qmilankoviccalendar.cpp

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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
 
#include "qglobal.h"
#include "qmilankoviccalendar_p.h"
#include "qcalendarmath_p.h"
 
#include <QtCore/qdatetime.h>
 
QT_BEGIN_NAMESPACE
 
using namespace QRoundingDown;
 
QString QMilankovicCalendar::name() const
{
    return QStringLiteral("Milankovic");
}
 
QStringList QMilankovicCalendar::nameList()
{
    return { QStringLiteral("Milankovic") };
}
 
bool QMilankovicCalendar::isLeapYear(int year) const
{
    if (year == QCalendar::Unspecified)
        return false;
    if (year <= 0)
        ++year;
    if (qMod<4>(year))
        return false;
    const auto yeardm = qDivMod<100>(year);
    if (yeardm.remainder == 0) {
        const qint16 century = qMod<9>(yeardm.quotient);
        if (century != 2 && century != 6)
            return false;
    }
    return true;
}
 
using namespace QRomanCalendrical;
// End a Milankovic nine-century cycle on 1 BC, Feb 28 (Gregorian Feb 29):
constexpr qint64 MilankovicBaseJd = LeapDayGregorian1Bce;
// Leap years every 4 years, except for 7 turn-of-century years per nine centuries:
constexpr unsigned NineCenturies = 365 * 900 + 900 / 4 - 7;
// When the turn-of-century is a leap year, the century has 25 leap years in it:
constexpr unsigned LeapCentury = 365 * 100 + 25;
 
bool QMilankovicCalendar::dateToJulianDay(int year, int month, int day, qint64 *jd) const
{
    Q_ASSERT(jd);
    if (!isDateValid(year, month, day))
        return false;
 
    const auto yearDays = yearMonthToYearDays(year, month);
    const auto centuryYear = qDivMod<100>(yearDays.year);
    const qint64 fromYear = qDiv<9>(NineCenturies * centuryYear.quotient + 6)
                          + qDiv<100>(LeapCentury * centuryYear.remainder);
    *jd = fromYear + yearDays.days + day + MilankovicBaseJd;
    return true;
}
 
QCalendar::YearMonthDay  QMilankovicCalendar::julianDayToDate(qint64 jd) const
{
    const auto century9Day = qDivMod<NineCenturies>(9 * (jd - MilankovicBaseJd) - 7);
    // Its remainder changes by 9 per day, except roughly once per century.
    const auto year100Day = qDivMod<LeapCentury>(100 * qDiv<9>(century9Day.remainder) + 99);
    // Its remainder changes by 100 per day, except roughly once per year.
    const auto ymd = dayInYearToYmd(qDiv<100>(year100Day.remainder));
    const int y = 100 * century9Day.quotient + year100Day.quotient + ymd.year;
    return QCalendar::YearMonthDay(y > 0 ? y : y - 1, ymd.month, ymd.day);
}
 
QT_END_NAMESPACE