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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
   * 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) 2008-2012, 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;
  
  import static java.time.temporal.ChronoUnit.DAYS;
  import static java.time.temporal.ChronoUnit.MONTHS;
  import static java.time.temporal.ChronoUnit.YEARS;
  
  import java.io.DataInput;
  import java.io.DataOutput;
  import java.io.IOException;
  import java.io.InvalidObjectException;
  import java.io.ObjectInputStream;
  import java.io.Serializable;
  import java.time.chrono.ChronoLocalDate;
  import java.time.chrono.ChronoPeriod;
  import java.time.chrono.Chronology;
  import java.time.chrono.IsoChronology;
  import java.time.format.DateTimeParseException;
  import java.time.temporal.ChronoUnit;
  import java.time.temporal.Temporal;
  import java.time.temporal.TemporalAccessor;
  import java.time.temporal.TemporalAmount;
  import java.time.temporal.TemporalQueries;
  import java.time.temporal.TemporalUnit;
  import java.time.temporal.UnsupportedTemporalTypeException;
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.List;
  import java.util.Objects;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  
  // Android-changed: removed ValueBased paragraph.
  
  /**
   * A date-based amount of time in the ISO-8601 calendar system,
   * such as '2 years, 3 months and 4 days'.
   * <p>
   * This class models a quantity or amount of time in terms of years, months and days.
   * See {@link Duration} for the time-based equivalent to this class.
   * <p>
   * Durations and periods differ in their treatment of daylight savings time
   * when added to {@link ZonedDateTime}. A {@code Duration} will add an exact
   * number of seconds, thus a duration of one day is always exactly 24 hours.
   * By contrast, a {@code Period} will add a conceptual day, trying to maintain
   * the local time.
   * <p>
   * For example, consider adding a period of one day and a duration of one day to
   * 18:00 on the evening before a daylight savings gap. The {@code Period} will add
   * the conceptual day and result in a {@code ZonedDateTime} at 18:00 the following day.
   * By contrast, the {@code Duration} will add exactly 24 hours, resulting in a
   * {@code ZonedDateTime} at 19:00 the following day (assuming a one hour DST gap).
   * <p>
   * The supported units of a period are {@link ChronoUnit#YEARS YEARS},
   * {@link ChronoUnit#MONTHS MONTHS} and {@link ChronoUnit#DAYS DAYS}.
   * All three fields are always present, but may be set to zero.
   * <p>
   * The ISO-8601 calendar system is the modern civil calendar system used today
   * in most of the world. It is equivalent to the proleptic Gregorian calendar
   * system, in which today's rules for leap years are applied for all time.
   * <p>
   * The period is modeled as a directed amount of time, meaning that individual parts of the
   * period may be negative.
   *
   * @implSpec
   * This class is immutable and thread-safe.
   *
   * @since 1.8
   */
  public final class Period
          implements ChronoPeriod, Serializable {
  
      /**
       * A constant for a period of zero.
       */
      public static final Period ZERO = new Period(0, 0, 0);
      /**
       * Serialization version.
       */
      private static final long serialVersionUID = -3587258372562876L;
      /**
       * The pattern for parsing.
       */
      private static final Pattern PATTERN =
              Pattern.compile("([-+]?)P(?:([-+]?[0-9]+)Y)?(?:([-+]?[0-9]+)M)?(?:([-+]?[0-9]+)W)?(?:([-+]?[0-9]+)D)?", Pattern.CASE_INSENSITIVE);
  
      /**
       * The set of supported units.
       */
      private static final List<TemporalUnit> SUPPORTED_UNITS =
              Collections.unmodifiableList(Arrays.<TemporalUnit>asList(YEARS, MONTHS, DAYS));
  
      /**
       * The number of years.
       */
      private final int years;
      /**
       * The number of months.
       */
      private final int months;
      /**
       * The number of days.
       */
      private final int days;
  
      //-----------------------------------------------------------------------
      /**
       * Obtains a {@code Period} representing a number of years.
       * <p>
       * The resulting period will have the specified years.
       * The months and days units will be zero.
       *
       * @param years  the number of years, positive or negative
       * @return the period of years, not null
       */
      public static Period ofYears(int years) {
          return create(years, 0, 0);
      }
  
      /**
       * Obtains a {@code Period} representing a number of months.
       * <p>
       * The resulting period will have the specified months.
       * The years and days units will be zero.
       *
       * @param months  the number of months, positive or negative
       * @return the period of months, not null
       */
      public static Period ofMonths(int months) {
          return create(0, months, 0);
      }
  
      /**
       * Obtains a {@code Period} representing a number of weeks.
       * <p>
       * The resulting period will be day-based, with the amount of days
       * equal to the number of weeks multiplied by 7.
       * The years and months units will be zero.
       *
       * @param weeks  the number of weeks, positive or negative
       * @return the period, with the input weeks converted to days, not null
       */
      public static Period ofWeeks(int weeks) {
          return create(0, 0, Math.multiplyExact(weeks, 7));
      }
  
      /**
       * Obtains a {@code Period} representing a number of days.
       * <p>
       * The resulting period will have the specified days.
       * The years and months units will be zero.
       *
       * @param days  the number of days, positive or negative
       * @return the period of days, not null
       */
      public static Period ofDays(int days) {
          return create(0, 0, days);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains a {@code Period} representing a number of years, months and days.
       * <p>
       * This creates an instance based on years, months and days.
       *
       * @param years  the amount of years, may be negative
       * @param months  the amount of months, may be negative
       * @param days  the amount of days, may be negative
       * @return the period of years, months and days, not null
       */
      public static Period of(int years, int months, int days) {
          return create(years, months, days);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains an instance of {@code Period} from a temporal amount.
       * <p>
       * This obtains a period based on the specified amount.
       * A {@code TemporalAmount} represents an  amount of time, which may be
       * date-based or time-based, which this factory extracts to a {@code Period}.
       * <p>
       * The conversion loops around the set of units from the amount and uses
       * the {@link ChronoUnit#YEARS YEARS}, {@link ChronoUnit#MONTHS MONTHS}
       * and {@link ChronoUnit#DAYS DAYS} units to create a period.
       * If any other units are found then an exception is thrown.
       * <p>
       * If the amount is a {@code ChronoPeriod} then it must use the ISO chronology.
       *
       * @param amount  the temporal amount to convert, not null
       * @return the equivalent period, not null
       * @throws DateTimeException if unable to convert to a {@code Period}
       * @throws ArithmeticException if the amount of years, months or days exceeds an int
       */
      public static Period from(TemporalAmount amount) {
          if (amount instanceof Period) {
              return (Period) amount;
          }
          if (amount instanceof ChronoPeriod) {
              if (IsoChronology.INSTANCE.equals(((ChronoPeriod) amount).getChronology()) == false) {
                  throw new DateTimeException("Period requires ISO chronology: " + amount);
              }
          }
          Objects.requireNonNull(amount, "amount");
          int years = 0;
          int months = 0;
          int days = 0;
          for (TemporalUnit unit : amount.getUnits()) {
              long unitAmount = amount.get(unit);
              if (unit == ChronoUnit.YEARS) {
                  years = Math.toIntExact(unitAmount);
              } else if (unit == ChronoUnit.MONTHS) {
                  months = Math.toIntExact(unitAmount);
              } else if (unit == ChronoUnit.DAYS) {
                  days = Math.toIntExact(unitAmount);
              } else {
                  throw new DateTimeException("Unit must be Years, Months or Days, but was " + unit);
              }
          }
          return create(years, months, days);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains a {@code Period} from a text string such as {@code PnYnMnD}.
       * <p>
       * This will parse the string produced by {@code toString()} which is
       * based on the ISO-8601 period formats {@code PnYnMnD} and {@code PnW}.
       * <p>
       * The string starts with an optional sign, denoted by the ASCII negative
       * or positive symbol. If negative, the whole period is negated.
       * The ASCII letter "P" is next in upper or lower case.
       * There are then four sections, each consisting of a number and a suffix.
       * At least one of the four sections must be present.
       * The sections have suffixes in ASCII of "Y", "M", "W" and "D" for
       * years, months, weeks and days, accepted in upper or lower case.
       * The suffixes must occur in order.
       * The number part of each section must consist of ASCII digits.
       * The number may be prefixed by the ASCII negative or positive symbol.
       * The number must parse to an {@code int}.
       * <p>
       * The leading plus/minus sign, and negative values for other units are
       * not part of the ISO-8601 standard. In addition, ISO-8601 does not
       * permit mixing between the {@code PnYnMnD} and {@code PnW} formats.
       * Any week-based input is multiplied by 7 and treated as a number of days.
       * <p>
       * For example, the following are valid inputs:
       * <pre>
       *   "P2Y"             -- Period.ofYears(2)
       *   "P3M"             -- Period.ofMonths(3)
       *   "P4W"             -- Period.ofWeeks(4)
       *   "P5D"             -- Period.ofDays(5)
       *   "P1Y2M3D"         -- Period.of(1, 2, 3)
       *   "P1Y2M3W4D"       -- Period.of(1, 2, 25)
       *   "P-1Y2M"          -- Period.of(-1, 2, 0)
       *   "-P1Y2M"          -- Period.of(-1, -2, 0)
       * </pre>
       *
       * @param text  the text to parse, not null
       * @return the parsed period, not null
       * @throws DateTimeParseException if the text cannot be parsed to a period
       */
      public static Period parse(CharSequence text) {
          Objects.requireNonNull(text, "text");
          Matcher matcher = PATTERN.matcher(text);
          if (matcher.matches()) {
              int negate = ("-".equals(matcher.group(1)) ? -1 : 1);
              String yearMatch = matcher.group(2);
              String monthMatch = matcher.group(3);
              String weekMatch = matcher.group(4);
              String dayMatch = matcher.group(5);
              if (yearMatch != null || monthMatch != null || dayMatch != null || weekMatch != null) {
                  try {
                      int years = parseNumber(text, yearMatch, negate);
                      int months = parseNumber(text, monthMatch, negate);
                      int weeks = parseNumber(text, weekMatch, negate);
                      int days = parseNumber(text, dayMatch, negate);
                      days = Math.addExact(days, Math.multiplyExact(weeks, 7));
                      return create(years, months, days);
                  } catch (NumberFormatException ex) {
                      throw new DateTimeParseException("Text cannot be parsed to a Period", text, 0, ex);
                  }
              }
          }
          throw new DateTimeParseException("Text cannot be parsed to a Period", text, 0);
      }
  
      private static int parseNumber(CharSequence text, String str, int negate) {
          if (str == null) {
              return 0;
          }
          int val = Integer.parseInt(str);
          try {
              return Math.multiplyExact(val, negate);
          } catch (ArithmeticException ex) {
              throw new DateTimeParseException("Text cannot be parsed to a Period", text, 0, ex);
          }
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains a {@code Period} consisting of the number of years, months,
       * and days between two dates.
       * <p>
       * The start date is included, but the end date is not.
       * The period is calculated by removing complete months, then calculating
       * the remaining number of days, adjusting to ensure that both have the same sign.
       * The number of months is then split into years and months based on a 12 month year.
       * A month is considered if the end day-of-month is greater than or equal to the start day-of-month.
       * For example, from {@code 2010-01-15} to {@code 2011-03-18} is one year, two months and three days.
       * <p>
       * The result of this method can be a negative period if the end is before the start.
       * The negative sign will be the same in each of year, month and day.
       *
       * @param startDateInclusive  the start date, inclusive, not null
       * @param endDateExclusive  the end date, exclusive, not null
       * @return the period between this date and the end date, not null
       * @see ChronoLocalDate#until(ChronoLocalDate)
       */
      public static Period between(LocalDate startDateInclusive, LocalDate endDateExclusive) {
          return startDateInclusive.until(endDateExclusive);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Creates an instance.
       *
       * @param years  the amount
       * @param months  the amount
       * @param days  the amount
       */
      private static Period create(int years, int months, int days) {
          if ((years | months | days) == 0) {
              return ZERO;
          }
          return new Period(years, months, days);
      }
  
      /**
       * Constructor.
       *
       * @param years  the amount
       * @param months  the amount
       * @param days  the amount
       */
      private Period(int years, int months, int days) {
          this.years = years;
          this.months = months;
          this.days = days;
      }
  
      //-----------------------------------------------------------------------
      /**
       * Gets the value of the requested unit.
       * <p>
       * This returns a value for each of the three supported units,
       * {@link ChronoUnit#YEARS YEARS}, {@link ChronoUnit#MONTHS MONTHS} and
       * {@link ChronoUnit#DAYS DAYS}.
       * All other units throw an exception.
       *
       * @param unit the {@code TemporalUnit} for which to return the value
       * @return the long value of the unit
       * @throws DateTimeException if the unit is not supported
       * @throws UnsupportedTemporalTypeException if the unit is not supported
       */
      @Override
      public long get(TemporalUnit unit) {
          if (unit == ChronoUnit.YEARS) {
              return getYears();
          } else if (unit == ChronoUnit.MONTHS) {
              return getMonths();
          } else if (unit == ChronoUnit.DAYS) {
              return getDays();
          } else {
              throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
          }
      }
  
      /**
       * Gets the set of units supported by this period.
       * <p>
       * The supported units are {@link ChronoUnit#YEARS YEARS},
       * {@link ChronoUnit#MONTHS MONTHS} and {@link ChronoUnit#DAYS DAYS}.
       * They are returned in the order years, months, days.
       * <p>
       * This set can be used in conjunction with {@link #get(TemporalUnit)}
       * to access the entire state of the period.
       *
       * @return a list containing the years, months and days units, not null
       */
      @Override
      public List<TemporalUnit> getUnits() {
          return SUPPORTED_UNITS;
      }
  
      /**
       * Gets the chronology of this period, which is the ISO calendar system.
       * <p>
       * The {@code Chronology} represents the calendar system in use.
       * The ISO-8601 calendar system is the modern civil calendar system used today
       * in most of the world. It is equivalent to the proleptic Gregorian calendar
       * system, in which today's rules for leap years are applied for all time.
       *
       * @return the ISO chronology, not null
       */
      @Override
      public IsoChronology getChronology() {
          return IsoChronology.INSTANCE;
      }
  
      //-----------------------------------------------------------------------
      /**
       * Checks if all three units of this period are zero.
       * <p>
       * A zero period has the value zero for the years, months and days units.
       *
       * @return true if this period is zero-length
       */
      public boolean isZero() {
          return (this == ZERO);
      }
  
      /**
       * Checks if any of the three units of this period are negative.
       * <p>
       * This checks whether the years, months or days units are less than zero.
       *
       * @return true if any unit of this period is negative
       */
      public boolean isNegative() {
          return years < 0 || months < 0 || days < 0;
      }
  
      //-----------------------------------------------------------------------
      /**
       * Gets the amount of years of this period.
       * <p>
       * This returns the years unit.
       * <p>
       * The months unit is not automatically normalized with the years unit.
       * This means that a period of "15 months" is different to a period
       * of "1 year and 3 months".
       *
       * @return the amount of years of this period, may be negative
       */
      public int getYears() {
          return years;
      }
  
      /**
       * Gets the amount of months of this period.
       * <p>
       * This returns the months unit.
       * <p>
       * The months unit is not automatically normalized with the years unit.
       * This means that a period of "15 months" is different to a period
       * of "1 year and 3 months".
       *
       * @return the amount of months of this period, may be negative
       */
      public int getMonths() {
          return months;
      }
  
      /**
       * Gets the amount of days of this period.
       * <p>
       * This returns the days unit.
       *
       * @return the amount of days of this period, may be negative
       */
      public int getDays() {
          return days;
      }
  
      //-----------------------------------------------------------------------
      /**
       * Returns a copy of this period with the specified amount of years.
       * <p>
       * This sets the amount of the years unit in a copy of this period.
       * The months and days units are unaffected.
       * <p>
       * The months unit is not automatically normalized with the years unit.
       * This means that a period of "15 months" is different to a period
       * of "1 year and 3 months".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param years  the years to represent, may be negative
       * @return a {@code Period} based on this period with the requested years, not null
       */
      public Period withYears(int years) {
          if (years == this.years) {
              return this;
          }
          return create(years, months, days);
      }
  
      /**
       * Returns a copy of this period with the specified amount of months.
       * <p>
       * This sets the amount of the months unit in a copy of this period.
       * The years and days units are unaffected.
       * <p>
       * The months unit is not automatically normalized with the years unit.
       * This means that a period of "15 months" is different to a period
       * of "1 year and 3 months".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param months  the months to represent, may be negative
       * @return a {@code Period} based on this period with the requested months, not null
       */
      public Period withMonths(int months) {
          if (months == this.months) {
              return this;
          }
          return create(years, months, days);
      }
  
      /**
       * Returns a copy of this period with the specified amount of days.
       * <p>
       * This sets the amount of the days unit in a copy of this period.
       * The years and months units are unaffected.
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param days  the days to represent, may be negative
       * @return a {@code Period} based on this period with the requested days, not null
       */
      public Period withDays(int days) {
          if (days == this.days) {
              return this;
          }
          return create(years, months, days);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Returns a copy of this period with the specified period added.
       * <p>
       * This operates separately on the years, months and days.
       * No normalization is performed.
       * <p>
       * For example, "1 year, 6 months and 3 days" plus "2 years, 2 months and 2 days"
       * returns "3 years, 8 months and 5 days".
       * <p>
       * The specified amount is typically an instance of {@code Period}.
       * Other types are interpreted using {@link Period#from(TemporalAmount)}.
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param amountToAdd  the amount to add, not null
       * @return a {@code Period} based on this period with the requested period added, not null
       * @throws DateTimeException if the specified amount has a non-ISO chronology or
       *  contains an invalid unit
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period plus(TemporalAmount amountToAdd) {
          Period isoAmount = Period.from(amountToAdd);
          return create(
                  Math.addExact(years, isoAmount.years),
                  Math.addExact(months, isoAmount.months),
                  Math.addExact(days, isoAmount.days));
      }
  
      /**
       * Returns a copy of this period with the specified years added.
       * <p>
       * This adds the amount to the years unit in a copy of this period.
       * The months and days units are unaffected.
       * For example, "1 year, 6 months and 3 days" plus 2 years returns "3 years, 6 months and 3 days".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param yearsToAdd  the years to add, positive or negative
       * @return a {@code Period} based on this period with the specified years added, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period plusYears(long yearsToAdd) {
          if (yearsToAdd == 0) {
              return this;
          }
          return create(Math.toIntExact(Math.addExact(years, yearsToAdd)), months, days);
      }
  
      /**
       * Returns a copy of this period with the specified months added.
       * <p>
       * This adds the amount to the months unit in a copy of this period.
       * The years and days units are unaffected.
       * For example, "1 year, 6 months and 3 days" plus 2 months returns "1 year, 8 months and 3 days".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param monthsToAdd  the months to add, positive or negative
       * @return a {@code Period} based on this period with the specified months added, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period plusMonths(long monthsToAdd) {
          if (monthsToAdd == 0) {
              return this;
          }
          return create(years, Math.toIntExact(Math.addExact(months, monthsToAdd)), days);
      }
  
      /**
       * Returns a copy of this period with the specified days added.
       * <p>
       * This adds the amount to the days unit in a copy of this period.
       * The years and months units are unaffected.
       * For example, "1 year, 6 months and 3 days" plus 2 days returns "1 year, 6 months and 5 days".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param daysToAdd  the days to add, positive or negative
       * @return a {@code Period} based on this period with the specified days added, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period plusDays(long daysToAdd) {
          if (daysToAdd == 0) {
              return this;
          }
          return create(years, months, Math.toIntExact(Math.addExact(days, daysToAdd)));
      }
  
      //-----------------------------------------------------------------------
      /**
       * Returns a copy of this period with the specified period subtracted.
       * <p>
       * This operates separately on the years, months and days.
       * No normalization is performed.
       * <p>
       * For example, "1 year, 6 months and 3 days" minus "2 years, 2 months and 2 days"
       * returns "-1 years, 4 months and 1 day".
       * <p>
       * The specified amount is typically an instance of {@code Period}.
       * Other types are interpreted using {@link Period#from(TemporalAmount)}.
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param amountToSubtract  the amount to subtract, not null
       * @return a {@code Period} based on this period with the requested period subtracted, not null
       * @throws DateTimeException if the specified amount has a non-ISO chronology or
       *  contains an invalid unit
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period minus(TemporalAmount amountToSubtract) {
          Period isoAmount = Period.from(amountToSubtract);
          return create(
                  Math.subtractExact(years, isoAmount.years),
                  Math.subtractExact(months, isoAmount.months),
                  Math.subtractExact(days, isoAmount.days));
      }
  
      /**
       * Returns a copy of this period with the specified years subtracted.
       * <p>
       * This subtracts the amount from the years unit in a copy of this period.
       * The months and days units are unaffected.
       * For example, "1 year, 6 months and 3 days" minus 2 years returns "-1 years, 6 months and 3 days".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param yearsToSubtract  the years to subtract, positive or negative
       * @return a {@code Period} based on this period with the specified years subtracted, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period minusYears(long yearsToSubtract) {
          return (yearsToSubtract == Long.MIN_VALUE ? plusYears(Long.MAX_VALUE).plusYears(1) : plusYears(-yearsToSubtract));
      }
  
      /**
       * Returns a copy of this period with the specified months subtracted.
       * <p>
       * This subtracts the amount from the months unit in a copy of this period.
       * The years and days units are unaffected.
       * For example, "1 year, 6 months and 3 days" minus 2 months returns "1 year, 4 months and 3 days".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param monthsToSubtract  the years to subtract, positive or negative
       * @return a {@code Period} based on this period with the specified months subtracted, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period minusMonths(long monthsToSubtract) {
          return (monthsToSubtract == Long.MIN_VALUE ? plusMonths(Long.MAX_VALUE).plusMonths(1) : plusMonths(-monthsToSubtract));
      }
  
      /**
       * Returns a copy of this period with the specified days subtracted.
       * <p>
       * This subtracts the amount from the days unit in a copy of this period.
       * The years and months units are unaffected.
       * For example, "1 year, 6 months and 3 days" minus 2 days returns "1 year, 6 months and 1 day".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param daysToSubtract  the months to subtract, positive or negative
       * @return a {@code Period} based on this period with the specified days subtracted, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period minusDays(long daysToSubtract) {
          return (daysToSubtract == Long.MIN_VALUE ? plusDays(Long.MAX_VALUE).plusDays(1) : plusDays(-daysToSubtract));
      }
  
      //-----------------------------------------------------------------------
      /**
       * Returns a new instance with each element in this period multiplied
       * by the specified scalar.
       * <p>
       * This returns a period with each of the years, months and days units
       * individually multiplied.
       * For example, a period of "2 years, -3 months and 4 days" multiplied by
       * 3 will return "6 years, -9 months and 12 days".
       * No normalization is performed.
       *
       * @param scalar  the scalar to multiply by, not null
       * @return a {@code Period} based on this period with the amounts multiplied by the scalar, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period multipliedBy(int scalar) {
          if (this == ZERO || scalar == 1) {
              return this;
          }
          return create(
                  Math.multiplyExact(years, scalar),
                  Math.multiplyExact(months, scalar),
                  Math.multiplyExact(days, scalar));
      }
  
      /**
       * Returns a new instance with each amount in this period negated.
       * <p>
       * This returns a period with each of the years, months and days units
       * individually negated.
       * For example, a period of "2 years, -3 months and 4 days" will be
       * negated to "-2 years, 3 months and -4 days".
       * No normalization is performed.
       *
       * @return a {@code Period} based on this period with the amounts negated, not null
       * @throws ArithmeticException if numeric overflow occurs, which only happens if
       *  one of the units has the value {@code Long.MIN_VALUE}
       */
      public Period negated() {
          return multipliedBy(-1);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Returns a copy of this period with the years and months normalized.
       * <p>
       * This normalizes the years and months units, leaving the days unit unchanged.
       * The months unit is adjusted to have an absolute value less than 11,
       * with the years unit being adjusted to compensate. For example, a period of
       * "1 Year and 15 months" will be normalized to "2 years and 3 months".
       * <p>
       * The sign of the years and months units will be the same after normalization.
       * For example, a period of "1 year and -25 months" will be normalized to
       * "-1 year and -1 month".
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @return a {@code Period} based on this period with excess months normalized to years, not null
       * @throws ArithmeticException if numeric overflow occurs
       */
      public Period normalized() {
          long totalMonths = toTotalMonths();
          long splitYears = totalMonths / 12;
          int splitMonths = (int) (totalMonths % 12);  // no overflow
          if (splitYears == years && splitMonths == months) {
              return this;
          }
          return create(Math.toIntExact(splitYears), splitMonths, days);
      }
  
      /**
       * Gets the total number of months in this period.
       * <p>
       * This returns the total number of months in the period by multiplying the
       * number of years by 12 and adding the number of months.
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @return the total number of months in the period, may be negative
       */
      public long toTotalMonths() {
          return years * 12L + months;  // no overflow
      }
  
      //-------------------------------------------------------------------------
      /**
       * Adds this period to the specified temporal object.
       * <p>
       * This returns a temporal object of the same observable type as the input
       * with this period added.
       * If the temporal has a chronology, it must be the ISO chronology.
       * <p>
       * In most cases, it is clearer to reverse the calling pattern by using
       * {@link Temporal#plus(TemporalAmount)}.
       * <pre>
       *   // these two lines are equivalent, but the second approach is recommended
       *   dateTime = thisPeriod.addTo(dateTime);
       *   dateTime = dateTime.plus(thisPeriod);
       * </pre>
       * <p>
       * The calculation operates as follows.
       * First, the chronology of the temporal is checked to ensure it is ISO chronology or null.
       * Second, if the months are zero, the years are added if non-zero, otherwise
       * the combination of years and months is added if non-zero.
       * Finally, any days are added.
       * <p>
       * This approach ensures that a partial period can be added to a partial date.
       * For example, a period of years and/or months can be added to a {@code YearMonth},
       * but a period including days cannot.
       * The approach also adds years and months together when necessary, which ensures
       * correct behaviour at the end of the month.
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param temporal  the temporal object to adjust, not null
       * @return an object of the same type with the adjustment made, not null
       * @throws DateTimeException if unable to add
       * @throws ArithmeticException if numeric overflow occurs
       */
      @Override
      public Temporal addTo(Temporal temporal) {
          validateChrono(temporal);
          if (months == 0) {
              if (years != 0) {
                  temporal = temporal.plus(years, YEARS);
              }
          } else {
              long totalMonths = toTotalMonths();
              if (totalMonths != 0) {
                  temporal = temporal.plus(totalMonths, MONTHS);
              }
          }
          if (days != 0) {
              temporal = temporal.plus(days, DAYS);
          }
          return temporal;
      }
  
      /**
       * Subtracts this period from the specified temporal object.
       * <p>
       * This returns a temporal object of the same observable type as the input
       * with this period subtracted.
       * If the temporal has a chronology, it must be the ISO chronology.
       * <p>
       * In most cases, it is clearer to reverse the calling pattern by using
       * {@link Temporal#minus(TemporalAmount)}.
       * <pre>
       *   // these two lines are equivalent, but the second approach is recommended
       *   dateTime = thisPeriod.subtractFrom(dateTime);
       *   dateTime = dateTime.minus(thisPeriod);
       * </pre>
       * <p>
       * The calculation operates as follows.
       * First, the chronology of the temporal is checked to ensure it is ISO chronology or null.
       * Second, if the months are zero, the years are subtracted if non-zero, otherwise
       * the combination of years and months is subtracted if non-zero.
       * Finally, any days are subtracted.
       * <p>
       * This approach ensures that a partial period can be subtracted from a partial date.
       * For example, a period of years and/or months can be subtracted from a {@code YearMonth},
       * but a period including days cannot.
       * The approach also subtracts years and months together when necessary, which ensures
       * correct behaviour at the end of the month.
       * <p>
       * This instance is immutable and unaffected by this method call.
       *
       * @param temporal  the temporal object to adjust, not null
       * @return an object of the same type with the adjustment made, not null
       * @throws DateTimeException if unable to subtract
       * @throws ArithmeticException if numeric overflow occurs
       */
      @Override
      public Temporal subtractFrom(Temporal temporal) {
          validateChrono(temporal);
          if (months == 0) {
              if (years != 0) {
                  temporal = temporal.minus(years, YEARS);
              }
          } else {
              long totalMonths = toTotalMonths();
              if (totalMonths != 0) {
                  temporal = temporal.minus(totalMonths, MONTHS);
              }
          }
          if (days != 0) {
              temporal = temporal.minus(days, DAYS);
          }
          return temporal;
      }
  
      /**
       * Validates that the temporal has the correct chronology.
       */
      private void validateChrono(TemporalAccessor temporal) {
          Objects.requireNonNull(temporal, "temporal");
          Chronology temporalChrono = temporal.query(TemporalQueries.chronology());
          if (temporalChrono != null && IsoChronology.INSTANCE.equals(temporalChrono) == false) {
              throw new DateTimeException("Chronology mismatch, expected: ISO, actual: " + temporalChrono.getId());
          }
      }
  
      //-----------------------------------------------------------------------
      /**
       * Checks if this period is equal to another period.
       * <p>
       * The comparison is based on the type {@code Period} and each of the three amounts.
       * To be equal, the years, months and days units must be individually equal.
       * Note that this means that a period of "15 Months" is not equal to a period
       * of "1 Year and 3 Months".
       *
       * @param obj  the object to check, null returns false
       * @return true if this is equal to the other period
       */
      @Override
      public boolean equals(Object obj) {
          if (this == obj) {
              return true;
          }
          if (obj instanceof Period) {
              Period other = (Period) obj;
              return years == other.years &&
                      months == other.months &&
                      days == other.days;
          }
          return false;
      }
  
      /**
       * A hash code for this period.
       *
       * @return a suitable hash code
       */
      @Override
      public int hashCode() {
          return years + Integer.rotateLeft(months, 8) + Integer.rotateLeft(days, 16);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Outputs this period as a {@code String}, such as {@code P6Y3M1D}.
       * <p>
       * The output will be in the ISO-8601 period format.
       * A zero period will be represented as zero days, 'P0D'.
       *
       * @return a string representation of this period, not null
       */
      @Override
      public String toString() {
          if (this == ZERO) {
              return "P0D";
          } else {
              StringBuilder buf = new StringBuilder();
              buf.append('P');
              if (years != 0) {
                  buf.append(years).append('Y');
              }
              if (months != 0) {
                  buf.append(months).append('M');
              }
              if (days != 0) {
                  buf.append(days).append('D');
              }
              return buf.toString();
          }
      }
  
      //-----------------------------------------------------------------------
      /**
       * Writes the object using a
       * <a href="../../serialized-form.html#java.time.Ser">dedicated serialized form</a>.
       * @serialData
       * <pre>
       *  out.writeByte(14);  // identifies a Period
       *  out.writeInt(years);
       *  out.writeInt(months);
       *  out.writeInt(days);
       * </pre>
       *
       * @return the instance of {@code Ser}, not null
       */
      private Object writeReplace() {
          return new Ser(Ser.PERIOD_TYPE, this);
      }
  
      /**
       * 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");
      }
  
      void writeExternal(DataOutput out) throws IOException {
          out.writeInt(years);
          out.writeInt(months);
          out.writeInt(days);
      }
  
      static Period readExternal(DataInput in) throws IOException {
          int years = in.readInt();
          int months = in.readInt();
          int days = in.readInt();
          return Period.of(years, months, days);
      }
  
  }
  