// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

namespace System.Globalization
{
    /// <remarks>
    /// This calendar recognizes two era values:
    /// 0 CurrentEra (AD)
    /// 1 BeforeCurrentEra (BC)
    /// </remarks>
    public class GregorianCalendar : Calendar
    {
        public const int ADEra = 1;

        // This is the min Gregorian year can be represented by the DateTime class.
        // The limitation is derived from the DateTime class.
        internal const int MinYear = 1;

        // This is the max Gregorian year can be represented by the DateTime class.
        // The limitation is derived from the DateTime class.
        internal const int MaxYear = 9999;

        private GregorianCalendarTypes _type;

        private static readonly int[] DaysToMonth365 = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 };

        private static readonly int[] DaysToMonth366 = { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 };

        private static volatile Calendar? s_defaultInstance;

        public override DateTime MinSupportedDateTime => DateTime.MinValue;

        public override DateTime MaxSupportedDateTime => DateTime.MaxValue;

        public override CalendarAlgorithmType AlgorithmType => CalendarAlgorithmType.SolarCalendar;

        /// <summary>
        /// Internal method to provide a default intance of GregorianCalendar.
        /// Used by NLS+ implementation
        /// </summary>
        internal static Calendar GetDefaultInstance() => s_defaultInstance ??= new GregorianCalendar();

        public GregorianCalendar() : this(GregorianCalendarTypes.Localized)
        {
        }

        public GregorianCalendar(GregorianCalendarTypes type)
        {
            if (type < GregorianCalendarTypes.Localized || type > GregorianCalendarTypes.TransliteratedFrench)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(type),
                    type,
                    SR.Format(SR.ArgumentOutOfRange_Range, GregorianCalendarTypes.Localized, GregorianCalendarTypes.TransliteratedFrench));
            }

            _type = type;
        }

        public virtual GregorianCalendarTypes CalendarType
        {
            get => _type;
            set
            {
                VerifyWritable();
                if (value < GregorianCalendarTypes.Localized || value > GregorianCalendarTypes.TransliteratedFrench)
                {
                    throw new ArgumentOutOfRangeException(
                        nameof(value),
                        value,
                        SR.Format(SR.ArgumentOutOfRange_Range, GregorianCalendarTypes.Localized, GregorianCalendarTypes.TransliteratedFrench));
                }

                _type = value;
            }
        }

        internal override CalendarId ID =>
            // By returning different ID for different variations of GregorianCalendar,
            // we can support the Transliterated Gregorian calendar.
            // DateTimeFormatInfo will use this ID to get formatting information about
            // the calendar.
            (CalendarId)_type;

        /// <summary>
        /// Gets the absolute date for the given Gregorian date. The absolute date means
        /// the number of days from January 1st, 1 A.D.
        /// </summary>
        /// <remarks>
        /// This is an internal method used by DateToTicks() and the calculations of Hijri and Hebrew calendars.
        /// Number of Days in Prior Years (both common and leap years) +
        /// Number of Days in Prior Months of Current Year +
        /// Number of Days in Current Month
        /// </remarks>
        internal static long GetAbsoluteDate(int year, int month, int day)
        {
            if (year >= 1 && year <= MaxYear && month >= 1 && month <= 12)
            {
                int[] days = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) ? DaysToMonth366 : DaysToMonth365;
                if (day >= 1 && (day <= days[month] - days[month - 1]))
                {
                    int y = year - 1;
                    return y * 365 + y / 4 - y / 100 + y / 400 + days[month - 1] + day - 1;
                }
            }

            throw new ArgumentOutOfRangeException(null, SR.ArgumentOutOfRange_BadYearMonthDay);
        }

        /// <summary>
        /// Returns the tick count corresponding to the given year, month, and day.
        /// Will check the if the parameters are valid.
        /// </summary>
        internal virtual long DateToTicks(int year, int month, int day)
        {
            return GetAbsoluteDate(year, month, day) * TicksPerDay;
        }

        /// <summary>
        /// Returns the DateTime resulting from adding the given number of
        /// months to the specified DateTime. The result is computed by incrementing
        /// (or decrementing) the year and month parts of the specified DateTime by
        /// value months, and, if required, adjusting the day part of the
        /// resulting date downwards to the last day of the resulting month in the
        /// resulting year. The time-of-day part of the result is the same as the
        /// time-of-day part of the specified DateTime.
        ///
        /// In more precise terms, considering the specified DateTime to be of the
        /// form y / m / d + t, where y is the
        /// year, m is the month, d is the day, and t is the
        /// time-of-day, the result is y1 / m1 / d1 + t,
        /// where y1 and m1 are computed by adding value months
        /// to y and m, and d1 is the largest value less than
        /// or equal to d that denotes a valid day in month m1 of year
        /// y1.
        /// </summary>
        public override DateTime AddMonths(DateTime time, int months)
        {
            if (months < -120000 || months > 120000)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(months),
                    months,
                    SR.Format(SR.ArgumentOutOfRange_Range, -120000, 120000));
            }

            time.GetDatePart(out int y, out int m, out int d);
            int i = m - 1 + months;
            if (i >= 0)
            {
                m = i % 12 + 1;
                y += i / 12;
            }
            else
            {
                m = 12 + (i + 1) % 12;
                y += (i - 11) / 12;
            }

            int[] daysArray = (y % 4 == 0 && (y % 100 != 0 || y % 400 == 0)) ? DaysToMonth366 : DaysToMonth365;
            int days = (daysArray[m] - daysArray[m - 1]);

            if (d > days)
            {
                d = days;
            }
            long ticks = DateToTicks(y, m, d) + time.Ticks % TicksPerDay;
            Calendar.CheckAddResult(ticks, MinSupportedDateTime, MaxSupportedDateTime);

            return new DateTime(ticks);
        }

        /// <summary>
        /// Returns the DateTime resulting from adding the given number of
        /// years to the specified DateTime. The result is computed by incrementing
        /// (or decrementing) the year part of the specified DateTime by value
        /// years. If the month and day of the specified DateTime is 2/29, and if the
        /// resulting year is not a leap year, the month and day of the resulting
        /// DateTime becomes 2/28. Otherwise, the month, day, and time-of-day
        /// parts of the result are the same as those of the specified DateTime.
        /// </summary>
        public override DateTime AddYears(DateTime time, int years)
        {
            return AddMonths(time, years * 12);
        }

        /// <summary>
        /// Returns the day-of-month part of the specified DateTime. The returned
        /// value is an integer between 1 and 31.
        /// </summary>
        public override int GetDayOfMonth(DateTime time) => time.Day;

        /// <summary>
        /// Returns the day-of-week part of the specified DateTime. The returned value
        /// is an integer between 0 and 6, where 0 indicates Sunday, 1 indicates
        /// Monday, 2 indicates Tuesday, 3 indicates Wednesday, 4 indicates
        /// Thursday, 5 indicates Friday, and 6 indicates Saturday.
        /// </summary>
        public override DayOfWeek GetDayOfWeek(DateTime time)
        {
            return (DayOfWeek)((int)(time.Ticks / TicksPerDay + 1) % 7);
        }

        /// <summary>
        /// Returns the day-of-year part of the specified DateTime. The returned value
        /// is an integer between 1 and 366.
        /// </summary>
        public override int GetDayOfYear(DateTime time) => time.DayOfYear;

        /// <summary>
        /// Returns the number of days in the month given by the year and
        /// month arguments.
        /// </summary>
        public override int GetDaysInMonth(int year, int month, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }

            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }
            if (month < 1 || month > 12)
            {
                throw new ArgumentOutOfRangeException(nameof(month), month, SR.ArgumentOutOfRange_Month);
            }

            int[] days = ((year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) ? DaysToMonth366 : DaysToMonth365);
            return days[month] - days[month - 1];
        }

        /// <summary>
        /// Returns the number of days in the year given by the year argument for
        /// the current era.
        /// </summary>
        public override int GetDaysInYear(int year, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }

            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }

            return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) ? 366 : 365;
        }

        public override int GetEra(DateTime time) => ADEra;

        public override int[] Eras => new int[] { ADEra };

        /// <summary>
        /// Returns the month part of the specified DateTime.
        /// The returned value is an integer between 1 and 12.
        /// </summary>
        public override int GetMonth(DateTime time) => time.Month;

        /// <summary>
        /// Returns the number of months in the specified year and era.
        /// </summary>
        public override int GetMonthsInYear(int year, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }
            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }

            return 12;
        }

        /// <summary>
        /// Returns the year part of the specified DateTime. The returned value is an
        /// integer between 1 and 9999.
        /// </summary>
        public override int GetYear(DateTime time) => time.Year;

        internal override bool IsValidYear(int year, int era) => year >= 1 && year <= MaxYear;

        internal override bool IsValidDay(int year, int month, int day, int era)
        {
            if ((era != CurrentEra && era != ADEra) ||
                year < 1 || year > MaxYear ||
                month < 1 || month > 12 ||
                day < 1)
            {
                return false;
            }

            int[] days = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) ? DaysToMonth366 : DaysToMonth365;
            return day <= (days[month] - days[month - 1]);
        }

        /// <summary>
        /// Checks whether a given day in the specified era is a leap day. This method returns true if
        /// the date is a leap day, or false if not.
        /// </summary>
        public override bool IsLeapDay(int year, int month, int day, int era)
        {
            if (month < 1 || month > 12)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(month),
                    month,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, 12));
            }

            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }
            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }
            if (day < 1 || day > GetDaysInMonth(year, month))
            {
                throw new ArgumentOutOfRangeException(
                    nameof(day),
                    day,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, GetDaysInMonth(year, month)));
            }

            return IsLeapYear(year) && month == 2 && day == 29;
        }

        /// <summary>
        /// Returns the leap month in a calendar year of the specified era.
        /// This method returns 0 if this calendar does not have leap month, or
        /// this year is not a leap year.
        /// </summary>
        public override int GetLeapMonth(int year, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }
            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }

            return 0;
        }

        /// <summary>
        /// Checks whether a given month in the specified era is a leap month.
        /// This method returns true if month is a leap month, or false if not.
        /// </summary>
        public override bool IsLeapMonth(int year, int month, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }
            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }
            if (month < 1 || month > 12)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(month),
                    month,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, 12));
            }

            return false;
        }

        /// <summary>
        /// Checks whether a given year in the specified era is a leap year. This method returns true if
        /// year is a leap year, or false if not.
        /// </summary>
        public override bool IsLeapYear(int year, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }
            if (year < 1 || year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }

            return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
        }

        /// <summary>
        /// Returns the date and time converted to a DateTime value.
        /// Throws an exception if the n-tuple is invalid.
        /// </summary>
        public override DateTime ToDateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int era)
        {
            if (era != CurrentEra && era != ADEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }

            return new DateTime(year, month, day, hour, minute, second, millisecond);
        }

        internal override bool TryToDateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int era, out DateTime result)
        {
            if (era != CurrentEra && era != ADEra)
            {
                result = DateTime.MinValue;
                return false;
            }

            return DateTime.TryCreate(year, month, day, hour, minute, second, millisecond, out result);
        }

        private const int DefaultTwoDigitYearMax = 2029;

        public override int TwoDigitYearMax
        {
            get
            {
                if (_twoDigitYearMax == -1)
                {
                    _twoDigitYearMax = GetSystemTwoDigitYearSetting(ID, DefaultTwoDigitYearMax);
                }

                return _twoDigitYearMax;
            }
            set
            {
                VerifyWritable();
                if (value < 99 || value > MaxYear)
                {
                    throw new ArgumentOutOfRangeException(
                        nameof(value),
                        value,
                        SR.Format(SR.ArgumentOutOfRange_Range, 99, MaxYear));
                }
                _twoDigitYearMax = value;
            }
        }

        public override int ToFourDigitYear(int year)
        {
            if (year < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(year), year, SR.ArgumentOutOfRange_NeedNonNegNum);
            }
            if (year > MaxYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, MaxYear));
            }

            return base.ToFourDigitYear(year);
        }
    }
}