/*
 * 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
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * 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,
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 * 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");
    }
}
