HebrewCalendar.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics;

namespace System.Globalization
{
    /// <remarks>
    /// Rules for the Hebrew calendar:
    ///   - The Hebrew calendar is both a Lunar (months) and Solar (years)
    ///       calendar, but allows for a week of seven days.
    ///   - Days begin at sunset.
    ///   - Leap Years occur in the 3, 6, 8, 11, 14, 17, &amp; 19th years of a
    ///       19-year cycle.  Year = leap iff ((7y+1) mod 19 &lt; 7).
    ///   - There are 12 months in a common year and 13 months in a leap year.
    ///   - In a common year, the 6th month, Adar, has 29 days.  In a leap
    ///       year, the 6th month, Adar I, has 30 days and the leap month,
    ///       Adar II, has 29 days.
    ///   - Common years have 353-355 days.  Leap years have 383-385 days.
    ///   - The Hebrew new year (Rosh HaShanah) begins on the 1st of Tishri,
    ///       the 7th month in the list below.
    ///       - The new year may not begin on Sunday, Wednesday, or Friday.
    ///       - If the new year would fall on a Tuesday and the conjunction of
    ///           the following year were at midday or later, the new year is
    ///           delayed until Thursday.
    ///       - If the new year would fall on a Monday after a leap year, the
    ///           new year is delayed until Tuesday.
    ///   - The length of the 8th and 9th months vary from year to year,
    ///       depending on the overall length of the year.
    ///       - The length of a year is determined by the dates of the new
    ///           years (Tishri 1) preceding and following the year in question.
    ///       - The 2th month is long (30 days) if the year has 355 or 385 days.
    ///       - The 3th month is short (29 days) if the year has 353 or 383 days.
    ///   - The Hebrew months are:
    ///       1.  Tishri        (30 days)
    ///       2.  Heshvan       (29 or 30 days)
    ///       3.  Kislev        (29 or 30 days)
    ///       4.  Teveth        (29 days)
    ///       5.  Shevat        (30 days)
    ///       6.  Adar I        (30 days)
    ///       7.  Adar {II}     (29 days, this only exists if that year is a leap year)
    ///       8.  Nisan         (30 days)
    ///       9.  Iyyar         (29 days)
    ///       10. Sivan         (30 days)
    ///       11. Tammuz        (29 days)
    ///       12. Av            (30 days)
    ///       13. Elul          (29 days)
    /// Calendar support range:
    ///     Calendar    Minimum     Maximum
    ///     ==========  ==========  ==========
    ///     Gregorian   1583/01/01  2239/09/29
    ///     Hebrew      5343/04/07  5999/13/29
    ///
    /// Includes CHebrew implementation;i.e All the code necessary for converting
    /// Gregorian to Hebrew Lunar from 1583 to 2239.
    /// </remarks>
    public class HebrewCalendar : Calendar
    {
        public static readonly int HebrewEra = 1;

        private const int DatePartYear = 0;
        private const int DatePartMonth = 2;
        private const int DatePartDay = 3;

        // Hebrew Translation Table.
        //
        //  This table is used to get the following Hebrew calendar information for a
        //  given Gregorian year:
        //      1. The day of the Hebrew month corresponding to Gregorian January 1st
        //         for a given Gregorian year.
        //      2. The month of the Hebrew month corresponding to Gregorian January 1st
        //         for a given Gregorian year.
        //         The information is not directly in the table.  Instead, the info is decoded
        //          by special values (numbers above 29 and below 1).
        //      3. The type of the Hebrew year for a given Gregorian year.
        //
        //  More notes:
        //  This table includes 2 numbers for each year.
        //  The offset into the table determines the year. (offset 0 is Gregorian year 1500)
        //  1st number determines the day of the Hebrew month coresponeds to January 1st.
        //  2nd number determines the type of the Hebrew year. (the type determines how
        //   many days are there in the year.)
        //
        //   normal years : 1 = 353 days   2 = 354 days   3 = 355 days.
        //   Leap years   : 4 = 383        5   384        6 = 385 days.
        //
        //   A 99 means the year is not supported for translation.
        //   for convenience the table was defined for 750 year,
        //   but only 640 years are supported. (from 1583 to 2239)
        //   the years before 1582 (starting of Georgian calendar)
        //   and after 2239, are filled with 99.
        //
        //   Greogrian January 1st falls usually in Tevet (4th month). Tevet has always 29 days.
        //   That's why, there no nead to specify the lunar month in the table.
        //   There are exceptions, these are coded by giving numbers above 29 and below 1.
        //   Actual decoding is takenig place whenever fetching information from the table.
        //   The function for decoding is in GetLunarMonthDay().
        //
        //   Example:
        //      The data for 2000 - 2005 A.D. is:
        //          23,6,6,1,17,2,27,6,7,3,         // 2000 - 2004
        //
        //      For year 2000, we know it has a Hebrew year type 6, which means it has 385 days.
        //      And 1/1/2000 A.D. is Hebrew year 5760, 23rd day of 4th month.
        //
        //  Jewish Era in use today is dated from the supposed year of the
        //  Creation with its beginning in 3761 B.C.
        //
        // The Hebrew year of Gregorian 1st year AD.
        // 0001/01/01 AD is Hebrew 3760/01/01
        private const int HebrewYearOf1AD = 3760;

        private const int FirstGregorianTableYear = 1583;   // == Hebrew Year 5343
        private const int LastGregorianTableYear = 2239;    // == Hebrew Year 5999

        private const int TableSize = (LastGregorianTableYear - FirstGregorianTableYear);

        private const int MinHebrewYear = HebrewYearOf1AD + FirstGregorianTableYear;   // == 5343
        private const int MaxHebrewYear = HebrewYearOf1AD + LastGregorianTableYear;    // == 5999

        private static ReadOnlySpan<byte> HebrewTable => new byte[] // rely on C# compiler optimization to reference static data
        {
            7, 3, 17, 3,         // 1583-1584  (Hebrew year: 5343 - 5344)
            0, 4, 11, 2, 21, 6, 1, 3, 13, 2,             // 1585-1589
            25, 4, 5, 3, 16, 2, 27, 6, 9, 1,             // 1590-1594
            20, 2, 0, 6, 11, 3, 23, 4, 4, 2,             // 1595-1599
            14, 3, 27, 4, 8, 2, 18, 3, 28, 6,            // 1600
            11, 1, 22, 5, 2, 3, 12, 3, 25, 4,      // 1605
            6, 2, 16, 3, 26, 6, 8, 2, 20, 1,      // 1610
            0, 6, 11, 2, 24, 4, 4, 3, 15, 2,      // 1615
            25, 6, 8, 1, 19, 2, 29, 6, 9, 3,      // 1620
            22, 4, 3, 2, 13, 3, 25, 4, 6, 3,      // 1625
            17, 2, 27, 6, 7, 3, 19, 2, 31, 4,      // 1630
            11, 3, 23, 4, 5, 2, 15, 3, 25, 6,      // 1635
            6, 2, 19, 1, 29, 6, 10, 2, 22, 4,      // 1640
            3, 3, 14, 2, 24, 6, 6, 1, 17, 3,      // 1645
            28, 5, 8, 3, 20, 1, 32, 5, 12, 3,      // 1650
            22, 6, 4, 1, 16, 2, 26, 6, 6, 3,      // 1655
            17, 2, 0, 4, 10, 3, 22, 4, 3, 2,      // 1660
            14, 3, 24, 6, 5, 2, 17, 1, 28, 6,      // 1665
            9, 2, 19, 3, 31, 4, 13, 2, 23, 6,      // 1670
            3, 3, 15, 1, 27, 5, 7, 3, 17, 3,      // 1675
            29, 4, 11, 2, 21, 6, 3, 1, 14, 2,      // 1680
            25, 6, 5, 3, 16, 2, 28, 4, 9, 3,      // 1685
            20, 2, 0, 6, 12, 1, 23, 6, 4, 2,      // 1690
            14, 3, 26, 4, 8, 2, 18, 3, 0, 4,      // 1695
            10, 3, 21, 5, 1, 3, 13, 1, 24, 5,      // 1700
            5, 3, 15, 3, 27, 4, 8, 2, 19, 3,      // 1705
            29, 6, 10, 2, 22, 4, 3, 3, 14, 2,      // 1710
            26, 4, 6, 3, 18, 2, 28, 6, 10, 1,      // 1715
            20, 6, 2, 2, 12, 3, 24, 4, 5, 2,      // 1720
            16, 3, 28, 4, 8, 3, 19, 2, 0, 6,      // 1725
            12, 1, 23, 5, 3, 3, 14, 3, 26, 4,      // 1730
            7, 2, 17, 3, 28, 6, 9, 2, 21, 4,      // 1735
            1, 3, 13, 2, 25, 4, 5, 3, 16, 2,      // 1740
            27, 6, 9, 1, 19, 3, 0, 5, 11, 3,      // 1745
            23, 4, 4, 2, 14, 3, 25, 6, 7, 1,      // 1750
            18, 2, 28, 6, 9, 3, 21, 4, 2, 2,      // 1755
            12, 3, 25, 4, 6, 2, 16, 3, 26, 6,      // 1760
            8, 2, 20, 1, 0, 6, 11, 2, 22, 6,      // 1765
            4, 1, 15, 2, 25, 6, 6, 3, 18, 1,      // 1770
            29, 5, 9, 3, 22, 4, 2, 3, 13, 2,      // 1775
            23, 6, 4, 3, 15, 2, 27, 4, 7, 3,      // 1780
            19, 2, 31, 4, 11, 3, 21, 6, 3, 2,      // 1785
            15, 1, 25, 6, 6, 2, 17, 3, 29, 4,      // 1790
            10, 2, 20, 6, 3, 1, 13, 3, 24, 5,      // 1795
            4, 3, 16, 1, 27, 5, 7, 3, 17, 3,      // 1800
            0, 4, 11, 2, 21, 6, 1, 3, 13, 2,      // 1805
            25, 4, 5, 3, 16, 2, 29, 4, 9, 3,      // 1810
            19, 6, 30, 2, 13, 1, 23, 6, 4, 2,      // 1815
            14, 3, 27, 4, 8, 2, 18, 3, 0, 4,      // 1820
            11, 3, 22, 5, 2, 3, 14, 1, 26, 5,      // 1825
            6, 3, 16, 3, 28, 4, 10, 2, 20, 6,      // 1830
            30, 3, 11, 2, 24, 4, 4, 3, 15, 2,      // 1835
            25, 6, 8, 1, 19, 2, 29, 6, 9, 3,      // 1840
            22, 4, 3, 2, 13, 3, 25, 4, 7, 2,      // 1845
            17, 3, 27, 6, 9, 1, 21, 5, 1, 3,      // 1850
            11, 3, 23, 4, 5, 2, 15, 3, 25, 6,      // 1855
            6, 2, 19, 1, 29, 6, 10, 2, 22, 4,      // 1860
            3, 3, 14, 2, 24, 6, 6, 1, 18, 2,      // 1865
            28, 6, 8, 3, 20, 4, 2, 2, 12, 3,      // 1870
            24, 4, 4, 3, 16, 2, 26, 6, 6, 3,      // 1875
            17, 2, 0, 4, 10, 3, 22, 4, 3, 2,      // 1880
            14, 3, 24, 6, 5, 2, 17, 1, 28, 6,      // 1885
            9, 2, 21, 4, 1, 3, 13, 2, 23, 6,      // 1890
            5, 1, 15, 3, 27, 5, 7, 3, 19, 1,      // 1895
            0, 5, 10, 3, 22, 4, 2, 3, 13, 2,      // 1900
            24, 6, 4, 3, 15, 2, 27, 4, 8, 3,      // 1905
            20, 4, 1, 2, 11, 3, 22, 6, 3, 2,      // 1910
            15, 1, 25, 6, 7, 2, 17, 3, 29, 4,      // 1915
            10, 2, 21, 6, 1, 3, 13, 1, 24, 5,      // 1920
            5, 3, 15, 3, 27, 4, 8, 2, 19, 6,      // 1925
            1, 1, 12, 2, 22, 6, 3, 3, 14, 2,      // 1930
            26, 4, 6, 3, 18, 2, 28, 6, 10, 1,      // 1935
            20, 6, 2, 2, 12, 3, 24, 4, 5, 2,      // 1940
            16, 3, 28, 4, 9, 2, 19, 6, 30, 3,      // 1945
            12, 1, 23, 5, 3, 3, 14, 3, 26, 4,      // 1950
            7, 2, 17, 3, 28, 6, 9, 2, 21, 4,      // 1955
            1, 3, 13, 2, 25, 4, 5, 3, 16, 2,      // 1960
            27, 6, 9, 1, 19, 6, 30, 2, 11, 3,      // 1965
            23, 4, 4, 2, 14, 3, 27, 4, 7, 3,      // 1970
            18, 2, 28, 6, 11, 1, 22, 5, 2, 3,      // 1975
            12, 3, 25, 4, 6, 2, 16, 3, 26, 6,      // 1980
            8, 2, 20, 4, 30, 3, 11, 2, 24, 4,      // 1985
            4, 3, 15, 2, 25, 6, 8, 1, 18, 3,      // 1990
            29, 5, 9, 3, 22, 4, 3, 2, 13, 3,      // 1995
            23, 6, 6, 1, 17, 2, 27, 6, 7, 3,         // 2000 - 2004
            20, 4, 1, 2, 11, 3, 23, 4, 5, 2,         // 2005 - 2009
            15, 3, 25, 6, 6, 2, 19, 1, 29, 6,        // 2010
            10, 2, 20, 6, 3, 1, 14, 2, 24, 6,      // 2015
            4, 3, 17, 1, 28, 5, 8, 3, 20, 4,      // 2020
            1, 3, 12, 2, 22, 6, 2, 3, 14, 2,      // 2025
            26, 4, 6, 3, 17, 2, 0, 4, 10, 3,      // 2030
            20, 6, 1, 2, 14, 1, 24, 6, 5, 2,      // 2035
            15, 3, 28, 4, 9, 2, 19, 6, 1, 1,      // 2040
            12, 3, 23, 5, 3, 3, 15, 1, 27, 5,      // 2045
            7, 3, 17, 3, 29, 4, 11, 2, 21, 6,      // 2050
            1, 3, 12, 2, 25, 4, 5, 3, 16, 2,      // 2055
            28, 4, 9, 3, 19, 6, 30, 2, 12, 1,      // 2060
            23, 6, 4, 2, 14, 3, 26, 4, 8, 2,      // 2065
            18, 3, 0, 4, 10, 3, 22, 5, 2, 3,      // 2070
            14, 1, 25, 5, 6, 3, 16, 3, 28, 4,      // 2075
            9, 2, 20, 6, 30, 3, 11, 2, 23, 4,      // 2080
            4, 3, 15, 2, 27, 4, 7, 3, 19, 2,      // 2085
            29, 6, 11, 1, 21, 6, 3, 2, 13, 3,      // 2090
            25, 4, 6, 2, 17, 3, 27, 6, 9, 1,      // 2095
            20, 5, 30, 3, 10, 3, 22, 4, 3, 2,      // 2100
            14, 3, 24, 6, 5, 2, 17, 1, 28, 6,      // 2105
            9, 2, 21, 4, 1, 3, 13, 2, 23, 6,      // 2110
            5, 1, 16, 2, 27, 6, 7, 3, 19, 4,      // 2115
            30, 2, 11, 3, 23, 4, 3, 3, 14, 2,      // 2120
            25, 6, 5, 3, 16, 2, 28, 4, 9, 3,      // 2125
            21, 4, 2, 2, 12, 3, 23, 6, 4, 2,      // 2130
            16, 1, 26, 6, 8, 2, 20, 4, 30, 3,      // 2135
            11, 2, 22, 6, 4, 1, 14, 3, 25, 5,      // 2140
            6, 3, 18, 1, 29, 5, 9, 3, 22, 4,      // 2145
            2, 3, 13, 2, 23, 6, 4, 3, 15, 2,      // 2150
            27, 4, 7, 3, 20, 4, 1, 2, 11, 3,      // 2155
            21, 6, 3, 2, 15, 1, 25, 6, 6, 2,      // 2160
            17, 3, 29, 4, 10, 2, 20, 6, 3, 1,      // 2165
            13, 3, 24, 5, 4, 3, 17, 1, 28, 5,      // 2170
            8, 3, 18, 6, 1, 1, 12, 2, 22, 6,      // 2175
            2, 3, 14, 2, 26, 4, 6, 3, 17, 2,      // 2180
            28, 6, 10, 1, 20, 6, 1, 2, 12, 3,    // 2185
            24, 4, 5, 2, 15, 3, 28, 4, 9, 2,     // 2190
            19, 6, 33, 3, 12, 1, 23, 5, 3, 3,    // 2195
            13, 3, 25, 4, 6, 2, 16, 3, 26, 6,    // 2200
            8, 2, 20, 4, 30, 3, 11, 2, 24, 4,    // 2205
            4, 3, 15, 2, 25, 6, 8, 1, 18, 6,     // 2210
            33, 2, 9, 3, 22, 4, 3, 2, 13, 3,     // 2215
            25, 4, 6, 3, 17, 2, 27, 6, 9, 1,     // 2220
            21, 5, 1, 3, 11, 3, 23, 4, 5, 2,     // 2225
            15, 3, 25, 6, 6, 2, 19, 4, 33, 3,    // 2230
            10, 2, 22, 4, 3, 3, 14, 2, 24, 6,    // 2235
            6, 1    // 2240 (Hebrew year: 6000)
        };

        private const int MaxMonthPlusOne = 14;

        // The lunar calendar has 6 different variations of month lengths
        // within a year.
        private static ReadOnlySpan<byte> LunarMonthLen => new byte[] // rely on C# compiler optimization to reference static data
        {
            0, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 0,
            0, 30, 29, 29, 29, 30, 29, 30, 29, 30, 29, 30, 29, 0,     // 3 common year variations
            0, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 0,
            0, 30, 30, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 0,
            0, 30, 29, 29, 29, 30, 30, 29, 30, 29, 30, 29, 30, 29,    // 3 leap year variations
            0, 30, 29, 30, 29, 30, 30, 29, 30, 29, 30, 29, 30, 29,
            0, 30, 30, 30, 29, 30, 30, 29, 30, 29, 30, 29, 30, 29
        };

        private static readonly DateTime s_calendarMinValue = new DateTime(1583, 1, 1);

        // Gregorian 2239/9/29 = Hebrew 5999/13/29 (last day in Hebrew year 5999).
        // We can only format/parse Hebrew numbers up to 999, so we limit the max range to Hebrew year 5999.
        private static readonly DateTime s_calendarMaxValue = new DateTime((new DateTime(2239, 9, 29, 23, 59, 59, 999)).Ticks + 9999);

        public override DateTime MinSupportedDateTime => s_calendarMinValue;

        public override DateTime MaxSupportedDateTime => s_calendarMaxValue;

        public override CalendarAlgorithmType AlgorithmType => CalendarAlgorithmType.LunisolarCalendar;

        public HebrewCalendar()
        {
        }

        internal override CalendarId ID => CalendarId.HEBREW;

        private static void CheckHebrewYearValue(int y, int era, string varName)
        {
            CheckEraRange(era);
            if (y > MaxHebrewYear || y < MinHebrewYear)
            {
                throw new ArgumentOutOfRangeException(
                    varName,
                    y,
                    SR.Format(SR.ArgumentOutOfRange_Range, MinHebrewYear, MaxHebrewYear));
            }
        }

        /// <summary>
        /// Check if the Hebrew month value is valid.
        /// </summary>
        /// <remarks>
        /// Call CheckHebrewYearValue() before calling this to verify the year value is supported.
        /// </remarks>
        private void CheckHebrewMonthValue(int year, int month, int era)
        {
            int monthsInYear = GetMonthsInYear(year, era);
            if (month < 1 || month > monthsInYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(month),
                    month,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, monthsInYear));
            }
        }

        /// <summary>
        /// Check if the Hebrew day value is valid.
        /// </summary>
        /// <remarks>
        /// Call CheckHebrewYearValue()/CheckHebrewMonthValue() before calling this to verify the year/month values are valid.
        /// </remarks>
        private void CheckHebrewDayValue(int year, int month, int day, int era)
        {
            int daysInMonth = GetDaysInMonth(year, month, era);
            if (day < 1 || day > daysInMonth)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(day),
                    day,
                    SR.Format(SR.ArgumentOutOfRange_Range, 1, daysInMonth));
            }
        }

        private static void CheckEraRange(int era)
        {
            if (era != CurrentEra && era != HebrewEra)
            {
                throw new ArgumentOutOfRangeException(nameof(era), era, SR.ArgumentOutOfRange_InvalidEraValue);
            }
        }

        private static void CheckTicksRange(long ticks)
        {
            if (ticks < s_calendarMinValue.Ticks || ticks > s_calendarMaxValue.Ticks)
            {
                throw new ArgumentOutOfRangeException(
                    "time",
                    ticks,
                    // Print out the date in Gregorian using InvariantCulture since the DateTime is based on GreograinCalendar.
                    SR.Format(
                        CultureInfo.InvariantCulture,
                        SR.ArgumentOutOfRange_CalendarRange,
                        s_calendarMinValue,
                        s_calendarMaxValue));
            }
        }

        private static int GetResult(DateBuffer result, int part)
        {
            return part switch
            {
                DatePartYear => result.year,
                DatePartMonth => result.month,
                DatePartDay => result.day,
                _ => throw new InvalidOperationException(SR.InvalidOperation_DateTimeParsing),
            };
        }

        /// <summary>
        /// Using the Hebrew table (HebrewTable) to get the Hebrew month/day value for Gregorian January 1st
        /// in a given Gregorian year.
        /// Greogrian January 1st falls usually in Tevet (4th month). Tevet has always 29 days.
        /// That's why, there no nead to specify the lunar month in the table.  There are exceptions, and these
        /// are coded by giving numbers above 29 and below 1.
        /// Actual decoding is takenig place in the switch statement below.
        /// </summary>
        /// <returns>
        /// The Hebrew year type. The value is from 1 to 6.
        /// normal years : 1 = 353 days   2 = 354 days   3 = 355 days.
        /// Leap years   : 4 = 383        5   384        6 = 385 days.
        /// </returns>
        internal static int GetLunarMonthDay(int gregorianYear, DateBuffer lunarDate)
        {
            // Get the offset into the LunarMonthLen array and the lunar day
            //  for January 1st.
            int index = gregorianYear - FirstGregorianTableYear;
            if (index < 0 || index > TableSize)
            {
                throw new ArgumentOutOfRangeException(nameof(gregorianYear));
            }

            index *= 2;
            lunarDate.day = HebrewTable[index];

            // Get the type of the year. The value is from 1 to 6
            int lunarYearType = HebrewTable[index + 1];

            // Get the Lunar Month.
            switch (lunarDate.day)
            {
                case 0:                   // 1/1 is on Shvat 1
                    lunarDate.month = 5;
                    lunarDate.day = 1;
                    break;
                case 30:                  // 1/1 is on Kislev 30
                    lunarDate.month = 3;
                    break;
                case 31:                  // 1/1 is on Shvat 2
                    lunarDate.month = 5;
                    lunarDate.day = 2;
                    break;
                case 32:                  // 1/1 is on Shvat 3
                    lunarDate.month = 5;
                    lunarDate.day = 3;
                    break;
                case 33:                  // 1/1 is on Kislev 29
                    lunarDate.month = 3;
                    lunarDate.day = 29;
                    break;
                default:                      // 1/1 is on Tevet (This is the general case)
                    lunarDate.month = 4;
                    break;
            }

            return lunarYearType;
        }

        /// <summary>
        /// Returns a given date part of this DateTime. This method is used
        /// to compute the year, day-of-year, month, or day part.
        /// </summary>
        internal virtual int GetDatePart(long ticks, int part)
        {
            // The Gregorian year, month, day value for ticks.
            int hebrewYearType;                // lunar year type
            long AbsoluteDate;                // absolute date - absolute date 1/1/1600

            // Make sure we have a valid Gregorian date that will fit into our
            // Hebrew conversion limits.
            CheckTicksRange(ticks);

            DateTime time = new DateTime(ticks);

            // Save the Gregorian date values.
            time.GetDate(out int gregorianYear, out int gregorianMonth, out int gregorianDay);

            DateBuffer lunarDate = new DateBuffer();    // lunar month and day for Jan 1

            // From the table looking-up value of HebrewTable[index] (stored in lunarDate.day), we get the
            // lunar month and lunar day where the Gregorian date 1/1 falls.
            lunarDate.year = gregorianYear + HebrewYearOf1AD;
            hebrewYearType = GetLunarMonthDay(gregorianYear, lunarDate);

            // This is the buffer used to store the result Hebrew date.
            DateBuffer result = new DateBuffer();

            // Store the values for the start of the new year - 1/1.
            result.year = lunarDate.year;
            result.month = lunarDate.month;
            result.day = lunarDate.day;

            // Get the absolute date from 1/1/1600.
            AbsoluteDate = GregorianCalendar.GetAbsoluteDate(gregorianYear, gregorianMonth, gregorianDay);

            // If the requested date was 1/1, then we're done.
            if ((gregorianMonth == 1) && (gregorianDay == 1))
            {
                return GetResult(result, part);
            }

            // Calculate the number of days between 1/1 and the requested date.
            long numDays = AbsoluteDate - GregorianCalendar.GetAbsoluteDate(gregorianYear, 1, 1);

            // If the requested date is within the current lunar month, then
            // we're done.
            if ((numDays + (long)lunarDate.day) <= (long)(LunarMonthLen[hebrewYearType * MaxMonthPlusOne + lunarDate.month]))
            {
                result.day += (int)numDays;
                return GetResult(result, part);
            }

            // Adjust for the current partial month.
            result.month++;
            result.day = 1;

            // Adjust the Lunar Month and Year (if necessary) based on the number
            // of days between 1/1 and the requested date.
            // Assumes Jan 1 can never translate to the last Lunar month, which
            // is true.
            numDays -= (long)(LunarMonthLen[hebrewYearType * MaxMonthPlusOne + lunarDate.month] - lunarDate.day);
            Debug.Assert(numDays >= 1, "NumDays >= 1");

            // If NumDays is 1, then we are done.  Otherwise, find the correct Hebrew month
            // and day.
            if (numDays > 1)
            {
                // See if we're on the correct Lunar month.
                while (numDays > (long)(LunarMonthLen[hebrewYearType * MaxMonthPlusOne + result.month]))
                {
                    // Adjust the number of days and move to the next month.
                    numDays -= (long)(LunarMonthLen[hebrewYearType * MaxMonthPlusOne + result.month++]);

                    // See if we need to adjust the Year.
                    // Must handle both 12 and 13 month years.
                    if ((result.month > 13) || (LunarMonthLen[hebrewYearType * MaxMonthPlusOne + result.month] == 0))
                    {
                        // Adjust the Year.
                        result.year++;
                        hebrewYearType = HebrewTable[(gregorianYear + 1 - FirstGregorianTableYear) * 2 + 1];

                        // Adjust the Month.
                        result.month = 1;
                    }
                }

                // Found the right Lunar month.
                result.day += (int)(numDays - 1);
            }

            return GetResult(result, part);
        }

        public override DateTime AddMonths(DateTime time, int months)
        {
            try
            {
                int y = GetDatePart(time.Ticks, DatePartYear);
                int m = GetDatePart(time.Ticks, DatePartMonth);
                int d = GetDatePart(time.Ticks, DatePartDay);

                int monthsInYear;
                int i;
                if (months >= 0)
                {
                    i = m + months;
                    while (i > (monthsInYear = GetMonthsInYear(y, CurrentEra)))
                    {
                        y++;
                        i -= monthsInYear;
                    }
                }
                else
                {
                    if ((i = m + months) <= 0)
                    {
                        months = -months;
                        months -= m;
                        y--;

                        while (months > (monthsInYear = GetMonthsInYear(y, CurrentEra)))
                        {
                            y--;
                            months -= monthsInYear;
                        }
                        monthsInYear = GetMonthsInYear(y, CurrentEra);
                        i = monthsInYear - months;
                    }
                }

                int days = GetDaysInMonth(y, i);
                if (d > days)
                {
                    d = days;
                }

                return new DateTime(ToDateTime(y, i, d, 0, 0, 0, 0).Ticks + (time.Ticks % TicksPerDay));
            }
            // We expect ArgumentException and ArgumentOutOfRangeException (which is subclass of ArgumentException)
            // If exception is thrown in the calls above, we are out of the supported range of this calendar.
            catch (ArgumentException)
            {
                throw new ArgumentOutOfRangeException(nameof(months), months, SR.ArgumentOutOfRange_AddValue);
            }
        }

        public override DateTime AddYears(DateTime time, int years)
        {
            int y = GetDatePart(time.Ticks, DatePartYear);
            int m = GetDatePart(time.Ticks, DatePartMonth);
            int d = GetDatePart(time.Ticks, DatePartDay);

            y += years;
            CheckHebrewYearValue(y, Calendar.CurrentEra, nameof(years));

            int months = GetMonthsInYear(y, CurrentEra);
            if (m > months)
            {
                m = months;
            }

            int days = GetDaysInMonth(y, m);
            if (d > days)
            {
                d = days;
            }

            long ticks = ToDateTime(y, m, d, 0, 0, 0, 0).Ticks + (time.Ticks % TicksPerDay);
            Calendar.CheckAddResult(ticks, MinSupportedDateTime, MaxSupportedDateTime);
            return new DateTime(ticks);
        }

        public override int GetDayOfMonth(DateTime time)
        {
            return GetDatePart(time.Ticks, DatePartDay);
        }

        public override DayOfWeek GetDayOfWeek(DateTime time)
        {
            // If we calculate back, the Hebrew day of week for Gregorian 0001/1/1 is Monday (1).
            return time.DayOfWeek;
        }

        internal static int GetHebrewYearType(int year, int era)
        {
            CheckHebrewYearValue(year, era, nameof(year));
            // The HebrewTable is indexed by Gregorian year and starts from FirstGregorianYear.
            // So we need to convert year (Hebrew year value) to Gregorian Year below.
            return HebrewTable[(year - HebrewYearOf1AD - FirstGregorianTableYear) * 2 + 1];
        }

        public override int GetDayOfYear(DateTime time)
        {
            // Get Hebrew year value of the specified time.
            int year = GetYear(time);
            DateTime beginOfYearDate;
            if (year == 5343)
            {
                // Gregorian 1583/01/01 corresponds to Hebrew 5343/04/07 (MinSupportedDateTime)
                // To figure out the Gregorian date associated with Hebrew 5343/01/01, we need to
                // count the days from 5343/01/01 to 5343/04/07 and subtract that from Gregorian
                // 1583/01/01.
                //        1.  Tishri        (30 days)
                //        2.  Heshvan       (30 days since 5343 has 355 days)
                //        3.  Kislev        (30 days since 5343 has 355 days)
                // 96 days to get from 5343/01/01 to 5343/04/07
                // Gregorian 1583/01/01 - 96 days = 1582/9/27

                // the beginning of Hebrew year 5343 corresponds to Gregorian September 27, 1582.
                beginOfYearDate = new DateTime(1582, 9, 27);
            }
            else
            {
                // following line will fail when year is 5343 (first supported year)
                beginOfYearDate = ToDateTime(year, 1, 1, 0, 0, 0, 0, CurrentEra);
            }

            return (int)((time.Ticks - beginOfYearDate.Ticks) / TicksPerDay) + 1;
        }

        public override int GetDaysInMonth(int year, int month, int era)
        {
            CheckEraRange(era);
            int hebrewYearType = GetHebrewYearType(year, era);
            CheckHebrewMonthValue(year, month, era);

            Debug.Assert(hebrewYearType >= 1 && hebrewYearType <= 6,
                $"hebrewYearType should be from  1 to 6, but now hebrewYearType = {hebrewYearType} for hebrew year {year}");
            int monthDays = LunarMonthLen[hebrewYearType * MaxMonthPlusOne + month];
            if (monthDays == 0)
            {
                ThrowHelper.ThrowArgumentOutOfRange_Month(month);
            }

            return monthDays;
        }

        public override int GetDaysInYear(int year, int era)
        {
            CheckEraRange(era);
            // normal years : 1 = 353 days   2 = 354 days   3 = 355 days.
            // Leap years   : 4 = 383        5   384        6 = 385 days.

            // LunarYearType is from 1 to 6
            int LunarYearType = GetHebrewYearType(year, era);
            if (LunarYearType < 4)
            {
                // common year: LunarYearType = 1, 2, 3
                return 352 + LunarYearType;
            }

            return 382 + (LunarYearType - 3);
        }

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

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

        public override int GetMonth(DateTime time)
        {
            return GetDatePart(time.Ticks, DatePartMonth);
        }

        public override int GetMonthsInYear(int year, int era)
        {
            return IsLeapYear(year, era) ? 13 : 12;
        }

        public override int GetYear(DateTime time)
        {
            return GetDatePart(time.Ticks, DatePartYear);
        }

        public override bool IsLeapDay(int year, int month, int day, int era)
        {
            if (IsLeapMonth(year, month, era))
            {
                // Every day in a leap month is a leap day.
                CheckHebrewDayValue(year, month, day, era);
                return true;
            }
            else if (IsLeapYear(year, Calendar.CurrentEra))
            {
                // There is an additional day in the 6th month in the leap year (the extra day is the 30th day in the 6th month),
                // so we should return true for 6/30 if that's in a leap year.
                if (month == 6 && day == 30)
                {
                    return true;
                }
            }

            CheckHebrewDayValue(year, month, day, era);
            return false;
        }

        public override int GetLeapMonth(int year, int era)
        {
            // Year/era values are checked in IsLeapYear().
            if (IsLeapYear(year, era))
            {
                // The 7th month in a leap year is a leap month.
                return 7;
            }
            return 0;
        }

        public override bool IsLeapMonth(int year, int month, int era)
        {
            // Year/era values are checked in IsLeapYear().
            bool isLeapYear = IsLeapYear(year, era);
            CheckHebrewMonthValue(year, month, era);

            // The 7th month in a leap year is a leap month.
            return isLeapYear && month == 7;
        }

        public override bool IsLeapYear(int year, int era)
        {
            CheckHebrewYearValue(year, era, nameof(year));
            return ((7 * (long)year + 1) % 19) < 7;
        }

        /// <summary>
        /// (month1, day1) - (month2, day2)
        /// </summary>
        private static int GetDayDifference(int lunarYearType, int month1, int day1, int month2, int day2)
        {
            if (month1 == month2)
            {
                return day1 - day2;
            }

            // Make sure that (month1, day1) < (month2, day2)
            bool swap = (month1 > month2);
            if (swap)
            {
                // (month1, day1) < (month2, day2).  Swap the values.
                // The result will be a negative number.
                int tempMonth, tempDay;
                tempMonth = month1; tempDay = day1;
                month1 = month2; day1 = day2;
                month2 = tempMonth; day2 = tempDay;
            }

            // Get the number of days from (month1,day1) to (month1, end of month1)
            int days = LunarMonthLen[lunarYearType * MaxMonthPlusOne + month1] - day1;

            // Move to next month.
            month1++;

            // Add up the days.
            while (month1 < month2)
            {
                days += LunarMonthLen[lunarYearType * MaxMonthPlusOne + month1++];
            }
            days += day2;

            return swap ? days : -days;
        }

        /// <summary>
        /// Convert Hebrew date to Gregorian date.
        ///  The algorithm is like this:
        ///     The hebrew year has an offset to the Gregorian year, so we can guess the Gregorian year for
        ///     the specified Hebrew year.  That is, GreogrianYear = HebrewYear - FirstHebrewYearOf1AD.
        ///
        ///     From the Gregorian year and HebrewTable, we can get the Hebrew month/day value
        ///     of the Gregorian date January 1st.  Let's call this month/day value [hebrewDateForJan1]
        ///
        ///     If the requested Hebrew month/day is less than [hebrewDateForJan1], we know the result
        ///     Gregorian date falls in previous year.  So we decrease the Gregorian year value, and
        ///     retrieve the Hebrew month/day value of the Gregorian date january 1st again.
        ///
        ///     Now, we get the answer of the Gregorian year.
        ///
        ///     The next step is to get the number of days between the requested Hebrew month/day
        ///     and [hebrewDateForJan1].  When we get that, we can create the DateTime by adding/subtracting
        ///     the ticks value of the number of days.
        /// </summary>
        private static DateTime HebrewToGregorian(int hebrewYear, int hebrewMonth, int hebrewDay, int hour, int minute, int second, int millisecond)
        {
            // Get the rough Gregorian year for the specified hebrewYear.
            int gregorianYear = hebrewYear - HebrewYearOf1AD;

            DateBuffer hebrewDateOfJan1 = new DateBuffer(); // year value is unused.
            int lunarYearType = GetLunarMonthDay(gregorianYear, hebrewDateOfJan1);

            if ((hebrewMonth == hebrewDateOfJan1.month) && (hebrewDay == hebrewDateOfJan1.day))
            {
                return new DateTime(gregorianYear, 1, 1, hour, minute, second, millisecond);
            }

            int days = GetDayDifference(lunarYearType, hebrewMonth, hebrewDay, hebrewDateOfJan1.month, hebrewDateOfJan1.day);

            DateTime gregorianNewYear = new DateTime(gregorianYear, 1, 1);
            return new DateTime(gregorianNewYear.Ticks + days * TicksPerDay + TimeToTicks(hour, minute, second, millisecond));
        }

        public override DateTime ToDateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int era)
        {
            CheckHebrewYearValue(year, era, nameof(year));
            CheckHebrewMonthValue(year, month, era);
            CheckHebrewDayValue(year, month, day, era);
            DateTime dt = HebrewToGregorian(year, month, day, hour, minute, second, millisecond);
            CheckTicksRange(dt.Ticks);
            return dt;
        }

        private const int DefaultTwoDigitYearMax = 5790;

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

                return _twoDigitYearMax;
            }
            set
            {
                VerifyWritable();
                if (value == 99)
                {
                    // Do nothing here. Year 99 is allowed so that TwoDitYearMax is disabled.
                }
                else
                {
                    CheckHebrewYearValue(value, HebrewEra, nameof(value));
                }

                _twoDigitYearMax = value;
            }
        }

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

            if (year < 100)
            {
                return base.ToFourDigitYear(year);
            }

            if (year > MaxHebrewYear || year < MinHebrewYear)
            {
                throw new ArgumentOutOfRangeException(
                    nameof(year),
                    year,
                    SR.Format(SR.ArgumentOutOfRange_Range, MinHebrewYear, MaxHebrewYear));
            }
            return year;
        }

        internal sealed class DateBuffer
        {
            internal int year;
            internal int month;
            internal int day;
        }
    }
}