2004-11-07 Andrew John Hughes <gnu_andrew@member.fsf.org>

[official-gcc.git] / libjava / java / util / GregorianCalendar.java

/* java.util.GregorianCalendar
   Copyright (C) 1998, 1999, 2001, 2002, 2003, 2004
   Free Software Foundation, Inc.

This file is part of GNU Classpath.

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it under the terms of the GNU General Public License as published by
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02111-1307 USA.

Linking this library statically or dynamically with other modules is
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permission to link this library with independent modules to produce an
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exception statement from your version. */


package java.util;

import java.io.IOException;
import java.io.ObjectInputStream;

/**
 * <p>
 * This class represents the Gregorian calendar, that is used in most
 * countries all over the world.  It does also handle the Julian calendar
 * for dates smaller than the date of the change to the Gregorian calendar.
 * The Gregorian calendar differs from the Julian calendar by a different
 * leap year rule (no leap year every 100 years, except if year is divisible
 * by 400).  
 * </p>
 * <p>
 * This change date is different from country to country, and can be changed with
 * <code>setGregorianChange</code>.  The first countries to adopt the Gregorian
 * calendar did so on the 15th of October, 1582.  This date followed October
 * the 4th, 1582 in the Julian calendar system.  The non-existant days that were
 * omitted when the change took place are interpreted as Gregorian dates.
 * </p>
 * <p>
 * Prior to the changeover date, New Year's Day occurred on the 25th of March.
 * However, this class always takes New Year's Day as being the 1st of January.
 * Client code should manually adapt the year value, if required, for dates
 * between January the 1st and March the 24th in years prior to the changeover.
 * </p>
 * <p>
 * Any date infinitely forwards or backwards in time can be represented by
 * this class.  A <em>proleptic</em> calendar system is used, which allows
 * future dates to be created via the existing rules.  This allows meaningful
 * and consistent dates to be produced for all years.  However, dates are only
 * historically accurate following March the 1st, 4AD when the Julian calendar
 * system was adopted.  Prior to this, leap year rules were applied erraticly.
 * </p>
 * <p>
 * There are two eras available for the Gregorian calendar, namely BC and AD.
 * </p>
 * <p>
 * Weeks are defined as a period of seven days, beginning on the first day
 * of the week, as returned by <code>getFirstDayOfWeek()</code>, and ending
 * on the day prior to this.
 * </p>
 * <p>
 * The weeks of the year are numbered from 1 to a possible 53.  The first week
 * of the year is defined as the first week that contains at least the minimum
 * number of days of the first week in the new year (retrieved via
 * <code>getMinimalDaysInFirstWeek()</code>).  All weeks after this are numbered
 * from 2 onwards.
 * </p>
 * <p>
 * For example, take the year 2004.  It began on a Thursday.  The first week
 * of 2004 depends both on where a week begins and how long it must minimally
 * last.  Let's say that the week begins on a Monday and must have a minimum
 * of 5 days.  In this case, the first week begins on Monday, the 5th of January.
 * The first 4 days (Thursday to Sunday) are not eligible, as they are too few
 * to make up the minimum number of days of the first week which must be in
 * the new year.  If the minimum was lowered to 4 days, then the first week
 * would instead begin on Monday, the 29th of December, 2003.  This first week
 * has 4 of its days in the new year, and is now eligible.
 * </p>
 * <p>
 * The weeks of the month are numbered from 0 to a possible 6.  The first week
 * of the month (numbered 1) is a set of days, prior to the first day of the week,
 * which number at least the minimum number of days in a week.  Unlike the first
 * week of the year, the first week of the month only uses days from that particular
 * month.  As a consequence, it may have a variable number of days (from the minimum
 * number required up to a full week of 7) and it need not start on the first day of
 * the week.  It must, however, be following by the first day of the week, as this
 * marks the beginning of week 2.  Any days of the month which occur prior to the
 * first week (because the first day of the week occurs before the minimum number
 * of days is met) are seen as week 0.
 * </p>
 * <p>
 * Again, we will take the example of the year 2004 to demonstrate this.  September
 * 2004 begins on a Wednesday.  Taking our first day of the week as Monday, and the
 * minimum length of the first week as 6, we find that week 1 runs from Monday,
 * the 6th of September to Sunday the 12th.  Prior to the 6th, there are only
 * 5 days (Wednesday through to Sunday).  This is too small a number to meet the
 * minimum, so these are classed as being days in week 0.  Week 2 begins on the
 * 13th, and so on.  This changes if we reduce the minimum to 5.  In this case,
 * week 1 is a truncated week from Wednesday the 1st to Sunday the 5th, and week
 * 0 doesn't exist.  The first seven day week is week 2, starting on the 6th.
 * </p>
 * <p>
 * On using the <code>clear()</code> method, the Gregorian calendar returns
 * to its default value of the 1st of January, 1970 AD 00:00:00 (the epoch).
 * The day of the week is set to the correct day for that particular time.
 * The day is also the first of the month, and the date is in week 0.
 * </p>
 *
 * @see Calendar
 * @see TimeZone
 * @see Calendar#getFirstDayOfWeek()
 * @see Calendar#getMinimalDaysInFirstWeek()
 */
public class GregorianCalendar extends Calendar
{
  /**
   * Constant representing the era BC (Before Christ).
   */
  public static final int BC = 0;
  
  /**
   * Constant representing the era AD (Anno Domini).
   */
  public static final int AD = 1;

  /**
   * The point at which the Gregorian calendar rules were used.
   * This is locale dependent; the default for most catholic
   * countries is midnight (UTC) on October 5, 1582 (Julian),
   * or October 15, 1582 (Gregorian).
   *
   * @serial the changeover point from the Julian calendar
   *         system to the Gregorian.
   */
  private long gregorianCutover;

  /**
   * For compatability with Sun's JDK.
   */
  static final long serialVersionUID = -8125100834729963327L;

  /**
   * The name of the resource bundle. Used only by getBundle()
   */
  private static final String bundleName = "gnu.java.locale.Calendar";

  /**
   * Retrieves the resource bundle.  The resources should be loaded
   * via this method only. Iff an application uses this method, the
   * resourcebundle is required.
   *
   * @param locale the locale in use for this calendar.
   * @return A resource bundle for the calendar for the specified locale.
   */
  private static ResourceBundle getBundle(Locale locale) 
  {
    return ResourceBundle.getBundle(bundleName, locale,
      ClassLoader.getSystemClassLoader());
  }

  /**
   * Constructs a new GregorianCalender representing the current
   * time, using the default time zone and the default locale.  
   */
  public GregorianCalendar()
  {
    this(TimeZone.getDefault(), Locale.getDefault());
  }
  
  /**
   * Constructs a new GregorianCalender representing the current
   * time, using the specified time zone and the default locale. 
   * 
   * @param zone a time zone.
   */
  public GregorianCalendar(TimeZone zone)
  {
    this(zone, Locale.getDefault());
  }
  
  /**
   * Constructs a new GregorianCalender representing the current
   * time, using the default time zone and the specified locale.
   *  
   * @param locale a locale.
   */
  public GregorianCalendar(Locale locale)
  {
    this(TimeZone.getDefault(), locale);
  }

  /**
   * Constructs a new GregorianCalender representing the current
   * time with the given time zone and the given locale.
   *
   * @param zone a time zone.  
   * @param locale a locale.  
   */
  public GregorianCalendar(TimeZone zone, Locale locale)
  {
    super(zone, locale);
    ResourceBundle rb = getBundle(locale);
    gregorianCutover = ((Date) rb.getObject("gregorianCutOver")).getTime();
    setTimeInMillis(System.currentTimeMillis());
  }

  /**
   * Constructs a new GregorianCalendar representing midnight on the
   * given date with the default time zone and locale.
   * @param year corresponds to the YEAR time field.
   * @param month corresponds to the MONTH time field.
   * @param day corresponds to the DAY time field.
   */
  public GregorianCalendar(int year, int month, int day)
  {
    super();
    set(year, month, day);
  }

  /**
   * Constructs a new GregorianCalendar representing midnight on the
   * given date with the default time zone and locale.
   *
   * @param year corresponds to the YEAR time field.
   * @param month corresponds to the MONTH time field.
   * @param day corresponds to the DAY time field.
   * @param hour corresponds to the HOUR_OF_DAY time field.
   * @param minute corresponds to the MINUTE time field.
   */
  public GregorianCalendar(int year, int month, int day, int hour, int minute)
  {
    super();
    set(year, month, day, hour, minute);
  }

  /**
   * Constructs a new GregorianCalendar representing midnight on the
   * given date with the default time zone and locale.
   *
   *
   * @param year corresponds to the YEAR time field.
   * @param month corresponds to the MONTH time field.
   * @param day corresponds to the DAY time field.
   * @param hour corresponds to the HOUR_OF_DAY time field.
   * @param minute corresponds to the MINUTE time field.
   * @param second corresponds to the SECOND time field.
   */
  public GregorianCalendar(int year, int month, int day,
                           int hour, int minute, int second)
  {
    super();
    set(year, month, day, hour, minute, second);
  }

  /**
   * Sets the date of the switch from Julian dates to Gregorian dates.
   * You can use <code>new Date(Long.MAX_VALUE)</code> to use a pure
   * Julian calendar, or <code>Long.MIN_VALUE</code> for a pure Gregorian
   * calendar.
   *
   * @param date the date of the change.
   */
  public void setGregorianChange(Date date)
  {
    gregorianCutover = date.getTime();
  }

  /**
   * Gets the date of the switch from Julian dates to Gregorian dates.
   *
   * @return the date of the change.
   */
  public final Date getGregorianChange()
  {
    return new Date(gregorianCutover);
  }

  /**
   * <p>
   * Determines if the given year is a leap year.  The result is
   * undefined if the Gregorian change took place in 1800, so that
   * the end of February is skipped, and that year is specified.
   * (well...).
   * </p>
   * <p>
   * To specify a year in the BC era, use a negative value calculated
   * as 1 - y, where y is the required year in BC.  So, 1 BC is 0,
   * 2 BC is -1, 3 BC is -2, etc.
   * </p>
   *
   * @param year a year (use a negative value for BC).
   * @return true, if the given year is a leap year, false otherwise.  
   */
  public boolean isLeapYear(int year)
  {
    if ((year & 3) != 0)
      // Only years divisible by 4 can be leap years
      return false;

    // compute the linear day of the 29. February of that year.
    // The 13 is the number of days, that were omitted in the Gregorian
    // Calender until the epoch.
    int julianDay = (((year-1) * (365*4+1)) >> 2) + (31+29 - 
        (((1970-1) * (365*4+1)) / 4 + 1 - 13));
    
    // If that day is smaller than the gregorianChange the julian
    // rule applies:  This is a leap year since it is divisible by 4.
    if (julianDay * (24 * 60 * 60 * 1000L) < gregorianCutover)
      return true;

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

  /**
   * Get the linear time in milliseconds since the epoch.  If you
   * specify a nonpositive year it is interpreted as BC as
   * following: 0 is 1 BC, -1 is 2 BC and so on.  The date is
   * interpreted as gregorian if the change occurred before that date.
   *
   * @param year the year of the date.
   * @param dayOfYear the day of year of the date; 1 based.
   * @param millis the millisecond in that day.
   * @return the days since the epoch, may be negative.  
   */
  private long getLinearTime(int year, int dayOfYear, int millis)
  {
    // The 13 is the number of days, that were omitted in the Gregorian
    // Calendar until the epoch.
    // We shift right by 2 instead of dividing by 4, to get correct
    // results for negative years (and this is even more efficient).
    int julianDay = ((year * (365 * 4 + 1)) >> 2) + dayOfYear -
      ((1970 * (365 * 4 + 1)) / 4 + 1 - 13);
    long time = julianDay * (24 * 60 * 60 * 1000L) + millis;

    if (time >= gregorianCutover)
      {
        // subtract the days that are missing in gregorian calendar
        // with respect to julian calendar.
        //
        // Okay, here we rely on the fact that the gregorian
        // calendar was introduced in the AD era.  This doesn't work
        // with negative years.
        //
        // The additional leap year factor accounts for the fact that
        // a leap day is not seen on Jan 1 of the leap year.
        // And on and after the leap day, the leap day has already been
        // included in dayOfYear. 
        int gregOffset = (year / 400) - (year / 100) + 2;
        if (isLeapYear (year, true))
          --gregOffset;
        time += gregOffset * (24 * 60 * 60 * 1000L);
      }
    return time;
  }

  /**
   * Retrieves the day of the week corresponding to the specified
   * day of the specified year.
   *
   * @param year the year in which the dayOfYear occurs.
   * @param dayOfYear the day of the year (an integer between 0 and
   *        and 366)
   */
  private int getWeekDay(int year, int dayOfYear)
  {
    int day =
      (int) (getLinearTime(year, dayOfYear, 0) / (24 * 60 * 60 * 1000L));

    // The epoch was a thursday.
    int weekday = (day + THURSDAY) % 7;
    if (weekday <= 0)
      weekday += 7;
    return weekday;
  }

  /**
   * <p>
   * Calculate the dayOfYear from the fields array.  
   * The relativeDays is used, to account for weeks that begin before
   * the Gregorian change and end after it.
   * </p>
   * <p>
   * We return two values.  The first is used to determine, if we
   * should use the Gregorian calendar or the Julian calendar, in order
   * to handle the change year. The second is a relative day after the given
   * day.  This is necessary for week calculation in the year in
   * which the Gregorian change occurs. 
   * </p>
   * 
   * @param year the year, negative for BC.
   * @return an array of two integer values, the first containing a reference
   * day in the current year, the second a relative count since this reference
   * day.  
   */
  private int[] getDayOfYear(int year)
  {
    if (isSet[MONTH])
      {
        int dayOfYear;
        if (fields[MONTH] > FEBRUARY)
          {

            // The months after February are regular:
            // 9 is an offset found by try and error.
            dayOfYear = (fields[MONTH] * (31 + 30 + 31 + 30 + 31) - 9) / 5;
            if (isLeapYear(year))
              dayOfYear++;
          }
        else
            dayOfYear = 31 * fields[MONTH];

        if (isSet[DAY_OF_MONTH])
          {
            return new int[]
            {
            dayOfYear + fields[DAY_OF_MONTH], 0};
          }
        if (isSet[WEEK_OF_MONTH] && isSet[DAY_OF_WEEK])
          {
            // the weekday of the first day in that month is:
            int weekday = getWeekDay(year, ++dayOfYear);

            return new int[]
            {
              dayOfYear,
                // the day of week in the first week
                // (weeks starting on sunday) is:
              fields[DAY_OF_WEEK] - weekday +
                // Now jump to the right week and correct the possible
                // error made by assuming sunday is the first week day.
              7 * (fields[WEEK_OF_MONTH]
                   + (fields[DAY_OF_WEEK] < getFirstDayOfWeek()? 0 : -1)
                   + (weekday < getFirstDayOfWeek()? -1 : 0))};
          }
        if (isSet[DAY_OF_WEEK] && isSet[DAY_OF_WEEK_IN_MONTH])
          {
            // the weekday of the first day in that month is:
            int weekday = getWeekDay(year, ++dayOfYear);
            return new int[] { 
                  dayOfYear,
                  fields[DAY_OF_WEEK] - weekday +
                  7 * (fields[DAY_OF_WEEK_IN_MONTH]
                       + (fields[DAY_OF_WEEK] < weekday ? 0 : -1))};
          }
      }

    // MONTH + something did not succeed.
    if (isSet[DAY_OF_YEAR])
      {
        return new int[] {0, fields[DAY_OF_YEAR]};
      }
      
    if (isSet[DAY_OF_WEEK] && isSet[WEEK_OF_YEAR])
      {
        int dayOfYear = getMinimalDaysInFirstWeek();
        // the weekday of the day, that begins the first week 
        // in that year is:
        int weekday = getWeekDay(year, dayOfYear);

        return new int[] { 
            dayOfYear,
              // the day of week in the first week
              // (weeks starting on sunday) is:
            fields[DAY_OF_WEEK] - weekday
              // Now jump to the right week and correct the possible
              // error made by assuming sunday is the first week day.
            + 7 * (fields[WEEK_OF_YEAR]
                   + (fields[DAY_OF_WEEK] < getFirstDayOfWeek()? 0 : -1)
                   + (weekday < getFirstDayOfWeek()? -1 : 0))};
      }

    // As last resort return Jan, 1st.
    return new int[] {1, 0};
  }

  /**
   * Converts the time field values (<code>fields</code>) to
   * milliseconds since the epoch UTC (<code>time</code>). 
   *
   * @throws IllegalArgumentException if any calendar fields
   *         are invalid.
   */
  protected synchronized void computeTime()
  {
    int era = isSet[ERA] ? fields[ERA] : AD;
    int year = isSet[YEAR] ? fields[YEAR] : 1970;
    if (era == BC)
      year = 1 - year;

    int[] daysOfYear = getDayOfYear(year);

    int hour = 0;
    if (isSet[HOUR_OF_DAY])
      hour = fields[HOUR_OF_DAY];
    else if (isSet[HOUR])
      {
        hour = fields[HOUR];
        if (isSet[AM_PM] && fields[AM_PM] == PM)
          if (hour != 12) /* not Noon */
            hour += 12;
        /* Fix the problem of the status of 12:00 AM (midnight). */
        if (isSet[AM_PM] && fields[AM_PM] == AM && hour == 12)
          hour = 0;
      }

    int minute = isSet[MINUTE] ? fields[MINUTE] : 0;
    int second = isSet[SECOND] ? fields[SECOND] : 0;
    int millis = isSet[MILLISECOND] ? fields[MILLISECOND] : 0;
    int millisInDay;

    if (isLenient())
      {
        // prevent overflow
        long allMillis = (((hour * 60L) + minute) * 60L + second) * 1000L
          + millis;
        daysOfYear[1] += allMillis / (24 * 60 * 60 * 1000L);
        millisInDay = (int) (allMillis % (24 * 60 * 60 * 1000L));
      }
    else
      {
        if (hour < 0 || hour >= 24 || minute < 0 || minute > 59
            || second < 0 || second > 59 || millis < 0 || millis >= 1000)
          throw new IllegalArgumentException();
        millisInDay = (((hour * 60) + minute) * 60 + second) * 1000 + millis;
      }
    time = getLinearTime(year, daysOfYear[0], millisInDay);

    // Add the relative days after calculating the linear time, to
    // get right behaviour when jumping over the gregorianCutover.
    time += daysOfYear[1] * (24 * 60 * 60 * 1000L);


    TimeZone zone = getTimeZone();
    int rawOffset = isSet[ZONE_OFFSET]
      ? fields[ZONE_OFFSET] : zone.getRawOffset();

    int dayOfYear = daysOfYear[0] + daysOfYear[1];
    // This formula isn't right, so check for month as a quick fix.
    // It doesn't compensate for leap years and puts day 30 in month 1
    // instead of month 0.
    int month = isSet[MONTH]
        ? fields[MONTH] : (dayOfYear * 5 + 3) / (31 + 30 + 31 + 30 + 31);
    // This formula isn't right, so check for day as a quick fix.  It
    // doesn't compensate for leap years, either.
    int day = isSet[DAY_OF_MONTH] ? fields[DAY_OF_MONTH]
        : (6 + (dayOfYear * 5 + 3) % (31 + 30 + 31 + 30 + 31)) / 5;
    int weekday = ((int) (time / (24 * 60 * 60 * 1000L)) + THURSDAY) % 7;
    if (weekday <= 0)
      weekday += 7;
    int dstOffset = isSet[DST_OFFSET]
      ? fields[DST_OFFSET] : (zone.getOffset((year < 0) ? BC : AD,
                                             (year < 0) ? 1 - year : year,
                                             month, day, weekday, millisInDay)
                              - zone.getRawOffset());
    time -= rawOffset + dstOffset;
    isTimeSet = true;
  }

  /**
   * <p>
   * Determines if the given year is a leap year.  
   * </p>
   * <p>
   * To specify a year in the BC era, use a negative value calculated
   * as 1 - y, where y is the required year in BC.  So, 1 BC is 0,
   * 2 BC is -1, 3 BC is -2, etc.
   * </p>
   *
   * @param year a year (use a negative value for BC).
   * @param gregorian if true, use the gregorian leap year rule.
   * @return true, if the given year is a leap year, false otherwise.  
   */
  private boolean isLeapYear(int year, boolean gregorian)
  {
    if ((year & 3) != 0)
      // Only years divisible by 4 can be leap years
      return false;

    if (!gregorian)
      return true;

    // We rely on AD area here.
    return ((year % 100) != 0 || (year % 400) == 0);
  }

  /**
   * Get the linear day in days since the epoch, using the
   * Julian or Gregorian calendar as specified.  If you specify a
   * nonpositive year it is interpreted as BC as following: 0 is 1
   * BC, -1 is 2 BC and so on.  
   *
   * @param year the year of the date.
   * @param dayOfYear the day of year of the date; 1 based.
   * @param gregorian <code>true</code>, if we should use the Gregorian rules.
   * @return the days since the epoch, may be negative.
   */
  private int getLinearDay(int year, int dayOfYear, boolean gregorian)
  {
    // The 13 is the number of days, that were omitted in the Gregorian
    // Calender until the epoch.
    // We shift right by 2 instead of dividing by 4, to get correct
    // results for negative years (and this is even more efficient).
    int julianDay = ((year * (365 * 4 + 1)) >> 2) + dayOfYear -
      ((1970 * (365 * 4 + 1)) / 4 + 1 - 13);

    if (gregorian)
      {
        // subtract the days that are missing in gregorian calendar
        // with respect to julian calendar.
        //
        // Okay, here we rely on the fact that the gregorian
        // calendar was introduced in the AD era.  This doesn't work
        // with negative years.
        //
        // The additional leap year factor accounts for the fact that
        // a leap day is not seen on Jan 1 of the leap year.
        int gregOffset = (year / 400) - (year / 100) + 2;
        if (isLeapYear (year, true) && dayOfYear < 31 + 29)
          --gregOffset;
        julianDay += gregOffset;
      }
    return julianDay;
  }

  /**
   * Converts the given linear day into era, year, month,
   * day_of_year, day_of_month, day_of_week, and writes the result
   * into the fields array.
   *
   * @param day the linear day.  
   * @param gregorian true, if we should use Gregorian rules.
   */
  private void calculateDay(int day, boolean gregorian)
  {
    // the epoch is a Thursday.
    int weekday = (day + THURSDAY) % 7;
    if (weekday <= 0)
      weekday += 7;
    fields[DAY_OF_WEEK] = weekday;

    // get a first approximation of the year.  This may be one 
    // year too big.
    int year = 1970 + (gregorian
                       ? ((day - 100) * 400) / (365 * 400 + 100 - 4 + 1)
                       : ((day - 100) * 4) / (365 * 4 + 1));
    if (day >= 0)
      year++;

    int firstDayOfYear = getLinearDay(year, 1, gregorian);

    // Now look in which year day really lies.
    if (day < firstDayOfYear)
      {
        year--;
        firstDayOfYear = getLinearDay(year, 1, gregorian);
      }

    day -= firstDayOfYear - 1;  // day of year,  one based.

    fields[DAY_OF_YEAR] = day;
    if (year <= 0)
      {
        fields[ERA] = BC;
        fields[YEAR] = 1 - year;
      }
    else
      {
        fields[ERA] = AD;
        fields[YEAR] = year;
      }

    int leapday = isLeapYear(year, gregorian) ? 1 : 0;
    if (day <= 31 + 28 + leapday)
      {
        fields[MONTH] = day / 32;       // 31->JANUARY, 32->FEBRUARY
        fields[DAY_OF_MONTH] = day - 31 * fields[MONTH];
      }
    else
      {
        // A few more magic formulas
        int scaledDay = (day - leapday) * 5 + 8;
        fields[MONTH] = scaledDay / (31 + 30 + 31 + 30 + 31);
        fields[DAY_OF_MONTH] = (scaledDay % (31 + 30 + 31 + 30 + 31)) / 5 + 1;
      }
  }

  /**
   * Converts the milliseconds since the epoch UTC
   * (<code>time</code>) to time fields
   * (<code>fields</code>).
   */
  protected synchronized void computeFields()
  {
    boolean gregorian = (time >= gregorianCutover);

    TimeZone zone = getTimeZone();
    fields[ZONE_OFFSET] = zone.getRawOffset();
    long localTime = time + fields[ZONE_OFFSET];

    int day = (int) (localTime / (24 * 60 * 60 * 1000L));
    int millisInDay = (int) (localTime % (24 * 60 * 60 * 1000L));
    if (millisInDay < 0)
      {
        millisInDay += (24 * 60 * 60 * 1000);
        day--;
      }

    calculateDay(day, gregorian);
    fields[DST_OFFSET] =
      zone.getOffset(fields[ERA], fields[YEAR], fields[MONTH],
                     fields[DAY_OF_MONTH], fields[DAY_OF_WEEK],
                     millisInDay) - fields[ZONE_OFFSET];

    millisInDay += fields[DST_OFFSET];
    if (millisInDay >= 24 * 60 * 60 * 1000)
      {
        millisInDay -= 24 * 60 * 60 * 1000;
        calculateDay(++day, gregorian);
      }

    fields[DAY_OF_WEEK_IN_MONTH] = (fields[DAY_OF_MONTH] + 6) / 7;

    // which day of the week are we (0..6), relative to getFirstDayOfWeek
    int relativeWeekday = (7 + fields[DAY_OF_WEEK] - getFirstDayOfWeek()) % 7;

    fields[WEEK_OF_MONTH] = (fields[DAY_OF_MONTH] - relativeWeekday + 12) / 7;

    int weekOfYear = (fields[DAY_OF_YEAR] - relativeWeekday + 6) / 7;

    // Do the Correction: getMinimalDaysInFirstWeek() is always in the 
    // first week.
    int minDays = getMinimalDaysInFirstWeek();
    int firstWeekday =
      (7 + getWeekDay(fields[YEAR], minDays) - getFirstDayOfWeek()) % 7;
    if (minDays - firstWeekday < 1)
      weekOfYear++;
    fields[WEEK_OF_YEAR] = weekOfYear;


    int hourOfDay = millisInDay / (60 * 60 * 1000);
    fields[AM_PM] = (hourOfDay < 12) ? AM : PM;
    int hour = hourOfDay % 12;
    fields[HOUR] = (hour == 0) ? 12 : hour;
    fields[HOUR_OF_DAY] = hourOfDay;
    millisInDay %= (60 * 60 * 1000);
    fields[MINUTE] = millisInDay / (60 * 1000);
    millisInDay %= (60 * 1000);
    fields[SECOND] = millisInDay / (1000);
    fields[MILLISECOND] = millisInDay % 1000;


    areFieldsSet = isSet[ERA] = isSet[YEAR] = isSet[MONTH] =
      isSet[WEEK_OF_YEAR] = isSet[WEEK_OF_MONTH] =
      isSet[DAY_OF_MONTH] = isSet[DAY_OF_YEAR] = isSet[DAY_OF_WEEK] =
      isSet[DAY_OF_WEEK_IN_MONTH] = isSet[AM_PM] = isSet[HOUR] =
      isSet[HOUR_OF_DAY] = isSet[MINUTE] = isSet[SECOND] =
      isSet[MILLISECOND] = isSet[ZONE_OFFSET] = isSet[DST_OFFSET] = true;

  }

  /**
   * Compares the given calendar with this.  An object, o, is
   * equivalent to this if it is also a <code>GregorianCalendar</code>
   * with the same time since the epoch under the same conditions
   * (same change date and same time zone).
   *  
   * @param o the object to that we should compare.
   * @return true, if the given object is a calendar, that represents
   * the same time (but doesn't necessarily have the same fields).
   * @throws IllegalArgumentException if one of the fields
   *         <code>ZONE_OFFSET</code> or <code>DST_OFFSET</code> is
   *         specified, if an unknown field is specified or if one
   *         of the calendar fields receives an illegal value when
   *         leniancy is not enabled.
   */
  public boolean equals(Object o)
  {
    if (!(o instanceof GregorianCalendar))
      return false;

    GregorianCalendar cal = (GregorianCalendar) o;
    return (cal.getTimeInMillis() == getTimeInMillis());
  }

//     /**
//      * Compares the given calender with this.  
//      * @param o the object to that we should compare.
//      * @return true, if the given object is a calendar, and this calendar
//      * represents a smaller time than the calender o.
//      */
//     public boolean before(Object o) {
//         if (!(o instanceof GregorianCalendar))
//             return false;

//         GregorianCalendar cal = (GregorianCalendar) o;
//         return (cal.getTimeInMillis() < getTimeInMillis());
//     }

//     /**
//      * Compares the given calender with this.  
//      * @param o the object to that we should compare.
//      * @return true, if the given object is a calendar, and this calendar
//      * represents a bigger time than the calender o.
//      */
//     public boolean after(Object o) {
//         if (!(o instanceof GregorianCalendar))
//             return false;

//         GregorianCalendar cal = (GregorianCalendar) o;
//         return (cal.getTimeInMillis() > getTimeInMillis());
//     }

  /**
   * Adds the specified amount of time to the given time field.  The
   * amount may be negative to subtract the time.  If the field overflows
   * it does what you expect: Jan, 25 + 10 Days is Feb, 4.
   * @param field one of the time field constants.
   * @param amount the amount of time to add.
   * @exception IllegalArgumentException if <code>field</code> is 
   *   <code>ZONE_OFFSET</code>, <code>DST_OFFSET</code>, or invalid; or
   *   if <code>amount</code> contains an out-of-range value and the calendar
   *   is not in lenient mode.
   */
  public void add(int field, int amount)
  {
    switch (field)
      {
      case YEAR:
        complete();
        fields[YEAR] += amount;
        isTimeSet = false;
        break;
      case MONTH:
        complete();
        int months = fields[MONTH] + amount;
        fields[YEAR] += months / 12;
        fields[MONTH] = months % 12;
        if (fields[MONTH] < 0)
          {
            fields[MONTH] += 12;
            fields[YEAR]--;
          }
        isTimeSet = false;
        int maxDay = getActualMaximum(DAY_OF_MONTH);
        if (fields[DAY_OF_MONTH] > maxDay)
          {
            fields[DAY_OF_MONTH] = maxDay;
            isTimeSet = false;
          }
        break;
      case DAY_OF_MONTH:
      case DAY_OF_YEAR:
      case DAY_OF_WEEK:
        if (!isTimeSet)
          computeTime();
        time += amount * (24 * 60 * 60 * 1000L);
        areFieldsSet = false;
        break;
      case WEEK_OF_YEAR:
      case WEEK_OF_MONTH:
      case DAY_OF_WEEK_IN_MONTH:
        if (!isTimeSet)
          computeTime();
        time += amount * (7 * 24 * 60 * 60 * 1000L);
        areFieldsSet = false;
        break;
      case AM_PM:
        if (!isTimeSet)
          computeTime();
        time += amount * (12 * 60 * 60 * 1000L);
        areFieldsSet = false;
        break;
      case HOUR:
      case HOUR_OF_DAY:
        if (!isTimeSet)
          computeTime();
        time += amount * (60 * 60 * 1000L);
        areFieldsSet = false;
        break;
      case MINUTE:
        if (!isTimeSet)
          computeTime();
        time += amount * (60 * 1000L);
        areFieldsSet = false;
        break;
      case SECOND:
        if (!isTimeSet)
          computeTime();
        time += amount * (1000L);
        areFieldsSet = false;
        break;
      case MILLISECOND:
        if (!isTimeSet)
          computeTime();
        time += amount;
        areFieldsSet = false;
        break;
      case ZONE_OFFSET:
      case DST_OFFSET:
      default:
        throw new IllegalArgumentException("Invalid or unknown field");
      }
  }


  /**
   * Rolls the specified time field up or down.  This means add one
   * to the specified field, but don't change the other fields.  If
   * the maximum for this field is reached, start over with the 
   * minimum value.  
   *
   * <strong>Note:</strong> There may be situation, where the other
   * fields must be changed, e.g rolling the month on May, 31. 
   * The date June, 31 is automatically converted to July, 1. 
   * This requires lenient settings.
   *
   * @param field the time field. One of the time field constants.
   * @param up the direction, true for up, false for down.
   * @throws IllegalArgumentException if one of the fields
   *         <code>ZONE_OFFSET</code> or <code>DST_OFFSET</code> is
   *         specified, if an unknown field is specified or if one
   *         of the calendar fields receives an illegal value when
   *         leniancy is not enabled.
   */
  public void roll(int field, boolean up)
  {
    roll(field, up ? 1 : -1);
  }

  /**
   * Checks that the fields are still within their legal bounds,
   * following use of the <code>roll()</code> method.
   *
   * @param field the field to check.
   * @param delta multipler for alterations to the <code>time</code>.
   * @see #roll(int, boolean)
   * @see #roll(int, int)
   */
  private void cleanUpAfterRoll(int field, int delta)
  {
    switch (field)
      {
      case ERA:
      case YEAR:
      case MONTH:
        // check that day of month is still in correct range
        if (fields[DAY_OF_MONTH] > getActualMaximum(DAY_OF_MONTH))
          fields[DAY_OF_MONTH] = getActualMaximum(DAY_OF_MONTH);
        isTimeSet = false;
        isSet[WEEK_OF_MONTH] = false;
        isSet[DAY_OF_WEEK] = false;
        isSet[DAY_OF_WEEK_IN_MONTH] = false;
        isSet[DAY_OF_YEAR] = false;
        isSet[WEEK_OF_YEAR] = false;
        break;

      case DAY_OF_MONTH:
        isSet[WEEK_OF_MONTH] = false;
        isSet[DAY_OF_WEEK] = false;
        isSet[DAY_OF_WEEK_IN_MONTH] = false;
        isSet[DAY_OF_YEAR] = false;
        isSet[WEEK_OF_YEAR] = false;
        time += delta * (24 * 60 * 60 * 1000L);
        break;

      case WEEK_OF_MONTH:
        isSet[DAY_OF_MONTH] = false;
        isSet[DAY_OF_WEEK_IN_MONTH] = false;
        isSet[DAY_OF_YEAR] = false;
        isSet[WEEK_OF_YEAR] = false;
        time += delta * (7 * 24 * 60 * 60 * 1000L);
        break;
      case DAY_OF_WEEK_IN_MONTH:
        isSet[DAY_OF_MONTH] = false;
        isSet[WEEK_OF_MONTH] = false;
        isSet[DAY_OF_YEAR] = false;
        isSet[WEEK_OF_YEAR] = false;
        time += delta * (7 * 24 * 60 * 60 * 1000L);
        break;
      case DAY_OF_YEAR:
        isSet[MONTH] = false;
        isSet[DAY_OF_MONTH] = false;
        isSet[WEEK_OF_MONTH] = false;
        isSet[DAY_OF_WEEK_IN_MONTH] = false;
        isSet[DAY_OF_WEEK] = false;
        isSet[WEEK_OF_YEAR] = false;
        time += delta * (24 * 60 * 60 * 1000L);
        break;
      case WEEK_OF_YEAR:
        isSet[MONTH] = false;
        isSet[DAY_OF_MONTH] = false;
        isSet[WEEK_OF_MONTH] = false;
        isSet[DAY_OF_WEEK_IN_MONTH] = false;
        isSet[DAY_OF_YEAR] = false;
        time += delta * (7 * 24 * 60 * 60 * 1000L);
        break;

      case AM_PM:
        isSet[HOUR_OF_DAY] = false;
        time += delta * (12 * 60 * 60 * 1000L);
        break;
      case HOUR:
        isSet[HOUR_OF_DAY] = false;
        time += delta * (60 * 60 * 1000L);
        break;
      case HOUR_OF_DAY:
        isSet[HOUR] = false;
        isSet[AM_PM] = false;
        time += delta * (60 * 60 * 1000L);
        break;

      case MINUTE:
        time += delta * (60 * 1000L);
        break;
      case SECOND:
        time += delta * (1000L);
        break;
      case MILLISECOND:
        time += delta;
        break;
      }
  }

  /**
   * Rolls the specified time field by the given amount.  This means
   * add amount to the specified field, but don't change the other
   * fields.  If the maximum for this field is reached, start over
   * with the minimum value and vice versa for negative amounts.
   *
   * <strong>Note:</strong> There may be situation, where the other
   * fields must be changed, e.g rolling the month on May, 31. 
   * The date June, 31 is automatically corrected to June, 30.
   *
   * @param field the time field. One of the time field constants.
   * @param amount the amount by which we should roll.
   * @throws IllegalArgumentException if one of the fields
   *         <code>ZONE_OFFSET</code> or <code>DST_OFFSET</code> is
   *         specified, if an unknown field is specified or if one
   *         of the calendar fields receives an illegal value when
   *         leniancy is not enabled.
   */
  public void roll(int field, int amount)
  {
    switch (field)
      {
      case DAY_OF_WEEK:
        // day of week is special: it rolls automatically
        add(field, amount);
        return;
      case ZONE_OFFSET:
      case DST_OFFSET:
        throw new IllegalArgumentException("Can't roll time zone");
      }
    complete();
    int min = getActualMinimum(field);
    int range = getActualMaximum(field) - min + 1;
    int oldval = fields[field];
    int newval = (oldval - min + range + amount) % range + min;
    if (newval < min)
      newval += range;
    fields[field] = newval;
    cleanUpAfterRoll(field, newval - oldval);
  }

  /**
   * The minimum values for the calendar fields.
   */
  private static final int[] minimums =
      { BC,       1,  0,  0, 1,  1,   1,   SUNDAY, 1, 
        AM,  1,  0,  1,  1,   1, -(12*60*60*1000),               0 };

  /**
   * The maximum values for the calendar fields.
   */
  private static final int[] maximums =
      { AD, 5000000, 11, 53, 5, 31, 366, SATURDAY, 5, 
        PM, 12, 23, 59, 59, 999, +(12*60*60*1000), (12*60*60*1000) };

  /**
   * Gets the smallest value that is allowed for the specified field.
   *
   * @param field one of the time field constants.
   * @return the smallest value for the specified field.
   */
  public int getMinimum(int field)
  {
    return minimums[field];
  }

  /**
   * Gets the biggest value that is allowed for the specified field.
   *
   * @param field one of the time field constants.
   * @return the biggest value.
   */
  public int getMaximum(int field)
  {
    return maximums[field];
  }


  /**
   * Gets the greatest minimum value that is allowed for the specified field.
   * This is the largest value returned by the <code>getActualMinimum(int)</code>
   * method.
   *
   * @param field the time field. One of the time field constants.
   * @return the greatest minimum value.
   * @see #getActualMinimum(int)
   */
  public int getGreatestMinimum(int field)
  {
    if (field == WEEK_OF_YEAR)
      return 1;
    return minimums[field];
  }

  /**
   * Gets the smallest maximum value that is allowed for the
   * specified field.  This is the smallest value returned
   * by the <code>getActualMaximum(int)</code>.  For example,
   * this is 28 for DAY_OF_MONTH (as all months have at least
   * 28 days).
   *
   * @param field the time field. One of the time field constants.
   * @return the least maximum value.  
   * @see #getActualMaximum(int)
   * @since 1.2
   */
  public int getLeastMaximum(int field)
  {
    switch (field)
      {
      case WEEK_OF_YEAR:
        return 52;
      case DAY_OF_MONTH:
        return 28;
      case DAY_OF_YEAR:
        return 365;
      case DAY_OF_WEEK_IN_MONTH:
      case WEEK_OF_MONTH:
        return 4;
      default:
        return maximums[field];
      }
  }

  /**
   * Gets the actual minimum value that is allowed for the specified field.
   * This value is dependent on the values of the other fields.  Note that
   * this calls <code>complete()</code> if not enough fields are set.  This
   * can have ugly side effects.  The value given depends on the current
   * time used by this instance.
   *
   * @param field the time field. One of the time field constants.
   * @return the actual minimum value.
   * @since 1.2
   */
  public int getActualMinimum(int field)
  {
    if (field == WEEK_OF_YEAR)
      {
        int min = getMinimalDaysInFirstWeek();
        if (min == 0)
          return 1;
        if (!areFieldsSet || !isSet[ERA] || !isSet[YEAR])
          complete();

        int year = fields[ERA] == AD ? fields[YEAR] : 1 - fields[YEAR];
        int weekday = getWeekDay(year, min);
        if ((7 + weekday - getFirstDayOfWeek()) % 7 >= min - 1)
          return 1;
        return 0;
      }
    return minimums[field];
  }

  /**
   * Gets the actual maximum value that is allowed for the specified field.
   * This value is dependent on the values of the other fields.  Note that
   * this calls <code>complete()</code> if not enough fields are set.  This
   * can have ugly side effects.  The value given depends on the current time
   * used by this instance; thus, leap years have a maximum day of month value of
   * 29, rather than 28.
   *
   * @param field the time field. One of the time field constants.
   * @return the actual maximum value.  
   */
  public int getActualMaximum(int field)
  {
    switch (field)
      {
      case WEEK_OF_YEAR:
        {
          if (!areFieldsSet || !isSet[ERA] || !isSet[YEAR])
            complete();
          // This is wrong for the year that contains the gregorian change.
          // I.e it gives the weeks in the julian year or in the gregorian
          // year in that case.
          int year = fields[ERA] == AD ? fields[YEAR] : 1 - fields[YEAR];
          int lastDay = isLeapYear(year) ? 366 : 365;
          int weekday = getWeekDay(year, lastDay);
          int week = (lastDay + 6
                      - (7 + weekday - getFirstDayOfWeek()) % 7) / 7;

          int minimalDays = getMinimalDaysInFirstWeek();
          int firstWeekday = getWeekDay(year, minimalDays);
          /* 
           * Is there a set of days at the beginning of the year, before the
           * first day of the week, equal to or greater than the minimum number
           * of days required in the first week?
           */
          if (minimalDays - (7 + firstWeekday - getFirstDayOfWeek()) % 7 < 1)
            return week + 1; /* Add week 1: firstWeekday through to firstDayOfWeek */
        }
        case DAY_OF_MONTH:
        {
          if (!areFieldsSet || !isSet[MONTH])
            complete();
          int month = fields[MONTH];
          // If you change this, you should also change 
          // SimpleTimeZone.getDaysInMonth();
          if (month == FEBRUARY)
            {
              if (!isSet[YEAR] || !isSet[ERA])
                complete();
              int year = fields[ERA] == AD ? fields[YEAR] : 1 - fields[YEAR];
              return isLeapYear(year) ? 29 : 28;
            }
          else if (month < AUGUST)
            return 31 - (month & 1);
          else
            return 30 + (month & 1);
        }
      case DAY_OF_YEAR:
        {
          if (!areFieldsSet || !isSet[ERA] || !isSet[YEAR])
            complete();
          int year = fields[ERA] == AD ? fields[YEAR] : 1 - fields[YEAR];
          return isLeapYear(year) ? 366 : 365;
        }
      case DAY_OF_WEEK_IN_MONTH:
        {
          // This is wrong for the month that contains the gregorian change.
          int daysInMonth = getActualMaximum(DAY_OF_MONTH);
          // That's black magic, I know
          return (daysInMonth - (fields[DAY_OF_MONTH] - 1) % 7 + 6) / 7;
        }
      case WEEK_OF_MONTH:
        {
          int daysInMonth = getActualMaximum(DAY_OF_MONTH);
          int weekday = (daysInMonth - fields[DAY_OF_MONTH]
                         + fields[DAY_OF_WEEK] - SUNDAY) % 7 + SUNDAY;
          return (daysInMonth + 6
                  - (7 + weekday - getFirstDayOfWeek()) % 7) / 7;
        }
      default:
        return maximums[field];
      }
  }


}