/*
    This file is part of the kholidays library.

    SPDX-FileCopyrightText: 2014 John Layt <john@layt.net>

    SPDX-License-Identifier: LGPL-2.0-or-later
*/

#include "qcalendarsystem_p.h"

#include <kholidays_debug.h>

#include <QDate>
#include <QSharedData>

class QCalendarSystemPrivate : public QSharedData
{
public:
    explicit QCalendarSystemPrivate(QCalendarSystem::CalendarSystem calendar);

    QCalendarSystem::CalendarSystem calendarSystem() const;
    qint64 epoch() const;
    qint64 earliestValidDate() const;
    int earliestValidYear() const;
    qint64 latestValidDate() const;
    int latestValidYear() const;
    int yearOffset() const;
    int maxMonthsInYear() const;
    int monthsInYear(int year) const;
    int maxDaysInYear() const;
    int daysInYear(int year) const;
    int maxDaysInMonth() const;
    int daysInMonth(int year, int month) const;
    bool hasYearZero() const;
    bool hasLeapMonths() const;

    int quarter(int month) const;
    bool isLeapYear(int year) const;
    void julianDayToDate(qint64 jd, int *year, int *month, int *day) const;
    qint64 julianDayFromDate(int year, int month, int day) const;

    bool isValidYear(int year) const;
    bool isValidMonth(int year, int month) const;
    int addYears(int y1, int years) const;
    int diffYears(int y1, int y2) const;

    QCalendarSystem::CalendarSystem m_calendarSystem;
};

static const char julianMonths[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

QCalendarSystemPrivate::QCalendarSystemPrivate(QCalendarSystem::CalendarSystem calendar)
    : QSharedData()
    , m_calendarSystem(calendar)
{
}

QCalendarSystem::CalendarSystem QCalendarSystemPrivate::calendarSystem() const
{
    if (m_calendarSystem == QCalendarSystem::DefaultCalendar) {
        return QCalendarSystem::GregorianCalendar;
    } else {
        return m_calendarSystem;
    }
}

qint64 QCalendarSystemPrivate::epoch() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
        return 1721426; //  0001-01-01 Gregorian
    case QCalendarSystem::CopticCalendar:
        return 1825030; //  0001-01-01 ==  0284-08-29 Gregorian
    case QCalendarSystem::EthiopicCalendar:
        return 1724221; //  0001-01-01 ==  0008-08-29 Gregorian
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        return -284655; //  0001-01-01 == -5492-08-29 Gregorian
    case QCalendarSystem::IndianNationalCalendar:
        return 1749994; //  0000-01-01 == 0078-03-22 Gregorian
    case QCalendarSystem::IslamicCivilCalendar:
        return 1948440; //  0001-01-01 == 0622-07-19 Gregorian
    case QCalendarSystem::ISO8601Calendar:
        return 1721060; //  0000-01-01 Gregorian
    case QCalendarSystem::JapaneseCalendar:
        return 1721426; //  0001-01-01 Gregorian
    case QCalendarSystem::JulianCalendar:
        return 1721424; //  0001-01-01 ==  Gregorian
    case QCalendarSystem::ROCCalendar:
        return 2419403; //  0001-01-01 ==  1912-01-01 Gregorian
    case QCalendarSystem::ThaiCalendar:
        return 1522734; //  0000-01-01 == -0544-01-01 Gregorian
    default:
        return 0;
    }
}

qint64 QCalendarSystemPrivate::earliestValidDate() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
        return -31738; // -4800-01-01 Gregorian
    case QCalendarSystem::CopticCalendar:
        return 1825030; //  0001-01-01 == 0284-08-29 Gregorian
    case QCalendarSystem::EthiopicCalendar:
        return 1724221; //  0001-01-01 == 0008-08-29 Gregorian
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        return -284655; //  0001-01-01 == -5492-08-29 Gregorian
    case QCalendarSystem::IndianNationalCalendar:
        return 1749994; //  0000-01-01 == 0078-03-22 Gregorian
    case QCalendarSystem::IslamicCivilCalendar:
        return 1948440; //  0001-01-01 == 0622-07-19 Gregorian
    case QCalendarSystem::ISO8601Calendar:
        return 1721060; //  0000-01-01 Gregorian
    case QCalendarSystem::JapaneseCalendar:
        return -31738; // -4800-01-01 Gregorian
    case QCalendarSystem::JulianCalendar:
        return -31776; // -4800-01-01 Julian
    case QCalendarSystem::ROCCalendar:
        return 2419403; //  0001-01-01 == 1912-01-01 Gregorian
    case QCalendarSystem::ThaiCalendar:
        return 1522734; //  0000-01-01 == -0544-01-01 Gregorian
    default:
        return 0;
    }
}

int QCalendarSystemPrivate::earliestValidYear() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
    case QCalendarSystem::JapaneseCalendar:
    case QCalendarSystem::JulianCalendar:
        return -4800;
    case QCalendarSystem::IndianNationalCalendar:
    case QCalendarSystem::ISO8601Calendar:
    case QCalendarSystem::ThaiCalendar:
        return 0;
    default:
        return 1;
    }
}

qint64 QCalendarSystemPrivate::latestValidDate() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
        return 5373484; //  9999-12-31 Gregorian
    case QCalendarSystem::CopticCalendar:
        return 5477164; //  9999-13-05 == 10283-11-12 Gregorian
    case QCalendarSystem::EthiopicCalendar:
        return 5376721; //  9999-13-05 == 10008-11-10 Gregorian
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        return 3367114; //  9999-13-05 ==  4506-09-29 Gregorian
    case QCalendarSystem::IndianNationalCalendar:
        return 5402054; //  9999-12-30 == 10078-03-21 Gregorian
    case QCalendarSystem::IslamicCivilCalendar:
        return 5491751; //  9999-12-29 == 10323-10-21 Gregorian
    case QCalendarSystem::ISO8601Calendar:
        return 5373484; //  9999-12-31 Gregorian
    case QCalendarSystem::JapaneseCalendar:
        return 5373484; //  9999-12-31 Gregorian
    case QCalendarSystem::JulianCalendar:
        return 5373557; //  9999-12-31 == 10000-03-13 Gregorian
    case QCalendarSystem::ROCCalendar:
        return 6071462; //  9999-12-31 == 11910-12-31 Gregorian
    case QCalendarSystem::ThaiCalendar:
        return 5175158; //  9999-12-31 ==  9456-12-31 Gregorian
    default:
        return 0;
    }
}

int QCalendarSystemPrivate::latestValidYear() const
{
    switch (calendarSystem()) {
    default:
        return 9999;
    }
}

int QCalendarSystemPrivate::yearOffset() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::ROCCalendar:
        return 1911; // 0001-01-01 == 1912-01-01 Gregorian
    case QCalendarSystem::ThaiCalendar:
        return -543; // 0000-01-01 == -544-01-01 Gregorian
    default:
        return 0;
    }
}

int QCalendarSystemPrivate::maxMonthsInYear() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        return 13;
    default:
        return 12;
    }
}

int QCalendarSystemPrivate::monthsInYear(int year) const
{
    // year = year + yearOffset();
    Q_UNUSED(year);

    switch (calendarSystem()) {
    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        return 13;
    default:
        return 12;
    }
}

int QCalendarSystemPrivate::maxDaysInYear() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::IslamicCivilCalendar:
        return 355;
    default:
        return 366;
    }
}

int QCalendarSystemPrivate::daysInYear(int year) const
{
    switch (calendarSystem()) {
    case QCalendarSystem::IslamicCivilCalendar:
        return isLeapYear(year) ? 355 : 354;
    default:
        return isLeapYear(year) ? 366 : 365;
    }
}

int QCalendarSystemPrivate::maxDaysInMonth() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
    case QCalendarSystem::IslamicCivilCalendar:
        return 30;
    default:
        return 31;
    }
}

int QCalendarSystemPrivate::daysInMonth(int year, int month) const
{
    if (month < 1 || month > monthsInYear(year)) {
        return 0;
    }

    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
    case QCalendarSystem::ISO8601Calendar:
    case QCalendarSystem::JapaneseCalendar:
    case QCalendarSystem::ROCCalendar:
    case QCalendarSystem::ThaiCalendar:
    case QCalendarSystem::JulianCalendar: {
        if (month == 2 && isLeapYear(year)) {
            return 29;
        } else {
            return julianMonths[month];
        }
    }
    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar: {
        if (month == 13) {
            return isLeapYear(year) ? 6 : 5;
        } else {
            return 30;
        }
    }
    case QCalendarSystem::IndianNationalCalendar: {
        if (month >= 7) {
            return 30;
        } else if (month >= 2) {
            return 31;
        } else if (isLeapYear(year)) {
            return 31;
        } else {
            return 30;
        }
    }
    case QCalendarSystem::IslamicCivilCalendar: {
        if (month == 12 && isLeapYear(year)) {
            return 30;
        } else if (month % 2 == 0) {
            return 29;
        } else {
            return 30;
        }
    }
    default:
        return 0;
    }
}

bool QCalendarSystemPrivate::hasYearZero() const
{
    switch (calendarSystem()) {
    case QCalendarSystem::IndianNationalCalendar:
    case QCalendarSystem::ISO8601Calendar:
    case QCalendarSystem::ThaiCalendar:
        return true;
    default:
        return false;
    }
}

bool QCalendarSystemPrivate::hasLeapMonths() const
{
    switch (calendarSystem()) {
    default:
        return false;
    }
}

int QCalendarSystemPrivate::quarter(int month) const
{
    switch (calendarSystem()) {
    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        if (month == 13) { // Consider the short epagomenal month as part of the 4th quarter
            return 4;
        }
        Q_FALLTHROUGH();
    default:
        return (((month - 1) / 3) + 1);
    }
}

bool QCalendarSystemPrivate::isLeapYear(int year) const
{
    year = year + yearOffset();

    // Uses same rule as Gregorian and in same years as Gregorian to keep in sync.
    // Can't use yearOffset() as this offset only applies for isLeapYear().
    if (calendarSystem() == QCalendarSystem::IndianNationalCalendar) {
        year = year + 78;
    }

    if (year < 1 && !hasYearZero()) {
        ++year;
    }

    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
    case QCalendarSystem::IndianNationalCalendar:
    case QCalendarSystem::ISO8601Calendar:
    case QCalendarSystem::JapaneseCalendar:
    case QCalendarSystem::ROCCalendar:
    case QCalendarSystem::ThaiCalendar:
        return ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0);
    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar:
        return (year % 4 == 3);
    case QCalendarSystem::JulianCalendar:
        return (year % 4 == 0);
    case QCalendarSystem::IslamicCivilCalendar:
        return ((((11 * year) + 14) % 30) < 11);
    default:
        return false;
    }
}

void QCalendarSystemPrivate::julianDayToDate(qint64 jd, int *year, int *month, int *day) const
{
    int yy = 0;
    int mm = 0;
    int dd = 0;

    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
    case QCalendarSystem::ISO8601Calendar:
    case QCalendarSystem::JapaneseCalendar:
    case QCalendarSystem::ROCCalendar:
    case QCalendarSystem::ThaiCalendar: {
        // Formula from The Calendar FAQ by Claus Tondering
        // https://www.tondering.dk/claus/calendar.html
        qint64 a = jd + 32044;
        qint64 b = ((4 * a) + 3) / 146097;
        qint64 c = a - ((146097 * b) / 4);
        qint64 d = ((4 * c) + 3) / 1461;
        qint64 e = c - ((1461 * d) / 4);
        qint64 m = ((5 * e) + 2) / 153;
        dd = e - (((153 * m) + 2) / 5) + 1;
        mm = m + 3 - (12 * (m / 10));
        yy = (100 * b) + d - 4800 + (m / 10);
        break;
    }

    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar: {
        // Formula derived from first principles by John Layt
        qint64 s = jd - (epoch() - 365);
        qint64 l = s / 1461;
        yy = (l * 4) + qMin(static_cast<qint64>(3), (s % 1461) / 365);
        qint64 diy = s - (yy * 365) + (yy / 4);
        mm = (diy / 30) + 1;
        dd = (diy % 30) + 1;
        break;
    }

    case QCalendarSystem::IndianNationalCalendar: {
        // Formula from the "Explanatory Supplement to the Astronomical Almanac"
        // Revised Edition 2006 section 12.94 pp 605-606, US Naval Observatory
        // Originally from the "Report of the Calendar Reform Committee" 1955, Indian Government
        qint64 l = jd + 68518;
        qint64 n = (4 * l) / 146097;
        l = l - (146097 * n + 3) / 4;
        qint64 i = (4000 * (l + 1)) / 1461001;
        l = l - (1461 * i) / 4 + 1;
        qint64 j = ((l - 1) / 31) * (1 - l / 185) + (l / 185) * ((l - 156) / 30 + 5) - l / 366;
        dd = l - 31 * j + ((j + 2) / 8) * (j - 5);
        l = j / 11;
        mm = j + 2 - 12 * l;
        yy = 100 * (n - 49) + l + i - 78;
        break;
    }

    case QCalendarSystem::IslamicCivilCalendar: {
        // Formula from the "Explanatory Supplement to the Astronomical Almanac"
        // Revised Edition 2006 section ??? pp ???, US Naval Observatory
        // Derived from Fliegel & Van Flandern 1968
        qint64 l = jd - epoch() + 10632;
        qint64 n = (l - 1) / 10631;
        l = l - 10631 * n + 354;
        int j = ((10985 - l) / 5316) * ((50 * l) / 17719) + (l / 5670) * ((43 * l) / 15238);
        l = l - ((30 - j) / 15) * ((17719 * j) / 50) - (j / 16) * ((15238 * j) / 43) + 29;
        yy = (30 * n) + j - 30;
        mm = (24 * l) / 709;
        dd = l - ((709 * mm) / 24);
        break;
    }

    case QCalendarSystem::JulianCalendar: {
        // Formula from The Calendar FAQ by Claus Tondering
        // https://www.tondering.dk/claus/calendar.html
        qint64 b = 0;
        qint64 c = jd + 32082;
        qint64 d = ((4 * c) + 3) / 1461;
        qint64 e = c - ((1461 * d) / 4);
        qint64 m = ((5 * e) + 2) / 153;
        dd = e - (((153 * m) + 2) / 5) + 1;
        mm = m + 3 - (12 * (m / 10));
        yy = (100 * b) + d - 4800 + (m / 10);
        break;
    }

    default:
        break;
    }

    if (!hasYearZero() && yy < 1) {
        yy -= 1;
    }

    yy = yy - yearOffset();

    if (year) {
        *year = yy;
    }
    if (month) {
        *month = mm;
    }
    if (day) {
        *day = dd;
    }
}

qint64 QCalendarSystemPrivate::julianDayFromDate(int year, int month, int day) const
{
    qint64 jd = 0;

    year = year + yearOffset();

    if (year < 1 && !hasYearZero()) {
        year = year + 1;
    }

    switch (calendarSystem()) {
    case QCalendarSystem::GregorianCalendar:
    case QCalendarSystem::ISO8601Calendar:
    case QCalendarSystem::JapaneseCalendar:
    case QCalendarSystem::ROCCalendar:
    case QCalendarSystem::ThaiCalendar: {
        // Formula from The Calendar FAQ by Claus Tondering
        // https://www.tondering.dk/claus/calendar.html
        int a = (14 - month) / 12;
        year = year + 4800 - a;
        int m = month + (12 * a) - 3;
        jd = day + (((153 * m) + 2) / 5) //
            + (365 * year) //
            + (year / 4) //
            - (year / 100) //
            + (year / 400) //
            - 32045;
        break;
    }

    case QCalendarSystem::CopticCalendar:
    case QCalendarSystem::EthiopicCalendar:
    case QCalendarSystem::EthiopicAmeteAlemCalendar: {
        // Formula derived from first principles by John Layt
        jd = epoch() - 1 //
            + ((year - 1) * 365) //
            + (year / 4) //
            + ((month - 1) * 30) //
            + day;
        break;
    }

    case QCalendarSystem::IndianNationalCalendar: {
        // Formula from the "Explanatory Supplement to the Astronomical Almanac"
        // Revised Edition 2006 section 12.94 pp 605-606, US Naval Observatory
        // Originally from the "Report of the Calendar Reform Committee" 1955, Indian Government
        jd = 365 * year + (year + 78 - 1 / month) / 4 //
            + 31 * month //
            - (month + 9) / 11 //
            - (month / 7) * (month - 7) //
            - (3 * ((year + 78 - 1 / month) / 100 + 1)) / 4 //
            + day //
            + 1749579;
        break;
    }

    case QCalendarSystem::IslamicCivilCalendar: {
        // Formula from the "Explanatory Supplement to the Astronomical Almanac"
        // Revised Edition 2006 section ??? pp ???, US Naval Observatory
        // Derived from Fliegel & Van Flandern 1968
        jd = (3 + (11 * year)) / 30 //
            + 354 * year //
            + 30 * month //
            - (month - 1) / 2 //
            + day //
            + epoch() //
            - 385;
        break;
    }

    case QCalendarSystem::JulianCalendar: {
        // Formula from The Calendar FAQ by Claus Tondering
        // https://www.tondering.dk/claus/calendar.html
        int a = (14 - month) / 12;
        year = year + 4800 - a;
        int m = month + (12 * a) - 3;
        jd = day //
            + (((153 * m) + 2) / 5) //
            + (365 * year) //
            + (year / 4) //
            - 32083;
        break;
    }

    default:
        break;
    }

    return jd;
}

// Some private utility rules

bool QCalendarSystemPrivate::isValidYear(int year) const
{
    return year >= earliestValidYear() //
        && year <= latestValidYear() //
        && (year == 0 ? hasYearZero() : true);
}

bool QCalendarSystemPrivate::isValidMonth(int year, int month) const
{
    return isValidYear(year) && month >= 1 && month <= monthsInYear(year);
}

int QCalendarSystemPrivate::addYears(int y1, int years) const
{
    int y2 = y1 + years;

    if (!hasYearZero()) {
        if (y1 > 0 && y2 <= 0) {
            --y2;
        } else if (y1 < 0 && y2 >= 0) {
            ++y2;
        }
    }

    return y2;
}

int QCalendarSystemPrivate::diffYears(int y1, int y2) const
{
    int dy = y2 - y1;

    if (!hasYearZero()) {
        if (y2 > 0 && y1 < 0) {
            dy -= 1;
        } else if (y2 < 0 && y1 > 0) {
            dy += 1;
        }
    }

    return dy;
}

// QCalendarSystem public api

QCalendarSystem::QCalendarSystem(QCalendarSystem::CalendarSystem calendar)
    : d(new QCalendarSystemPrivate(calendar))
{
}

QCalendarSystem::~QCalendarSystem()
{
}

QCalendarSystem &QCalendarSystem::operator=(const QCalendarSystem &other)
{
    d = other.d;
    return *this;
}

QCalendarSystem::CalendarSystem QCalendarSystem::calendarSystem() const
{
    return d->calendarSystem();
}

QDate QCalendarSystem::epoch() const
{
    return QDate::fromJulianDay(d->epoch());
}

QDate QCalendarSystem::earliestValidDate() const
{
    return QDate::fromJulianDay(d->earliestValidDate());
}

QDate QCalendarSystem::latestValidDate() const
{
    return QDate::fromJulianDay(d->latestValidDate());
}

int QCalendarSystem::maximumMonthsInYear() const
{
    return d->maxMonthsInYear();
}

int QCalendarSystem::maximumDaysInYear() const
{
    return d->maxDaysInYear();
}

int QCalendarSystem::maximumDaysInMonth() const
{
    return d->maxDaysInMonth();
}

bool QCalendarSystem::isValid(const QDate &date) const
{
    return date.isValid() && date >= earliestValidDate() && date <= latestValidDate();
}

bool QCalendarSystem::isValid(int year, int month, int day) const
{
    return d->isValidMonth(year, month) && day >= 1 && day <= d->daysInMonth(year, month);
}

bool QCalendarSystem::isValid(int year, int dayOfYear) const
{
    return d->isValidYear(year) && dayOfYear > 0 && dayOfYear <= d->daysInYear(year);
}

QDate QCalendarSystem::date(int year, int month, int day) const
{
    if (isValid(year, month, day)) {
        return QDate::fromJulianDay(d->julianDayFromDate(year, month, day));
    } else {
        return QDate();
    }
}

QDate QCalendarSystem::date(int year, int dayOfYear) const
{
    if (isValid(year, dayOfYear)) {
        return QDate::fromJulianDay(d->julianDayFromDate(year, 1, 1) + dayOfYear - 1);
    } else {
        return QDate();
    }
}

void QCalendarSystem::getDate(const QDate &date, int *year, int *month, int *day) const
{
    int yy = 0;
    int mm = 0;
    int dd = 0;

    if (isValid(date)) {
        d->julianDayToDate(date.toJulianDay(), &yy, &mm, &dd);
    }

    if (year) {
        *year = yy;
    }
    if (month) {
        *month = mm;
    }
    if (day) {
        *day = dd;
    }
}

int QCalendarSystem::year(const QDate &date) const
{
    int y = 0;

    if (isValid(date)) {
        d->julianDayToDate(date.toJulianDay(), &y, nullptr, nullptr);
    }

    return y;
}

int QCalendarSystem::month(const QDate &date) const
{
    int m = 0;

    if (isValid(date)) {
        d->julianDayToDate(date.toJulianDay(), nullptr, &m, nullptr);
    }

    return m;
}

int QCalendarSystem::day(const QDate &date) const
{
    int dd = 0;

    if (isValid(date)) {
        d->julianDayToDate(date.toJulianDay(), nullptr, nullptr, &dd);
    }

    return dd;
}

int QCalendarSystem::quarter(const QDate &date) const
{
    if (isValid(date)) {
        int month;
        d->julianDayToDate(date.toJulianDay(), nullptr, &month, nullptr);
        return d->quarter(month);
    } else {
        return 0;
    }
}

int QCalendarSystem::quarter(int year, int month, int day) const
{
    if (isValid(year, month, day)) {
        return d->quarter(month);
    } else {
        return 0;
    }
}

int QCalendarSystem::dayOfYear(const QDate &date) const
{
    if (isValid(date)) {
        return date.toJulianDay() - firstDayOfYear(date).toJulianDay() + 1;
    } else {
        return 0;
    }
}

int QCalendarSystem::dayOfYear(int year, int month, int day) const
{
    return dayOfYear(date(year, month, day));
}

int QCalendarSystem::dayOfWeek(const QDate &date) const
{
    // jd 0 = Monday = weekday 1.  We've never skipped weekdays.
    if (isValid(date)) {
        if (date.toJulianDay() >= 0) {
            return (date.toJulianDay() % daysInWeek()) + 1;
        } else {
            return ((date.toJulianDay() + 1) % daysInWeek()) + daysInWeek();
        }
    } else {
        return 0;
    }
}

int QCalendarSystem::dayOfWeek(int year, int month, int day) const
{
    return dayOfWeek(date(year, month, day));
}

// TODO These are ISO weeks, may need to localise
int QCalendarSystem::weekNumber(const QDate &date, int *yearNum) const
{
    if (isValid(date)) {
        int year;
        int month;
        int day;
        d->julianDayToDate(date.toJulianDay(), &year, &month, &day);
        return weekNumber(year, month, day, yearNum);
    } else {
        return 0;
    }
}

/*
    The following method is based on code from tzcode's strftime.c, which is turn said to be
    "Based on the UCB version with the ID appearing below."

    SPDX-FileCopyrightText: 1989 The Regents of the University of California. All rights reserved.

    SPDX-License-Identifier: BSD-3-Clause
*/
// TODO These are ISO weeks, may need to localise
// TODO Replace with cleanly licensed routine
int QCalendarSystem::weekNumber(int year, int month, int day, int *yearNum) const
{
    if (!isValid(year, month, day)) {
        if (yearNum) {
            *yearNum = 0;
        }
        return 0;
    }

    int yday = dayOfYear(year, month, day) - 1;
    int wday = dayOfWeek(year, month, day);
    if (wday == 7) {
        wday = 0;
    }
    int w;

    for (;;) {
        int len;
        int bot;
        int top;

        len = d->daysInYear(year);
        /*
        ** What yday (-3 ... 3) does
        ** the ISO year begin on?
        */
        bot = ((yday + 11 - wday) % 7) - 3;
        /*
        ** What yday does the NEXT
        ** ISO year begin on?
        */
        top = bot - (len % 7);
        if (top < -3) {
            top += 7;
        }
        top += len;
        if (yday >= top) {
            ++year;
            w = 1;
            break;
        }
        if (yday >= bot) {
            w = 1 + ((yday - bot) / 7);
            break;
        }
        --year;
        yday += d->daysInYear(year);
    }

    if (yearNum) {
        *yearNum = year;
    }

    return w;
}

int QCalendarSystem::monthsInYear(const QDate &date) const
{
    if (isValid(date)) {
        return d->monthsInYear(year(date));
    } else {
        return 0;
    }
}

int QCalendarSystem::monthsInYear(int year) const
{
    if (d->isValidYear(year)) {
        return d->monthsInYear(year);
    } else {
        return 0;
    }
}

int QCalendarSystem::weeksInYear(const QDate &date) const
{
    if (isValid(date)) {
        return weeksInYear(year(date));
    } else {
        return 0;
    }
}

// TODO This is ISO weeks, may need to localise
int QCalendarSystem::weeksInYear(int year) const
{
    if (d->isValidYear(year)) {
        int weekYear = year;
        int lastWeek = weekNumber(lastDayOfYear(year), &weekYear);
        if (lastWeek < 1 || weekYear != year) {
            lastWeek = weekNumber(addDays(lastDayOfYear(year), -7), &weekYear);
        }
        return lastWeek;
    } else {
        return 0;
    }
}

int QCalendarSystem::daysInYear(const QDate &date) const
{
    if (isValid(date)) {
        return d->daysInYear(year(date));
    } else {
        return 0;
    }
}

int QCalendarSystem::daysInYear(int year) const
{
    if (d->isValidYear(year)) {
        return d->daysInYear(year);
    } else {
        return 0;
    }
}

int QCalendarSystem::daysInMonth(const QDate &date) const
{
    if (isValid(date)) {
        int year;
        int month;
        d->julianDayToDate(date.toJulianDay(), &year, &month, nullptr);
        return d->daysInMonth(year, month);
    } else {
        return 0;
    }
}

int QCalendarSystem::daysInMonth(int year, int month) const
{
    if (d->isValidMonth(year, month)) {
        return d->daysInMonth(year, month);
    } else {
        return 0;
    }
}

int QCalendarSystem::daysInWeek() const
{
    return 7;
}

bool QCalendarSystem::isLeapYear(const QDate &date) const
{
    if (isValid(date)) {
        return d->isLeapYear(year(date));
    } else {
        return false;
    }
}

bool QCalendarSystem::isLeapYear(int year) const
{
    if (d->isValidYear(year)) {
        return d->isLeapYear(year);
    } else {
        return false;
    }
}

QDate QCalendarSystem::addYears(const QDate &dt, int years) const
{
    if (isValid(dt)) {
        int year;
        int month;
        int day;
        d->julianDayToDate(dt.toJulianDay(), &year, &month, &day);
        year = d->addYears(year, years);
        month = qMin(month, d->monthsInYear(year));
        return date(year, month, qMin(day, d->daysInMonth(year, month)));
    } else {
        return QDate();
    }
}

QDate QCalendarSystem::addMonths(const QDate &dt, int months) const
{
    if (isValid(dt)) {
        int year;
        int month;
        int day;
        d->julianDayToDate(dt.toJulianDay(), &year, &month, &day);
        while (months != 0) {
            if (months < 0) {
                if (month + months >= 1) {
                    month += months;
                    months = 0;
                } else {
                    year = d->addYears(year, -1);
                    months += d->monthsInYear(year);
                }
            } else {
                int miy = d->monthsInYear(year);
                if (month + months <= miy) {
                    month += months;
                    months = 0;
                } else {
                    year = d->addYears(year, 1);
                    months -= miy;
                }
            }
        }
        return date(year, month, qMin(day, d->daysInMonth(year, month)));
    } else {
        return QDate();
    }
}

QDate QCalendarSystem::addDays(const QDate &date, int days) const
{
    return date.addDays(days);
}

// Caters for Leap Months, but possibly not for Hebrew
int QCalendarSystem::yearsDifference(const QDate &fromDate, const QDate &toDate) const
{
    if (!isValid(fromDate) || !isValid(toDate) || toDate == fromDate) {
        return 0;
    }

    if (toDate < fromDate) {
        return -yearsDifference(toDate, fromDate);
    }

    int y1;
    int m1;
    int d1;
    int y2;
    int m2;
    int d2;
    d->julianDayToDate(fromDate.toJulianDay(), &y1, &m1, &d1);
    d->julianDayToDate(toDate.toJulianDay(), &y2, &m2, &d2);

    if (y2 == y1) {
        return 0;
    }

    if (m2 > m1) {
        return d->diffYears(y1, y2);
    }

    if (m2 < m1) {
        return d->diffYears(y1, y2) - 1;
    }

    // m2 == m1
    // Allow for last day of month to last day of month and leap days
    // e.g. 2000-02-29 to 2001-02-28 is 1 year not 0 years
    if (d2 >= d1 //
        || (d1 == d->daysInMonth(y1, m1) && d2 == d->daysInMonth(y2, m2))) {
        return d->diffYears(y1, y2);
    } else {
        return d->diffYears(y1, y2) - 1;
    }
}

// Caters for Leap Months, but possibly not for Hebrew
int QCalendarSystem::monthsDifference(const QDate &fromDate, const QDate &toDate) const
{
    if (!isValid(fromDate) || !isValid(toDate) || toDate == fromDate) {
        return 0;
    }

    if (toDate < fromDate) {
        return -monthsDifference(toDate, fromDate);
    }

    int y1;
    int m1;
    int d1;
    int y2;
    int m2;
    int d2;
    int my;
    d->julianDayToDate(fromDate.toJulianDay(), &y1, &m1, &d1);
    d->julianDayToDate(toDate.toJulianDay(), &y2, &m2, &d2);

    // Calculate the number of months in full years preceding y2.
    if (y2 == y1) {
        my = 0;
    } else if (d->hasLeapMonths()) {
        my = 0;
        for (int y = y1; y < y2; y = d->addYears(y, 1)) {
            my = my + monthsInYear(y);
        }
    } else {
        my = d->diffYears(y1, y2) * monthsInYear(y2);
    }

    // Allow for last day of month to last day of month and leap days
    // e.g. 2010-03-31 to 2010-04-30 is 1 month not 0 months
    // also 2000-02-29 to 2001-02-28 is 12 months not 11 months
    if (d2 >= d1 //
        || (d1 == d->daysInMonth(y1, m1) && d2 == d->daysInMonth(y2, m2))) {
        return my + m2 - m1;
    } else {
        return my + m2 - m1 - 1;
    }
}

qint64 QCalendarSystem::daysDifference(const QDate &fromDate, const QDate &toDate) const
{
    if (isValid(fromDate) && isValid(toDate)) {
        return toDate.toJulianDay() - fromDate.toJulianDay();
    } else {
        return 0;
    }
}

// Caters for Leap Months, but possibly not for Hebrew
void QCalendarSystem::dateDifference(const QDate &fromDate, const QDate &toDate, int *years, int *months, int *days, int *direction) const
{
    int dy = 0;
    int dm = 0;
    int dd = 0;
    int dir = 1;

    if (isValid(fromDate) && isValid(toDate) && fromDate != toDate) {
        if (toDate < fromDate) {
            dateDifference(toDate, fromDate, &dy, &dm, &dd, nullptr);
            dir = -1;
        } else {
            int y1;
            int m1;
            int d1;
            int y2;
            int m2;
            int d2;
            d->julianDayToDate(fromDate.toJulianDay(), &y1, &m1, &d1);
            d->julianDayToDate(toDate.toJulianDay(), &y2, &m2, &d2);

            dy = yearsDifference(fromDate, toDate);

            // Calculate months and days difference
            int miy0 = d->monthsInYear(d->addYears(y2, -1));
            if (d2 >= d1) {
                dm = (miy0 + m2 - m1) % miy0;
                dd = d2 - d1;
            } else { // d2 < d1
                // Allow for last day of month to last day of month and leap days
                // e.g. 2010-03-31 to 2010-04-30 is 1 month
                //      2000-02-29 to 2001-02-28 is 1 year
                //      2000-02-29 to 2001-03-01 is 1 year 1 day
                int dim0 = daysInMonth(addMonths(toDate, -1));
                int dim1 = d->daysInMonth(y1, m1);
                if (d1 == dim1 && d2 == d->daysInMonth(y2, m2)) {
                    dm = (miy0 + m2 - m1) % miy0;
                    dd = 0;
                } else if (month(addMonths(toDate, -1)) == m1 && dim0 < dim1) {
                    // Special case where fromDate = leap day and toDate in month following but non-leap year
                    // e.g. 2000-02-29 to 2001-03-01 needs to use 29 to calculate day number not 28
                    dm = (miy0 + m2 - m1 - 1) % miy0;
                    dd = (dim1 + d2 - d1) % dim1;
                } else {
                    dm = (miy0 + m2 - m1 - 1) % miy0;
                    dd = (dim0 + d2 - d1) % dim0;
                }
            }
        }
    }

    if (years) {
        *years = dy;
    }
    if (months) {
        *months = dm;
    }
    if (days) {
        *days = dd;
    }
    if (direction) {
        *direction = dir;
    }
}

QDate QCalendarSystem::firstDayOfYear(const QDate &dt) const
{
    if (isValid(dt)) {
        return date(year(dt), 1, 1);
    } else {
        return QDate();
    }
}

QDate QCalendarSystem::firstDayOfYear(int year) const
{
    return date(year, 1, 1);
}

QDate QCalendarSystem::lastDayOfYear(const QDate &dt) const
{
    if (isValid(dt)) {
        int y = year(dt);
        return date(y, d->daysInYear(y));
    } else {
        return QDate();
    }
}

QDate QCalendarSystem::lastDayOfYear(int year) const
{
    if (d->isValidYear(year)) {
        return date(year, d->daysInYear(year));
    } else {
        return QDate();
    }
}

QDate QCalendarSystem::firstDayOfMonth(const QDate &dt) const
{
    int year;
    int month;
    getDate(dt, &year, &month, nullptr);
    return date(year, month, 1);
}

QDate QCalendarSystem::firstDayOfMonth(int year, int month) const
{
    return date(year, month, 1);
}

QDate QCalendarSystem::lastDayOfMonth(const QDate &dt) const
{
    int year;
    int month;
    getDate(dt, &year, &month, nullptr);
    return date(year, month, daysInMonth(year, month));
}

QDate QCalendarSystem::lastDayOfMonth(int year, int month) const
{
    return date(year, month, daysInMonth(year, month));
}