
+/*!
+    Returns the total number of months in the year number \a year.
+    Returns zero if there is no such year in this calendar.
+
+    This base implementation returns 12 for any valid year.
+
+    \sa daysInYear(), maxMonthsInYear(), isDateValid()
+*/
+int QCalendarBackend::monthsInYear(int year) const
+{
+    return year > 0 || (year < 0 ? isProleptic() : hasYearZero()) ? 12 : 0;
+}
+
+/*!
+    Returns \c true if the date specified by \a year, \a month, and \a day is
+    valid for this calendar; otherwise returns \c false. For example,
+    the date 2018-04-19 is valid for the Gregorian calendar, but 2018-16-19 and
+    2018-04-38 are invalid.
+
+    Calendars with intercallary days may represent these as extra days of the
+    preceding month or as short months separate from the usual ones. In the
+    former case, a \a day value greater than \c{daysInMonth(\a{month},
+    \a{year})} may be valid.
+
+    \sa daysInMonth(), monthsInYear()
+*/
+bool QCalendarBackend::isDateValid(int year, int month, int day) const
+{
+    return day > 0 && day <= daysInMonth(month, year);
+}
+
+/*!
+    Returns \c true if this calendar is a proleptic calendar. Otherwise returns
+    \c false.
+
+    A proleptic calendar results from allowing negative year numbers to indicate
+    years before the nominal start of the calendar system.
+
+    \sa isLuniSolar(), isSolar(), isLunar(), hasYearZero()
+*/
+
+bool QCalendarBackend::isProleptic() const
+{
+    return true;
+}
+
+/*!
+    Returns \c true if year number \c 0 is considered a valid year in this
+    calendar. Otherwise returns \c false.
+
+    \sa isDateValid(), isProleptic()
+*/
+
+bool QCalendarBackend::hasYearZero() const
+{
+    return false;
+}
+
+/*!
+    Returns the maximum number of days in a month for any year.
+
+    This base implementation returns 31, as this is a common case.
+
+    For calendars with intercallary days, although daysInMonth() doesn't include
+    the intercallary days in its count for an individual month, maxDaysInMonth()
+    should include intercallary days, so that it is the maximum value of \c day
+    for which \c{isDateValid(year, month, day)} can be true.
+
+    \sa maxMonthsInYear(), daysInMonth()
+*/
+int QCalendarBackend::maxDaysInMonth() const
+{
+    return 31;
+}
+
+/*!
+    Returns the minimum number of days in any valid month of any valid year.
+
+    This base implementation returns 29, as this is a common case.
+
+    \sa maxMonthsInYear(), daysInMonth()
+*/
+int QCalendarBackend::minDaysInMonth() const
+{
+    return 29;
+}
+
+/*!
+    Returns the maximum number of months possible in any year.
+
+    This base implementation returns 12, as this is a common case.
+
+    \sa maxDaysInMonth(), monthsInYear()
+*/
+int QCalendarBackend::maxMonthsInYear() const
+{
+    return 12;
+}
+
+// Julian day number calculations
+
+/*!
+    \fn bool dateToJulianDay(int year, int month, int day, qint64 *jd) const
+
+    Computes the Julian day number corresponding to the specified \a year, \a
+    month, and \a day. Returns true and sets \a jd if there is such a date in
+    this calendar; otherwise, returns false.
+
+    \sa QCalendar::partsFromDate(), julianDayToDate()
+*/
+
+/*!
+    \fn QCalendar::YearMonthDay julianDayToDate(qint64 jd) const
+
+    Computes the year, month, and day in this calendar for the given Julian day
+    number \a jd. If the given day falls outside this calendar's scope
+    (e.g. before the start-date of a non-proleptic calendar), the returned
+    structure's isValid() is false; otherwise, its year, month, and day fields
+    provide this calendar's description of the date.
+
+    \sa QCalendar::dateFromParts(), dateToJulianDay()
+*/
+
+/*!
+   Returns the day of the week for a given Julian Day Number.
+
+   This is 1 for Monday through 7 for Sunday.
+
+   Calendars with intercallary days may return larger values for these
+   intercallary days. They should avoid using 0 for any special purpose (it is
+   already used in QDate::dayOfWeek() to mean an invalid date). The calendar
+   should treat the numbers used as an \c enum, whose values need not be
+   contiguous, nor need they follow closely from the 1 through 7 of the usual
+   returns. It suffices that weekDayName() can recognize each such number as
+   identifying a distinct name, that it returns to identify the particular
+   intercallary day.
+
+   This base implementation uses the day-numbering that various calendars have
+   borrowed off the Hebrew calendar.
+
+   \sa weekDayName(), standaloneWeekDayName(), QDate::dayOfWeek()
+ */
+int QCalendarBackend::dayOfWeek(qint64 jd) const
+{
+    return QRoundingDown::qMod(jd, 7) + 1;
+}
+
+// Month and week-day name look-ups (implemented in qlocale.cpp):
+/*!
+  \fn QString QCalendarBackend::monthName(const QLocale &locale, int month, int year,
+                                          QLocale::FormatType format) const
+
+  Returns the name of the specified \a month in the given \a year for the chosen
+  \a locale, using the given \a format to determine how complete the name is.
+
+  If \a year is Unspecified, return the name for the month that usually has this
+  number within a typical year. Calendars with a leap month that isn't always
+  the last may need to take account of the year to map the month number to the
+  particular year's month with that number.
+
+  \note Backends for which CLDR provides data can configure the default
+  implementation of the two month name look-up methods by arranging for
+  localeMonthIndexData() and localeMonthData() to provide access to the CLDR
+  data (see cldr2qlocalexml.py, qlocalexml2cpp.py and existing backends).
+  Conversely, backends that override both month name look-up methods need not
+  return anything meaningful from localeMonthIndexData() or localeMonthData().
+
+  \sa standaloneMonthName(), QLocale::monthName()
+*/
+
+/*!
+  \fn QString QCalendarBackend::standaloneMonthName(const QLocale &locale, int month, int year
+                                                    QLocale::FormatType format) const
+
+  Returns the standalone name of the specified \a month in the chosen \a locale,
+  using the specified \a format to determine how complete the name is.
+
+  If \a year is Unspecified, return the standalone name for the month that
+  usually has this number within a typical year. Calendars with a leap month
+  that isn't always the last may need to take account of the year to map the
+  month number to the particular year's month with that number.
+
+  \sa monthName(), QLocale::standaloneMonthName()
+*/
+
+/*!
+  \fn QString QCalendarBackend::weekDayName(const QLocale &locale, int day,
+                                            QLocale::FormatType format) const
+
+  Returns the name of the specified \a day of the week in the chosen \a locale,
+  using the specified \a format to determine how complete the name is.
+
+  The base implementation handles \a day values from 1 to 7 using the day names
+  CLDR provides, which are suitable for calendards that use the same
+  (Hebrew-derived) week as the Gregorian calendar.
+
+  Calendars whose dayOfWeek() returns a value outside the range from 1 to 7 need
+  to reimplement this method to handle such extra week-day values. They can
+  assume that \a day is a value returned by the same calendar's dayOfWeek().
+
+  \sa dayOfWeek(), standaloneWeekDayName(), QLocale::dayName()
+*/
+
+/*!
+  \fn QString QCalendarBackend::standaloneWeekDayName(const QLocale &locale, int day,
+                                                      QLocale::FormatType format) const
+
+  Returns the standalone name of the specified \a day of the week in the chosen
+  \a locale, using the specified \a format to determine how complete the name
+  is.
+
+  The base implementation handles \a day values from 1 to 7 using the standalone
+  day names CLDR provides, which are suitable for calendards that use the same
+  (Hebrew-derived) week as the Gregorian calendar.
+
+  Calendars whose dayOfWeek() returns a value outside the range from 1 to 7 need
+  to reimplement this method to handle such extra week-day values. They can
+  assume that \a day is a value returned by the same calendar's dayOfWeek().
+
+  \sa dayOfWeek(), weekDayName(), QLocale::standaloneDayName()
+*/
+
+/*!
+  \fn QString QCalendarBackend::dateTimeToString(QStringView format, const QDateTime &datetime,
+                                                 const QDate &dateOnly, const QTime &timeOnly,
+                                                 const QLocale &locale) const
+
+  Returns a string representing a given date, time or date-time.
+
+  If \a datetime is specified and valid, it is used and both date and time
+  format tokens are converted to appropriate representations of the parts of the
+  datetime. Otherwise, if \a dateOnly is valid, only date format tokens are
+  converted; else, if \a timeOnly is valid, only time format tokens are
+  converted. If none are valid, an empty string is returned.
+
+  The specified \a locale influences how some format tokens are converted; for
+  example, when substituting day and month names and their short-forms. For the
+  supported formatting tokens, see QDate::toString() and QTime::toString(). As
+  described above, the provided date, time and date-time determine which of
+  these tokens are recognized: where these appear in \a format they are replaced
+  by data. Any text in \a format not recognized as a format token is copied
+  verbatim into the result string.
+
+  \sa QDate::toString(), QTime::toString(), QDateTime::toString()
+*/
+// End of methods implemented in qlocale.cpp
+
+/*!
+    Returns a list of names of the available calendar systems. Any
+    QCalendarBackend sub-class must be registered before being exposed to Date
+    and Time APIs.
+
+    \sa registerCalendar(), fromName()
+*/
+QStringList QCalendarBackend::availableCalendars()
+{
+    if (calendarRegistry.isDestroyed())
+        return {};
+    calendarRegistry->populate();
+    return QStringList(calendarRegistry->byName.keyBegin(), calendarRegistry->byName.keyEnd());
+}
+
+/*!
+    Registers an alias for this calendar backend. Once a backend is registered,
+    its name will be included in the list of available calendars and the
+    calendar can be instantiated by name.
+
+    Returns \c false if the given \a name is already in use, otherwise it
+    registers this calendar backend and returns \c true.
+
+    \sa availableCalendars(), fromName()
+*/
+bool QCalendarBackend::registerAlias(const QString &name)
+{
+    if (calendarRegistry.isDestroyed())
+        return false;
+    return calendarRegistry->registerName(this, name);
+}
+
+/*!
+  Returns a pointer to a named calendar backend.
+
+  If the given \a name is present in availableCalendars(), the backend matching
+  it is returned; otherwise, \c nullptr is returned. Matching of names ignores
+  case. Note that this won't provoke construction of a calendar backend, it will
+  only return ones that have been instantiated (and not yet destroyed) by some
+  other means. However, calendars available via the QCalendar::System enum are
+  always registered when this is called.
+
+  \sa availableCalendars(), registerCalendar(), fromEnum()
+*/
+const QCalendarBackend *QCalendarBackend::fromName(QStringView name)
+{
+    if (calendarRegistry.isDestroyed())
+        return nullptr;
+    calendarRegistry->populate();
+    auto it = calendarRegistry->byName.find(name.toString());
+    return it == calendarRegistry->byName.end() ? nullptr : *it;
+}
+
+/*!
+  \overload
+ */
+const QCalendarBackend *QCalendarBackend::fromName(QLatin1String name)
+{
+    if (calendarRegistry.isDestroyed())
+        return nullptr;
+    calendarRegistry->populate();
+    auto it = calendarRegistry->byName.find(QString(name));
+    return it == calendarRegistry->byName.end() ? nullptr : *it;
+}
+
+/*!
+  Returns a pointer to a calendar backend, specified by enum.
+
+  This will instantiate the indicated calendar (which will enable fromName() to
+  return it subsequently), but only for the Qt-supported calendars for which
+  (where relevant) the appropriate feature has been enabled.
+*/
+const QCalendarBackend *QCalendarBackend::fromEnum(QCalendar::System system)
+{
+    if (calendarRegistry.isDestroyed() || system == QCalendar::System::User)
+        return nullptr;
+    Q_ASSERT(calendarRegistry->byId.size() >= size_t(system));
+    if (auto *c = calendarRegistry->byId.at(size_t(system)))
+        return c;
+    switch (system) {
+    case QCalendar::System::Gregorian:
+        return new QGregorianCalendar;
+    case QCalendar::System::User:
+        Q_UNREACHABLE();
+    }
+    return nullptr;
+}
+
+/*!
+  \since 5.14
+
+  \class QCalendar
+  \inmodule QtCore
+  \reentrant
+  \brief The QCalendar class describes calendar systems.
+
+  A QCalendar object maps a year, month, and day-number to a specific day
+  (ultimately identified by its Julian day number), using the rules of a
+  particular system.
+
+  The default QCalendar() is a proleptic Gregorian calendar, which has no year
+  zero. Other calendars may be supported by enabling suitable features or
+  loading plugins. Calendars supported as features can be constructed by passing
+  the QCalendar::System enumeration to the constructor. All supported calendars
+  may be constructed by name, once they have been constructed. (Thus plugins
+  instantiate their calendar backend to register it.) Built-in backends,
+  accessible via QCalendar::System, are also always available by name.
+
+  A QCalendar value is immutable.
+
+  \sa QCalendarBackend, QDate, QDateTime
+*/
+
+/*!
+    \enum QCalendar::System
+
+    This enumerated type is used to specify a choice of calendar system.
+
+    \value Gregorian The default calendar, used internationally.
+
+    \sa QCalendar
+*/
+
+/*!
+  \fn QCalendar::QCalendar()
+  \fn QCalendar::QCalendar(QCalendar::System system)
+  \fn QCalendar::QCalendar(QLatin1String name)
+  \fn QCalendar::QCalendar(QStringView name)
+
+  Constructs a calendar object.
+
+  The choice of calendar to use may be indicated as \a system, using the
+  enumeration QCalendar::System, or by \a name, using a string (either Unicode
+  or Latin 1). Construction by name may depend on an instance of the given
+  calendar being constructed by other means first. With no argument, the default
+  constructor returns the Gregorian calendar.
+
+  \sa QCalendar, System
+*/
+
+QCalendar::QCalendar()
+    : d(nullptr)
+{
+    if (calendarRegistry.isDestroyed())
+        return;
+    d = calendarRegistry->gregorianCalendar;
+    if (!d)
+        d = new QGregorianCalendar;
+}
+
+QCalendar::QCalendar(QCalendar::System system)
+    : d(QCalendarBackend::fromEnum(system)) {}
+
+QCalendar::QCalendar(QLatin1String name)
+    : d(QCalendarBackend::fromName(name)) {}
+
+QCalendar::QCalendar(QStringView name)
+    : d(QCalendarBackend::fromName(name)) {}
+
+// Date queries:
+
+/*!
+  Returns the number of days in the given \a month of the given \a year.
+
+  Months are numbered consecutively, starting with 1 for the first month of each
+  year.
+
+  \sa maxDaysInMonth(), minDaysInMonth()
+*/
+int QCalendar::daysInMonth(int month, int year) const
+{
+    return d ? d->daysInMonth(month, year) : 0;
+}
+
+/*!
+  Returns the number of days in the given \a year.
+*/
+int QCalendar::daysInYear(int year) const
+{
+    return d ? d->daysInYear(year) : 0;
+}
+
+/*!
+    Returns the number of months in the given \a year.
+*/
+int QCalendar::monthsInYear(int year) const
+{
+    return d ? d->monthsInYear(year) : 0;
+}
+
+/*!
+  Returns \c true precisely if the given \a year, \a month, and \a day specify a
+  valid date in this calendar.
+
+  Usually this means 1 <= month <= monthsInYear(year) and 1 <= day <=
+  daysInMonth(month, year). However, calendars with intercallary days or months
+  may complicate that.
+*/
+bool QCalendar::isDateValid(int year, int month, int day) const
+{
+    return d && d->isDateValid(year, month, day);
+}
+
+// properties of the calendar
+
+/*!
+    Returns \c true if this calendar object is the Gregorian calendar object
+    used as default calendar by other Qt APIs, e.g. in QDate.
+*/
+bool QCalendar::isGregorian() const
+{
+    Q_ASSERT(!calendarRegistry.isDestroyed());
+    return d == calendarRegistry->gregorianCalendar;
+}
+
+/*!
+  Returns \c true if the given year is a leap year.
+
+  Since the year is not a whole number of days long, some years are longer than
+  others. The difference may be a whole month or just a single day; the details
+  vary between calendars.
+
+  \sa isDateValid()
+*/
+bool QCalendar::isLeapYear(int year) const
+{
+    return d && d->isLeapYear(year);
+}
+
+/*!
+  Returns \c true if this calendar is a lunar calendar.
+
+  A lunar calendar is one based primarily on the phases of the moon.
+*/
+bool QCalendar::isLunar() const
+{
+    return d && d->isLunar();
+}
+
+/*!
+  Returns \c true if this calendar is luni-solar.
+
+  A luni-solar calendar expresses the phases of the moon but adapts itself to
+  also keep track of the Sun's varying position in the sky, relative to the
+  fixed stars.
+*/
+bool QCalendar::isLuniSolar() const
+{
+    return d && d->isLuniSolar();
+}
+
+/*!
+  Returns \c true if this calendar is solar.
+
+  A solar calendar is based primaril on the Sun's varying position in the sky,
+  relative to the fixed stars.
+*/
+bool QCalendar::isSolar() const
+{
+    return d && d->isSolar();
+}
+
+/*!
+  Returns \c true if this calendar is proleptic.
+
+  A proleptic calendar is able to describe years arbitrarily long before its
+  first. These are represented by negative year numbers and possibly by a year
+  zero.
+
+  \sa hasYearZero()
+*/
+bool QCalendar::isProleptic() const
+{
+    return d && d->isProleptic();
+}
+
+/*!
+  Returns \c true if this calendar has a year zero.
+
+  A non-proleptic calendar with no year zero represents years from its first
+  year onwards but provides no way to describe years before its first; such a
+  calendar has no year zero and is not proleptic.
+
+  A calendar which represents years before its first may number these years
+  simply by following the usual integer counting, so that the year before the
+  first is year zero, with negative-numbered years preceding this; such a
+  calendar is proleptic and has a year zero. A calendar might also have a year
+  zero (for example, the year of some great event, with subsequent years being
+  the first year after that event, the second year after, and so on) without
+  describing years before its year zero. Such a calendar would have a year zero
+  without being proleptic.
+
+  Some calendars, however, represent years before their first by an alternate
+  numbering; for example, the proleptic Gregorian calendar's first year is 1 CE
+  and the year before it is 1 BCE, preceded by 2 BCE and so on. In this case,
+  we use negative year numbers, with year -1 as the year before year 1, year -2
+  as the year before year -1 and so on. Such a calendar is proleptic but has no
+  year zero.
+
+  \sa isProleptic()
+*/
+bool QCalendar::hasYearZero() const
+{
+    return d && d->hasYearZero();
+}
+
+/*!
+  Returns the number of days in the longest month in the calendar, in any year.
+
+  \sa daysInMonth(), minDaysInMonth()
+*/
+int QCalendar::maxDaysInMonth() const
+{
+    return d ? d->maxDaysInMonth() : 0;
+}
+
+/*!
+  Returns the number of days in the shortest month in the calendar, in any year.
+
+  \sa daysInMonth(), maxDaysInMonth()
+*/
+int QCalendar::minDaysInMonth() const
+{
+    return d ? d->minDaysInMonth() : 0;
+}
+
+/*!
+  Returns the largest number of months that any year may contain.
+
+  \sa monthName(), standaloneMonthName(), monthsInYear()
+*/
+int QCalendar::maxMonthsInYear() const
+{
+    return d ?  d->maxMonthsInYear() : 0;
+}
+
+// Julian Day conversions:
+
+/*!
+    \fn QDate QCalendar::dateFromParts(int year, int month, int day) const
+    \fn QDate QCalendar::dateFromParts(QCalendar::YearMonthDay parts) const
+
+    Converts a year, month, and day to a QDate.
+
+    The \a year, \a month, and \a day may be passed as separate numbers or
+    packaged together as the members of \a parts. Returns a QDate with the given
+    year, month, and day of the month in this calendar, if there is one.
+    Otherwise, including the case where any of the values is
+    QCalendar::Unspecified, returns a QDate whose isNull() is true.
+
+    \sa isDateValid(), partsFromDate()
+*/
+QDate QCalendar::dateFromParts(int year, int month, int day) const
+{
+    qint64 jd;
+    return d && d->dateToJulianDay(year, month, day, &jd)
+        ? QDate::fromJulianDay(jd) : QDate();
+}
+
+QDate QCalendar::dateFromParts(const QCalendar::YearMonthDay &parts) const
+{
+    return parts.isValid() ? dateFromParts(parts.year, parts.month, parts.day) : QDate();
+}
+
+/*!
+    Converts a QDate to a year, month, and day of the month.
+
+    The returned structure's isValid() shall be false if the calendar is unable
+    to represent the given \a date. Otherwise its \a year, \a month, and \a day
+    members record the so-named parts of its representation.
+
+    \sa dateFromParts(), isProleptic(), hasYearZero()
+*/
+QCalendar::YearMonthDay QCalendar::partsFromDate(QDate date) const
+{
+    return d ? d->julianDayToDate(date.toJulianDay()) : YearMonthDay();
+}
+
+/*!
+  Returns the day of the week number for the given \a date.
+
+  Returns zero if the calendar is unable to represent the indicated date.
+  Returns 1 for Monday through 7 for Sunday. Calendars with intercallary days
+  may use other numbers to represent these.
+
+  \sa partsFromDate(), Qt::DayOfWeek
+*/
+int QCalendar::dayOfWeek(QDate date) const
+{
+    return d ? d->dayOfWeek(date.toJulianDay()) : 0;
+}
+
+// Locale data access
+
+/*!
+  Returns a suitably localised name for a month.
+
+  The month is indicated by a number, with \a month = 1 meaning the first month
+  of the year and subsequent months numbered accordingly. Returns an empty
+  string if the \a month number is unrecognized.
+
+  The \a year may be Unspecified, in which case the mapping from numbers to
+  names for a typical year's months should be used. Some calendars have leap
+  months that aren't always at the end of the year; their mapping of month
+  numbers to names may then depend on the placement of a leap month. Thus the
+  year should normally be specified, if known.
+
+  The name is returned in the form that would normally be used in a full date,
+  in the specified \a locale; the \a format determines how fully it shall be
+  expressed (i.e. to what extent it is abbreviated).
+
+  \sa standaloneMonthName(), maxMonthsInYear(), dateTimeString()
+*/
+QString QCalendar::monthName(const QLocale &locale, int month, int year,
+                             QLocale::FormatType format) const
+{
+    const int maxMonth = year == Unspecified ? maxMonthsInYear() : monthsInYear(year);
+    if (!d || month < 1 || month > maxMonth)
+        return QString();
+
+    return d->monthName(locale, month, year, format);
+}
+
+/*!
+  Returns a suitably localised standalone name for a month.
+
+  The month is indicated by a number, with \a month = 1 meaning the first month
+  of the year and subsequent months numbered accordingly. Returns an empty
+  string if the \a month number is unrecognized.
+
+  The \a year may be Unspecified, in which case the mapping from numbers to
+  names for a typical year's months should be used. Some calendars have leap
+  months that aren't always at the end of the year; their mapping of month
+  numbers to names may then depend on the placement of a leap month. Thus the
+  year should normally be specified, if known.
+
+  The name is returned in the form that would be used in isolation in the
+  specified \a locale; the \a format determines how fully it shall be expressed
+  (i.e. to what extent it is abbreviated).
+
+  \sa monthName(), maxMonthsInYear(), dateTimeString()
+*/
+QString QCalendar::standaloneMonthName(const QLocale &locale, int month, int year,
+                                       QLocale::FormatType format) const
+{
+    const int maxMonth = year == Unspecified ? maxMonthsInYear() : monthsInYear(year);
+    if (!d || month < 1 || month > maxMonth)
+        return QString();
+
+    return d->standaloneMonthName(locale, month, year, format);
+}
+
+/*!
+  Returns a suitably localised name for a day of the week.
+
+  The days of the week are numbered from 1 for Monday through 7 for Sunday. Some
+  calendars may support higher numbers for other days (e.g. intercallary days,
+  that are not part of any week). Returns an empty string if the \a day number
+  is unrecognized.
+
+  The name is returned in the form that would normally be used in a full date,
+  in the specified \a locale; the \a format determines how fully it shall be
+  expressed (i.e. to what extent it is abbreviated).
+
+  \sa standaloneWeekDayName(), dayOfWeek()
+*/
+QString QCalendar::weekDayName(const QLocale &locale, int day,
+                               QLocale::FormatType format) const
+{
+    return d ? d->weekDayName(locale, day, format) : QString();
+}
+
+/*!
+  Returns a suitably localised standalone name for a day of the week.
+
+  The days of the week are numbered from 1 for Monday through 7 for Sunday. Some
+  calendars may support higher numbers for other days (e.g. intercallary days,
+  that are not part of any week). Returns an empty string if the \a day number
+  is unrecognized.
+
+  The name is returned in the form that would be used in isolation (for example
+  as a column heading in a calendar's tabular display of a month with successive
+  weeks as rows) in the specified \a locale; the \a format determines how fully
+  it shall be expressed (i.e. to what extent it is abbreviated).
+
+  \sa weekDayName(), dayOfWeek()
+*/
+QString QCalendar::standaloneWeekDayName(const QLocale &locale, int day,
+                                         QLocale::FormatType format) const
+{
+    return d ? d->standaloneWeekDayName(locale, day, format) : QString();
+}
+
+/*!
+  Returns a string representing a given date, time or date-time.
+
+  If \a datetime is valid, it is represented and format specifiers for both date
+  and time fields are recognized; otherwise, if \a dateOnly is valid, it is
+  represented and only format specifiers for date fields are recognized;
+  finally, if \a timeOnly is valid, it is represented and only format specifiers
+  for time fields are recognized. If none of these is valid, an empty string is
+  returned.
+
+  See QDate::toString and QTime::toString() for the supported field specifiers.
+  Characters in \a format that are recognized as field specifiers are replaced
+  by text representing appropriate data from the date and/or time being
+  represented. The texts to represent them may depend on the \a locale
+  specified. Other charagers in \a format are copied verbatim into the returned
+  string.
+
+  \sa monthName(), weekDayName(), QDate::toString(), QTime::toString()
+*/
+QString QCalendar::dateTimeToString(QStringView format, const QDateTime &datetime,
+                                    const QDate &dateOnly, const QTime &timeOnly,
+                                    const QLocale &locale) const
+{
+    return d ? d->dateTimeToString(format, datetime, dateOnly, timeOnly, locale) : QString();
+}
+
+/*!
+    Returns a list of names of the available calendar systems.
+
+    These may be supplied by plugins or other code linked into an application,
+    in addition to the ones provided by Qt, some of which are controlled by
+    features.
+*/
+QStringList QCalendar::availableCalendars()
+{
+    return QCalendarBackend::availableCalendars();
+}
+
+QT_END_NAMESPACE