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++ // Copyright (C) 2021 The Qt Company Ltd.
++ // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
++ // Qt-Security score:significant reason:default
++
++ #include "qgregoriancalendar_p.h"
++ #include "qcalendarmath_p.h"
++
++ #include <QtCore/qdatetime.h>
++
++ QT_BEGIN_NAMESPACE
++
++ using namespace QRoundingDown;
++
++ // Verification that QRoundingDown::qDivMod() works correctly:
++ static_assert(qDivMod<2>(-86400).quotient == -43200);
++ static_assert(qDivMod<2>(-86400).remainder == 0);
++ static_assert(qDivMod<86400>(-86400).quotient == -1);
++ static_assert(qDivMod<86400>(-86400).remainder == 0);
++ static_assert(qDivMod<86400>(-86401).quotient == -2);
++ static_assert(qDivMod<86400>(-86401).remainder == 86399);
++ static_assert(qDivMod<86400>(-100000).quotient == -2);
++ static_assert(qDivMod<86400>(-100000).remainder == 72800);
++ static_assert(qDivMod<86400>(-172799).quotient == -2);
++ static_assert(qDivMod<86400>(-172799).remainder == 1);
++ static_assert(qDivMod<86400>(-172800).quotient == -2);
++ static_assert(qDivMod<86400>(-172800).remainder == 0);
++
++ // Uncomment to verify error on bad denominator is clear and intelligible:
++ // static_assert(qDivMod<1>(17).remainder == 0);
++ // static_assert(qDivMod<0>(17).remainder == 0);
++ // static_assert(qDivMod<std::numeric_limits<unsigned>::max()>(17).remainder == 0);
++
++ /*!
++ \since 5.14
++ \internal
++
++ \class QGregorianCalendar
++ \inmodule QtCore
++ \brief The QGregorianCalendar class implements the Gregorian calendar.
++
++ \section1 The Gregorian Calendar
++
++ The Gregorian calendar is a refinement of the earlier Julian calendar,
++ itself a late form of the Roman calendar. It is widely used.
++
++ \sa QRomanCalendar, QJulianCalendar, QCalendar
++ */
++
++ QString QGregorianCalendar::name() const
++ {
++ return QStringLiteral("Gregorian");
++ }
++
++ QStringList QGregorianCalendar::nameList()
++ {
++ return {
++ QStringLiteral("Gregorian"),
++ QStringLiteral("gregory"),
++ };
++ }
++
++ bool QGregorianCalendar::isLeapYear(int year) const
++ {
++ return leapTest(year);
++ }
++
++ bool QGregorianCalendar::leapTest(int year)
++ {
++ if (year == QCalendar::Unspecified)
++ return false;
++
++ // No year 0 in Gregorian calendar, so -1, -5, -9 etc are leap years
++ if (year < 1)
++ ++year;
++
++ return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
++ }
++
++ // Duplicating code from QRomanCalendar, but inlining isLeapYear() as leapTest():
++ int QGregorianCalendar::monthLength(int month, int year)
++ {
++ if (month < 1 || month > 12)
++ return 0;
++
++ if (month == 2)
++ return leapTest(year) ? 29 : 28;
++
++ return 30 | ((month & 1) ^ (month >> 3));
++ }
++
++ bool QGregorianCalendar::validParts(int year, int month, int day)
++ {
++ return year && 0 < day && day <= monthLength(month, year);
++ }
++
++ int QGregorianCalendar::weekDayOfJulian(qint64 jd)
++ {
++ return int(qMod<7>(jd) + 1);
++ }
++
++ bool QGregorianCalendar::dateToJulianDay(int year, int month, int day, qint64 *jd) const
++ {
++ const auto maybe = julianFromParts(year, month, day);
++ if (maybe)
++ *jd = *maybe;
++ return bool(maybe);
++ }
++
++ QCalendar::YearMonthDay QGregorianCalendar::julianDayToDate(qint64 jd) const
++ {
++ return partsFromJulian(jd);
++ }
++
++ qint64
++ QGregorianCalendar::matchCenturyToWeekday(const QCalendar::YearMonthDay &parts, int dow) const
++ {
++ /* The Gregorian four-century cycle is a whole number of weeks long, so we
++ only need to consider four centuries, from previous through next-but-one.
++ There are thus three days of the week that can't happen, for any given
++ day-of-month, month and year-mod-100. (Exception: '00 Feb 29 has only one
++ option.)
++ */
++ auto maybe = julianFromParts(parts.year, parts.month, parts.day);
++ if (maybe) {
++ int diff = weekDayOfJulian(*maybe) - dow;
++ if (!diff)
++ return *maybe;
++ int year = parts.year < 0 ? parts.year + 1 : parts.year;
++ // What matters is the placement of leap days, so dates before March
++ // effectively belong with the dates since the preceding March:
++ const auto yearSplit = qDivMod<100>(year - (parts.month < 3 ? 1 : 0));
++ const int centuryMod4 = qMod<4>(yearSplit.quotient);
++ // Week-day shift for a century is 5, unless crossing a multiple of 400's Feb 29th.
++ static_assert(qMod<7>(36524) == 5); // and (3 * 5) % 7 = 1
++ // Formulae arrived at by case-by-case analysis of the values of
++ // centuryMod4 and diff (and the above clue to multiply by -3 = 4):
++ if (qMod<7>(diff * 4 + centuryMod4) < 4) {
++ // Century offset maps qMod<7>(diff) in {5, 6} to -1, {3, 4} to +2, and {1, 2} to +1:
++ year += (((qMod<7>(diff) + 3) / 2) % 4 - 1) * 100;
++ maybe = julianFromParts(year > 0 ? year : year - 1, parts.month, parts.day);
++ if (maybe && weekDayOfJulian(*maybe) == dow)
++ return *maybe;
++ Q_ASSERT(parts.month == 2 && parts.day == 29
++ && dow != int(Qt::Tuesday) && !(year % 100));
++ }
++
++ } else if (parts.month == 2 && parts.day == 29) {
++ int year = parts.year < 0 ? parts.year + 1 : parts.year;
++ // Feb 29th on a century needs to resolve to a multiple of 400 years.
++ const auto yearSplit = qDivMod<100>(year);
++ if (!yearSplit.remainder) {
++ const auto centuryMod4 = qMod<4>(yearSplit.quotient);
++ Q_ASSERT(centuryMod4); // or we'd have got a valid date to begin with.
++ if (centuryMod4 == 1) // round down
++ year -= 100;
++ else // 2 or 3; round up
++ year += (4 - centuryMod4) * 100;
++ maybe = julianFromParts(year > 0 ? year : year - 1, parts.month, parts.day);
++ if (maybe && weekDayOfJulian(*maybe) == dow) // (Can only happen for Tuesday.)
++ return *maybe;
++ Q_ASSERT(dow != int(Qt::Tuesday));
++ }
++ }
++ return (std::numeric_limits<qint64>::min)();
++ }
++
++ int QGregorianCalendar::yearStartWeekDay(int year)
++ {
++ // Equivalent to weekDayOfJulian(julianForParts({year, 1, 1})
++ const int y = year - (year < 0 ? 800 : 801);
++ return qMod<7>(y + qDiv<4>(y) - qDiv<100>(y) + qDiv<400>(y)) + 1;
++ }
++
++ int QGregorianCalendar::yearSharingWeekDays(QDate date)
++ {
++ // Returns a post-epoch year, no later than 2400, that has the same pattern
++ // of week-days (in the proleptic Gregorian calendar) as the year in which
++ // the given date falls. This will be the year in question if it's in the
++ // given range. Otherwise, the returned year's last two (decimal) digits
++ // won't coincide with the month number or day-of-month of the given date.
++ // For positive years, except when necessary to avoid such a clash, the
++ // returned year's last two digits shall coincide with those of the original
++ // year.
++
++ // Needed when formatting dates using system APIs with limited year ranges
++ // and possibly only a two-digit year. (The need to be able to safely
++ // replace the two-digit form of the returned year with a suitable form of
++ // the true year, when they don't coincide, is why the last two digits are
++ // treated specially.)
++
++ static_assert((400 * 365 + 97) % 7 == 0);
++ // A full 400-year cycle of the Gregorian calendar has 97 + 400 * 365 days;
++ // as 365 is one more than a multiple of seven and 497 is a multiple of
++ // seven, that full cycle is a whole number of weeks. So adding a multiple
++ // of four hundred years should get us a result that meets our needs.
++
++ const int year = date.year();
++ int res = (year < 1970
++ ? 2400 - (2000 - (year < 0 ? year + 1 : year)) % 400
++ : year > 2399 ? 2000 + (year - 2000) % 400 : year);
++ Q_ASSERT(res > 0);
++ if (res != year) {
++ const int lastTwo = res % 100;
++ if (lastTwo == date.month() || lastTwo == date.day()) {
++ Q_ASSERT(lastTwo && !(lastTwo & ~31));
++ // Last two digits of these years are all > 31:
++ static constexpr int usual[] = { 2198, 2199, 2098, 2099, 2399, 2298, 2299 };
++ static constexpr int leaps[] = { 2396, 2284, 2296, 2184, 2196, 2084, 2096 };
++ // Indexing is: first day of year's day-of-week, Monday = 0, one less
++ // than Qt's, as it's simpler to subtract one than to s/7/0/.
++ res = (leapTest(year) ? leaps : usual)[yearStartWeekDay(year) - 1];
++ }
++ Q_ASSERT(QDate(res, 1, 1).dayOfWeek() == QDate(year, 1, 1).dayOfWeek());
++ Q_ASSERT(QDate(res, 12, 31).dayOfWeek() == QDate(year, 12, 31).dayOfWeek());
++ }
++ Q_ASSERT(res >= 1970 && res <= 2400);
++ return res;
++ }
++
++ /*
++ * Math from The Calendar FAQ at http://www.tondering.dk/claus/cal/julperiod.php
++ * This formula is correct for all julian days, when using mathematical integer
++ * division (round to negative infinity), not c++11 integer division (round to zero).
++ *
++ * The source given uses 4801 BCE as base date; the following adjusts that by
++ * 4800 years to simplify part of the arithmetic (and match more closely what we
++ * do for Milankovic).
++ */
++
++ using namespace QRomanCalendrical;
++ // End a Gregorian four-century cycle on 1 BC's leap day:
++ constexpr qint64 BaseJd = LeapDayGregorian1Bce;
++ // Every four centures there are 97 leap years:
++ constexpr unsigned FourCenturies = 400 * 365 + 97;
++
++ std::optional<qint64> QGregorianCalendar::julianFromParts(int year, int month, int day)
++ {
++ if (!validParts(year, month, day))
++ return std::nullopt;
++
++ const auto yearDays = yearMonthToYearDays(year, month);
++ const qint64 y = yearDays.year;
++ const qint64 fromYear = 365 * y + qDiv<4>(y) - qDiv<100>(y) + qDiv<400>(y);
++ return fromYear + yearDays.days + day + BaseJd;
++ }
++
++ QCalendar::YearMonthDay QGregorianCalendar::partsFromJulian(qint64 jd)
++ {
++ const qint64 dayNumber = jd - BaseJd;
++ const qint64 century = qDiv<FourCenturies>(4 * dayNumber - 1);
++ const int dayInCentury = dayNumber - qDiv<4>(FourCenturies * century);
++
++ const int yearInCentury = qDiv<FourYears>(4 * dayInCentury - 1);
++ const int dayInYear = dayInCentury - qDiv<4>(FourYears * yearInCentury);
++ const int m = qDiv<FiveMonths>(5 * dayInYear - 3);
++ Q_ASSERT(m < 12 && m >= 0);
++ // That m is a month adjusted to March = 0, with Jan = 10, Feb = 11 in the previous year.
++ const int yearOffset = m < 10 ? 0 : 1;
++
++ const int y = 100 * century + yearInCentury + yearOffset;
++ const int month = m + 3 - 12 * yearOffset;
++ const int day = dayInYear - qDiv<5>(FiveMonths * m + 2);
++
++ // Adjust for no year 0
++ return QCalendar::YearMonthDay(y > 0 ? y : y - 1, month, day);
++ }
++
++ QT_END_NAMESPACE