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   * Copyright (c) 2012, 2015, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Copyright (c) 2012, Stephen Colebourne & Michael Nascimento Santos
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   *  * Redistributions of source code must retain the above copyright notice,
   *    this list of conditions and the following disclaimer.
   *
   *  * Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   *
   *  * Neither the name of JSR-310 nor the names of its contributors
   *    may be used to endorse or promote products derived from this software
   *    without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
   * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
   * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  package java.time.chrono;
  
  import java.io.InvalidObjectException;
  import static java.time.temporal.ChronoField.DAY_OF_MONTH;
  import static java.time.temporal.ChronoField.ERA;
  import static java.time.temporal.ChronoField.MONTH_OF_YEAR;
  import static java.time.temporal.ChronoField.PROLEPTIC_MONTH;
  import static java.time.temporal.ChronoField.YEAR;
  import static java.time.temporal.ChronoField.YEAR_OF_ERA;
  
  import java.io.ObjectInputStream;
  import java.io.Serializable;
  import java.time.Clock;
  import java.time.DateTimeException;
  import java.time.Instant;
  import java.time.LocalDate;
  import java.time.LocalDateTime;
  import java.time.Month;
  import java.time.Period;
  import java.time.Year;
  import java.time.ZoneId;
  import java.time.ZonedDateTime;
  import java.time.format.ResolverStyle;
  import java.time.temporal.ChronoField;
  import java.time.temporal.TemporalAccessor;
  import java.time.temporal.TemporalField;
  import java.time.temporal.ValueRange;
  import java.util.Arrays;
  import java.util.List;
  import java.util.Locale;
  import java.util.Map;
  import java.util.Objects;
  
  /**
   * The ISO calendar system.
   * <p>
   * This chronology defines the rules of the ISO calendar system.
   * This calendar system is based on the ISO-8601 standard, which is the
   * <i>de facto</i> world calendar.
   * <p>
   * The fields are defined as follows:
   * <ul>
   * <li>era - There are two eras, 'Current Era' (CE) and 'Before Current Era' (BCE).
   * <li>year-of-era - The year-of-era is the same as the proleptic-year for the current CE era.
   *  For the BCE era before the ISO epoch the year increases from 1 upwards as time goes backwards.
   * <li>proleptic-year - The proleptic year is the same as the year-of-era for the
   *  current era. For the previous era, years have zero, then negative values.
   * <li>month-of-year - There are 12 months in an ISO year, numbered from 1 to 12.
   * <li>day-of-month - There are between 28 and 31 days in each of the ISO month, numbered from 1 to 31.
   *  Months 4, 6, 9 and 11 have 30 days, Months 1, 3, 5, 7, 8, 10 and 12 have 31 days.
   *  Month 2 has 28 days, or 29 in a leap year.
   * <li>day-of-year - There are 365 days in a standard ISO year and 366 in a leap year.
   *  The days are numbered from 1 to 365 or 1 to 366.
   * <li>leap-year - Leap years occur every 4 years, except where the year is divisble by 100 and not divisble by 400.
   * </ul>
   *
   * @implSpec
   * This class is immutable and thread-safe.
   *
   * @since 1.8
   */
  public final class IsoChronology extends AbstractChronology implements Serializable {
  
      /**
       * Singleton instance of the ISO chronology.
       */
      public static final IsoChronology INSTANCE = new IsoChronology();
  
      /**
       * Serialization version.
       */
      private static final long serialVersionUID = -1440403870442975015L;
  
      /**
       * Restricted constructor.
       */
      private IsoChronology() {
      }
  
      //-----------------------------------------------------------------------
      /**
       * Gets the ID of the chronology - 'ISO'.
       * <p>
       * The ID uniquely identifies the {@code Chronology}.
       * It can be used to lookup the {@code Chronology} using {@link Chronology#of(String)}.
       *
       * @return the chronology ID - 'ISO'
       * @see #getCalendarType()
       */
      @Override
      public String getId() {
          return "ISO";
      }
  
      /**
       * Gets the calendar type of the underlying calendar system - 'iso8601'.
       * <p>
       * The calendar type is an identifier defined by the
       * <em>Unicode Locale Data Markup Language (LDML)</em> specification.
       * It can be used to lookup the {@code Chronology} using {@link Chronology#of(String)}.
       * It can also be used as part of a locale, accessible via
       * {@link Locale#getUnicodeLocaleType(String)} with the key 'ca'.
       *
       * @return the calendar system type - 'iso8601'
       * @see #getId()
       */
      @Override
      public String getCalendarType() {
          return "iso8601";
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains an ISO local date from the era, year-of-era, month-of-year
       * and day-of-month fields.
       *
       * @param era  the ISO era, not null
       * @param yearOfEra  the ISO year-of-era
       * @param month  the ISO month-of-year
       * @param dayOfMonth  the ISO day-of-month
       * @return the ISO local date, not null
       * @throws DateTimeException if unable to create the date
       * @throws ClassCastException if the type of {@code era} is not {@code IsoEra}
       */
      @Override  // override with covariant return type
      public LocalDate date(Era era, int yearOfEra, int month, int dayOfMonth) {
          return date(prolepticYear(era, yearOfEra), month, dayOfMonth);
      }
  
      /**
       * Obtains an ISO local date from the proleptic-year, month-of-year
       * and day-of-month fields.
       * <p>
       * This is equivalent to {@link LocalDate#of(int, int, int)}.
       *
       * @param prolepticYear  the ISO proleptic-year
       * @param month  the ISO month-of-year
       * @param dayOfMonth  the ISO day-of-month
       * @return the ISO local date, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate date(int prolepticYear, int month, int dayOfMonth) {
          return LocalDate.of(prolepticYear, month, dayOfMonth);
      }
  
      /**
       * Obtains an ISO local date from the era, year-of-era and day-of-year fields.
       *
       * @param era  the ISO era, not null
       * @param yearOfEra  the ISO year-of-era
       * @param dayOfYear  the ISO day-of-year
       * @return the ISO local date, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate dateYearDay(Era era, int yearOfEra, int dayOfYear) {
          return dateYearDay(prolepticYear(era, yearOfEra), dayOfYear);
      }
  
      /**
       * Obtains an ISO local date from the proleptic-year and day-of-year fields.
       * <p>
       * This is equivalent to {@link LocalDate#ofYearDay(int, int)}.
       *
       * @param prolepticYear  the ISO proleptic-year
       * @param dayOfYear  the ISO day-of-year
       * @return the ISO local date, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate dateYearDay(int prolepticYear, int dayOfYear) {
          return LocalDate.ofYearDay(prolepticYear, dayOfYear);
      }
  
      /**
       * Obtains an ISO local date from the epoch-day.
       * <p>
       * This is equivalent to {@link LocalDate#ofEpochDay(long)}.
       *
       * @param epochDay  the epoch day
       * @return the ISO local date, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate dateEpochDay(long epochDay) {
          return LocalDate.ofEpochDay(epochDay);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains an ISO local date from another date-time object.
       * <p>
       * This is equivalent to {@link LocalDate#from(TemporalAccessor)}.
       *
       * @param temporal  the date-time object to convert, not null
       * @return the ISO local date, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate date(TemporalAccessor temporal) {
          return LocalDate.from(temporal);
      }
  
      /**
       * Obtains an ISO local date-time from another date-time object.
       * <p>
       * This is equivalent to {@link LocalDateTime#from(TemporalAccessor)}.
       *
       * @param temporal  the date-time object to convert, not null
       * @return the ISO local date-time, not null
       * @throws DateTimeException if unable to create the date-time
       */
      @Override  // override with covariant return type
      public LocalDateTime localDateTime(TemporalAccessor temporal) {
          return LocalDateTime.from(temporal);
      }
  
      /**
       * Obtains an ISO zoned date-time from another date-time object.
       * <p>
       * This is equivalent to {@link ZonedDateTime#from(TemporalAccessor)}.
       *
       * @param temporal  the date-time object to convert, not null
       * @return the ISO zoned date-time, not null
       * @throws DateTimeException if unable to create the date-time
       */
      @Override  // override with covariant return type
      public ZonedDateTime zonedDateTime(TemporalAccessor temporal) {
          return ZonedDateTime.from(temporal);
      }
  
      /**
       * Obtains an ISO zoned date-time in this chronology from an {@code Instant}.
       * <p>
       * This is equivalent to {@link ZonedDateTime#ofInstant(Instant, ZoneId)}.
       *
       * @param instant  the instant to create the date-time from, not null
       * @param zone  the time-zone, not null
       * @return the zoned date-time, not null
       * @throws DateTimeException if the result exceeds the supported range
       */
      @Override
      public ZonedDateTime zonedDateTime(Instant instant, ZoneId zone) {
          return ZonedDateTime.ofInstant(instant, zone);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains the current ISO local date from the system clock in the default time-zone.
       * <p>
       * This will query the {@link Clock#systemDefaultZone() system clock} in the default
       * time-zone to obtain the current date.
       * <p>
       * Using this method will prevent the ability to use an alternate clock for testing
       * because the clock is hard-coded.
       *
       * @return the current ISO local date using the system clock and default time-zone, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate dateNow() {
          return dateNow(Clock.systemDefaultZone());
      }
  
      /**
       * Obtains the current ISO local date from the system clock in the specified time-zone.
       * <p>
       * This will query the {@link Clock#system(ZoneId) system clock} to obtain the current date.
       * Specifying the time-zone avoids dependence on the default time-zone.
       * <p>
       * Using this method will prevent the ability to use an alternate clock for testing
       * because the clock is hard-coded.
       *
       * @return the current ISO local date using the system clock, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate dateNow(ZoneId zone) {
          return dateNow(Clock.system(zone));
      }
  
      /**
       * Obtains the current ISO local date from the specified clock.
       * <p>
       * This will query the specified clock to obtain the current date - today.
       * Using this method allows the use of an alternate clock for testing.
       * The alternate clock may be introduced using {@link Clock dependency injection}.
       *
       * @param clock  the clock to use, not null
       * @return the current ISO local date, not null
       * @throws DateTimeException if unable to create the date
       */
      @Override  // override with covariant return type
      public LocalDate dateNow(Clock clock) {
          Objects.requireNonNull(clock, "clock");
          return date(LocalDate.now(clock));
      }
  
      //-----------------------------------------------------------------------
      /**
       * Checks if the year is a leap year, according to the ISO proleptic
       * calendar system rules.
       * <p>
       * This method applies the current rules for leap years across the whole time-line.
       * In general, a year is a leap year if it is divisible by four without
       * remainder. However, years divisible by 100, are not leap years, with
       * the exception of years divisible by 400 which are.
       * <p>
       * For example, 1904 is a leap year it is divisible by 4.
       * 1900 was not a leap year as it is divisible by 100, however 2000 was a
       * leap year as it is divisible by 400.
       * <p>
       * The calculation is proleptic - applying the same rules into the far future and far past.
       * This is historically inaccurate, but is correct for the ISO-8601 standard.
       *
       * @param prolepticYear  the ISO proleptic year to check
       * @return true if the year is leap, false otherwise
       */
      @Override
      public boolean isLeapYear(long prolepticYear) {
          return ((prolepticYear & 3) == 0) && ((prolepticYear % 100) != 0 || (prolepticYear % 400) == 0);
      }
  
      @Override
      public int prolepticYear(Era era, int yearOfEra) {
          if (era instanceof IsoEra == false) {
              throw new ClassCastException("Era must be IsoEra");
          }
          return (era == IsoEra.CE ? yearOfEra : 1 - yearOfEra);
      }
  
      @Override
      public IsoEra eraOf(int eraValue) {
          return IsoEra.of(eraValue);
      }
  
      @Override
      public List<Era> eras() {
          return Arrays.<Era>asList(IsoEra.values());
      }
  
      //-----------------------------------------------------------------------
      /**
       * Resolves parsed {@code ChronoField} values into a date during parsing.
       * <p>
       * Most {@code TemporalField} implementations are resolved using the
       * resolve method on the field. By contrast, the {@code ChronoField} class
       * defines fields that only have meaning relative to the chronology.
       * As such, {@code ChronoField} date fields are resolved here in the
       * context of a specific chronology.
       * <p>
       * {@code ChronoField} instances on the ISO calendar system are resolved
       * as follows.
       * <ul>
       * <li>{@code EPOCH_DAY} - If present, this is converted to a {@code LocalDate}
       *  and all other date fields are then cross-checked against the date.
       * <li>{@code PROLEPTIC_MONTH} - If present, then it is split into the
       *  {@code YEAR} and {@code MONTH_OF_YEAR}. If the mode is strict or smart
       *  then the field is validated.
       * <li>{@code YEAR_OF_ERA} and {@code ERA} - If both are present, then they
       *  are combined to form a {@code YEAR}. In lenient mode, the {@code YEAR_OF_ERA}
       *  range is not validated, in smart and strict mode it is. The {@code ERA} is
       *  validated for range in all three modes. If only the {@code YEAR_OF_ERA} is
       *  present, and the mode is smart or lenient, then the current era (CE/AD)
       *  is assumed. In strict mode, no era is assumed and the {@code YEAR_OF_ERA} is
       *  left untouched. If only the {@code ERA} is present, then it is left untouched.
       * <li>{@code YEAR}, {@code MONTH_OF_YEAR} and {@code DAY_OF_MONTH} -
       *  If all three are present, then they are combined to form a {@code LocalDate}.
       *  In all three modes, the {@code YEAR} is validated. If the mode is smart or strict,
       *  then the month and day are validated, with the day validated from 1 to 31.
       *  If the mode is lenient, then the date is combined in a manner equivalent to
       *  creating a date on the first of January in the requested year, then adding
       *  the difference in months, then the difference in days.
       *  If the mode is smart, and the day-of-month is greater than the maximum for
       *  the year-month, then the day-of-month is adjusted to the last day-of-month.
       *  If the mode is strict, then the three fields must form a valid date.
       * <li>{@code YEAR} and {@code DAY_OF_YEAR} -
       *  If both are present, then they are combined to form a {@code LocalDate}.
       *  In all three modes, the {@code YEAR} is validated.
       *  If the mode is lenient, then the date is combined in a manner equivalent to
       *  creating a date on the first of January in the requested year, then adding
       *  the difference in days.
       *  If the mode is smart or strict, then the two fields must form a valid date.
       * <li>{@code YEAR}, {@code MONTH_OF_YEAR}, {@code ALIGNED_WEEK_OF_MONTH} and
       *  {@code ALIGNED_DAY_OF_WEEK_IN_MONTH} -
       *  If all four are present, then they are combined to form a {@code LocalDate}.
       *  In all three modes, the {@code YEAR} is validated.
       *  If the mode is lenient, then the date is combined in a manner equivalent to
       *  creating a date on the first of January in the requested year, then adding
       *  the difference in months, then the difference in weeks, then in days.
       *  If the mode is smart or strict, then the all four fields are validated to
       *  their outer ranges. The date is then combined in a manner equivalent to
       *  creating a date on the first day of the requested year and month, then adding
       *  the amount in weeks and days to reach their values. If the mode is strict,
       *  the date is additionally validated to check that the day and week adjustment
       *  did not change the month.
       * <li>{@code YEAR}, {@code MONTH_OF_YEAR}, {@code ALIGNED_WEEK_OF_MONTH} and
       *  {@code DAY_OF_WEEK} - If all four are present, then they are combined to
       *  form a {@code LocalDate}. The approach is the same as described above for
       *  years, months and weeks in {@code ALIGNED_DAY_OF_WEEK_IN_MONTH}.
       *  The day-of-week is adjusted as the next or same matching day-of-week once
       *  the years, months and weeks have been handled.
       * <li>{@code YEAR}, {@code ALIGNED_WEEK_OF_YEAR} and {@code ALIGNED_DAY_OF_WEEK_IN_YEAR} -
       *  If all three are present, then they are combined to form a {@code LocalDate}.
       *  In all three modes, the {@code YEAR} is validated.
       *  If the mode is lenient, then the date is combined in a manner equivalent to
       *  creating a date on the first of January in the requested year, then adding
       *  the difference in weeks, then in days.
       *  If the mode is smart or strict, then the all three fields are validated to
       *  their outer ranges. The date is then combined in a manner equivalent to
       *  creating a date on the first day of the requested year, then adding
       *  the amount in weeks and days to reach their values. If the mode is strict,
       *  the date is additionally validated to check that the day and week adjustment
       *  did not change the year.
       * <li>{@code YEAR}, {@code ALIGNED_WEEK_OF_YEAR} and {@code DAY_OF_WEEK} -
       *  If all three are present, then they are combined to form a {@code LocalDate}.
       *  The approach is the same as described above for years and weeks in
       *  {@code ALIGNED_DAY_OF_WEEK_IN_YEAR}. The day-of-week is adjusted as the
       *  next or same matching day-of-week once the years and weeks have been handled.
       * </ul>
       *
       * @param fieldValues  the map of fields to values, which can be updated, not null
       * @param resolverStyle  the requested type of resolve, not null
       * @return the resolved date, null if insufficient information to create a date
       * @throws DateTimeException if the date cannot be resolved, typically
       *  because of a conflict in the input data
       */
      @Override  // override for performance
      public LocalDate resolveDate(Map<TemporalField, Long> fieldValues, ResolverStyle resolverStyle) {
          return (LocalDate) super.resolveDate(fieldValues, resolverStyle);
      }
  
      @Override  // override for better proleptic algorithm
      void resolveProlepticMonth(Map<TemporalField, Long> fieldValues, ResolverStyle resolverStyle) {
          Long pMonth = fieldValues.remove(PROLEPTIC_MONTH);
          if (pMonth != null) {
              if (resolverStyle != ResolverStyle.LENIENT) {
                  PROLEPTIC_MONTH.checkValidValue(pMonth);
              }
              addFieldValue(fieldValues, MONTH_OF_YEAR, Math.floorMod(pMonth, 12) + 1);
              addFieldValue(fieldValues, YEAR, Math.floorDiv(pMonth, 12));
          }
      }
  
      @Override  // override for enhanced behaviour
      LocalDate resolveYearOfEra(Map<TemporalField, Long> fieldValues, ResolverStyle resolverStyle) {
          Long yoeLong = fieldValues.remove(YEAR_OF_ERA);
          if (yoeLong != null) {
              if (resolverStyle != ResolverStyle.LENIENT) {
                  YEAR_OF_ERA.checkValidValue(yoeLong);
              }
              Long era = fieldValues.remove(ERA);
              if (era == null) {
                  Long year = fieldValues.get(YEAR);
                  if (resolverStyle == ResolverStyle.STRICT) {
                      // do not invent era if strict, but do cross-check with year
                      if (year != null) {
                          addFieldValue(fieldValues, YEAR, (year > 0 ? yoeLong: Math.subtractExact(1, yoeLong)));
                      } else {
                          // reinstate the field removed earlier, no cross-check issues
                          fieldValues.put(YEAR_OF_ERA, yoeLong);
                      }
                  } else {
                      // invent era
                      addFieldValue(fieldValues, YEAR, (year == null || year > 0 ? yoeLong: Math.subtractExact(1, yoeLong)));
                  }
              } else if (era.longValue() == 1L) {
                  addFieldValue(fieldValues, YEAR, yoeLong);
              } else if (era.longValue() == 0L) {
                  addFieldValue(fieldValues, YEAR, Math.subtractExact(1, yoeLong));
              } else {
                  throw new DateTimeException("Invalid value for era: " + era);
              }
          } else if (fieldValues.containsKey(ERA)) {
              ERA.checkValidValue(fieldValues.get(ERA));  // always validated
          }
          return null;
      }
  
      @Override  // override for performance
      LocalDate resolveYMD(Map <TemporalField, Long> fieldValues, ResolverStyle resolverStyle) {
          int y = YEAR.checkValidIntValue(fieldValues.remove(YEAR));
          if (resolverStyle == ResolverStyle.LENIENT) {
              long months = Math.subtractExact(fieldValues.remove(MONTH_OF_YEAR), 1);
              long days = Math.subtractExact(fieldValues.remove(DAY_OF_MONTH), 1);
              return LocalDate.of(y, 1, 1).plusMonths(months).plusDays(days);
          }
          int moy = MONTH_OF_YEAR.checkValidIntValue(fieldValues.remove(MONTH_OF_YEAR));
          int dom = DAY_OF_MONTH.checkValidIntValue(fieldValues.remove(DAY_OF_MONTH));
          if (resolverStyle == ResolverStyle.SMART) {  // previous valid
              if (moy == 4 || moy == 6 || moy == 9 || moy == 11) {
                  dom = Math.min(dom, 30);
              } else if (moy == 2) {
                  dom = Math.min(dom, Month.FEBRUARY.length(Year.isLeap(y)));
  
              }
          }
          return LocalDate.of(y, moy, dom);
      }
  
      //-----------------------------------------------------------------------
      @Override
      public ValueRange range(ChronoField field) {
          return field.range();
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains a period for this chronology based on years, months and days.
       * <p>
       * This returns a period tied to the ISO chronology using the specified
       * years, months and days. See {@link Period} for further details.
       *
       * @param years  the number of years, may be negative
       * @param months  the number of years, may be negative
       * @param days  the number of years, may be negative
       * @return the period in terms of this chronology, not null
       * @return the ISO period, not null
       */
      @Override  // override with covariant return type
      public Period period(int years, int months, int days) {
          return Period.of(years, months, days);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Writes the Chronology using a
       * <a href="../../../serialized-form.html#java.time.chrono.Ser">dedicated serialized form</a>.
       * @serialData
       * <pre>
       *  out.writeByte(1);     // identifies a Chronology
       *  out.writeUTF(getId());
       * </pre>
       *
       * @return the instance of {@code Ser}, not null
       */
      @Override
      Object writeReplace() {
          return super.writeReplace();
      }
  
      /**
       * Defend against malicious streams.
       *
       * @param s the stream to read
       * @throws InvalidObjectException always
       */
      private void readObject(ObjectInputStream s) throws InvalidObjectException {
          throw new InvalidObjectException("Deserialization via serialization delegate");
      }
  }
  