// $Id: Duration.java,v 1.36.8.1.4.3 2004/06/07 06:33:50 jsuttor Exp $

/*
 * @(#)Duration.java	1.13 04/07/26
 * 
 * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package javax.xml.datatype;

import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

import javax.xml.namespace.QName;

/**
 * <p>Immutable representation of a time span as defined in
 * the W3C XML Schema 1.0 specification.</p>
 * 
 * <p>A Duration object represents a period of Gregorian time,
 * which consists of six fields (years, months, days, hours,
 * minutes, and seconds) plus a sign (+/-) field.</p>
 * 
 * <p>The first five fields have non-negative (>=0) integers or null
 * (which represents that the field is not set),
 * and the seconds field has a non-negative decimal or null.
 * A negative sign indicates a negative duration.</p> 
 * 
 * <p>This class provides a number of methods that make it easy
 * to use for the duration datatype of XML Schema 1.0 with
 * the errata.</p>
 * 
 * <h2>Order relationship</h2>
 * <p>Duration objects only have partial order, where two values A and B
 * maybe either:</p>
 * <ol>
 *  <li>A&lt;B (A is shorter than B)
 *  <li>A&gt;B (A is longer than B)
 *  <li>A==B   (A and B are of the same duration)
 *  <li>A&lt;>B (Comparison between A and B is indeterminate)
 * </ol>

 *  * <p>For example, 30 days cannot be meaningfully compared to one month.
 * The {@link #compare(Duration duration)} method implements this
 * relationship.</p>
 * 
 * <p>See the {@link #isLongerThan(Duration)} method for details about
 * the order relationship among <code>Duration</code> objects.</p>
 * 
 * <h2>Operations over Duration</h2>
 * <p>This class provides a set of basic arithmetic operations, such
 * as addition, subtraction and multiplication.
 * Because durations don't have total order, an operation could
 * fail for some combinations of operations. For example, you cannot
 * subtract 15 days from 1 month. See the javadoc of those methods
 * for detailed conditions where this could happen.</p>
 * 
 * <p>Also, division of a duration by a number is not provided because
 * the <code>Duration</code> class can only deal with finite precision
 * decimal numbers. For example, one cannot represent 1 sec divided by 3.</p> 
 * 
 * <p>However, you could substitute a division by 3 with multiplying
 * by numbers such as 0.3 or 0.333.</p>
 * 
 * <h2>Range of allowed values</h2>
 * <p>
 * Because some operations of <code>Duration</code> rely on {@link Calendar}
 * even though {@link Duration} can hold very large or very small values,
 * some of the methods may not work correctly on such <code>Duration</code>s.
 * The impacted methods document their dependency on {@link Calendar}.
 * 
 *  
 * @author <a href="mailto:Joseph.Fialli@Sun.COM">Joseph Fialli</a>
 * @author <a href="mailto:Kohsuke.Kawaguchi@Sun.com">Kohsuke Kawaguchi</a>
 * @author <a href="mailto:Jeff.Suttor@Sun.com">Jeff Suttor</a>
 * @version $Revision: 1.36.8.1.4.3 $, $Date: 2004/06/07 06:33:50 $    
 * @see XMLGregorianCalendar#add(Duration)
 * @since 1.5
 */
public abstract class Duration {
		
	/**
	 * <p>Debugging <code>true</code> or <code>false</code>.</p> 
	 */
	private static final boolean DEBUG = true;

	/**
	 * <p>Return the name of the XML Schema date/time type that this instance 
	 * maps to. Type is computed based on fields that are set,
	 * i.e. {@link #isSet(DatatypeConstants.Field field)} == <code>true</code>.</p>
	 *
	 * <table border="2" rules="all" cellpadding="2">
	 *   <thead>
	 *     <tr>
	 *       <th align="center" colspan="7">
	 *         Required fields for XML Schema 1.0 Date/Time Datatypes.<br/>
	 *         <i>(timezone is optional for all date/time datatypes)</i>
	 *       </th>
	 *     </tr>
	 *   </thead>
	 *   <tbody>
	 *     <tr>
	 *       <td>Datatype</td>
	 *       <td>year</td>
	 *       <td>month</td>
	 *       <td>day</td>
	 *       <td>hour</td>
	 *       <td>minute</td>
	 *       <td>second</td>
	 *     </tr>
	 *     <tr>
	 *       <td>{@link DatatypeConstants#DURATION}</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *     </tr>
	 *     <tr>
	 *       <td>{@link DatatypeConstants#DURATION_DAYTIME}</td>
	 *       <td></td>
	 *       <td></td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *     </tr>
	 *     <tr>
	 *       <td>{@link DatatypeConstants#DURATION_YEARMONTH}</td>
	 *       <td>X</td>
	 *       <td>X</td>
	 *       <td></td>
	 *       <td></td>
	 *       <td></td>
	 *       <td></td>
	 *     </tr>
	 *   </tbody>
	 * </table>
	 * 
	 * @return one of the following constants:
	 *   {@link DatatypeConstants#DURATION},
	 *   {@link DatatypeConstants#DURATION_DAYTIME} or
	 *   {@link DatatypeConstants#DURATION_YEARMONTH}.
	 *  
	 * @throws IllegalStateException If the combination of set fields does not match one of the XML Schema date/time datatypes.
	 */
	public QName getXMLSchemaType() {
		
		boolean yearSet = isSet(DatatypeConstants.YEARS);
		boolean monthSet = isSet(DatatypeConstants.MONTHS);
		boolean daySet = isSet(DatatypeConstants.DAYS);
		boolean hourSet = isSet(DatatypeConstants.HOURS);
		boolean minuteSet = isSet(DatatypeConstants.MINUTES);
		boolean secondSet = isSet(DatatypeConstants.SECONDS);
		
		// DURATION
		if (yearSet
			&& monthSet
			&& daySet
			&& hourSet
			&& minuteSet
			&& secondSet) {
			return DatatypeConstants.DURATION;
		}

		// DURATION_DAYTIME
		if (!yearSet
			&& !monthSet
			&& daySet
			&& hourSet
			&& minuteSet
			&& secondSet) {
			return DatatypeConstants.DURATION_DAYTIME;
		}

		// DURATION_YEARMONTH
		if (yearSet
			&& monthSet
			&& !daySet
			&& !hourSet
			&& !minuteSet
			&& !secondSet) {
			return DatatypeConstants.DURATION_YEARMONTH;
		}

		// nothing matches
		throw new IllegalStateException(
				"javax.xml.datatype.Duration#getXMLSchemaType():"
				+ " this Duration does not match one of the XML Schema date/time datatypes:"
				+ " year set = " + yearSet
				+ " month set = " + monthSet
				+ " day set = " + daySet
				+ " hour set = " + hourSet
				+ " minute set = " + minuteSet
				+ " second set = " + secondSet
		);
	}

	/**
	 * Returns the sign of this duration in -1,0, or 1.
	 * 
	 * @return
	 *      -1 if this duration is negative, 0 if the duration is zero,
	 *      and 1 if the duration is positive.
	 */
	public abstract int getSign();

	/**
	 * <p>Get the years value of this <code>Duration</code> as an <code>int</code> or <code>0</code> if not present.</p>
	 * 
	 * <p><code>getYears()</code> is a convenience method for
	 * {@link #getField(DatatypeConstants.Field field) getField(DatatypeConstants.YEARS)}.</p>
	 * 
	 * <p>As the return value is an <code>int</code>, an incorrect value will be returned for <code>Duration</code>s
	 * with years that go beyond the range of an <code>int</code>.
	 * Use {@link #getField(DatatypeConstants.Field field) getField(DatatypeConstants.YEARS)} to avoid possible loss of precision.</p>
	 * 
	 * @return If the years field is present, return its value as an <code>int</code>, else return <code>0</code>.
	 */
	public int getYears() {
		return getField(DatatypeConstants.YEARS).intValue();
	}
    
	/**
	 * Obtains the value of the MONTHS field as an integer value,
	 * or 0 if not present.
	 * 
	 * This method works just like {@link #getYears()} except
	 * that this method works on the MONTHS field.
	 * 
	 * @return Months of this <code>Duration</code>.
	 */
	public int getMonths() {
		return getField(DatatypeConstants.MONTHS).intValue();
	}
    
	/**
	 * Obtains the value of the DAYS field as an integer value,
	 * or 0 if not present.
	 * 
	 * This method works just like {@link #getYears()} except
	 * that this method works on the DAYS field.
	 * 
	 * @return Days of this <code>Duration</code>.
	 */
	public int getDays() {
		return getField(DatatypeConstants.DAYS).intValue();
	}
    
	/**
	 * Obtains the value of the HOURS field as an integer value,
	 * or 0 if not present.
	 * 
	 * This method works just like {@link #getYears()} except
	 * that this method works on the HOURS field.
	 * 
	 * @return Hours of this <code>Duration</code>.
	 * 
	 */
	public int getHours() {
		return getField(DatatypeConstants.HOURS).intValue();
	}
    
	/**
	 * Obtains the value of the MINUTES field as an integer value,
	 * or 0 if not present.
	 * 
	 * This method works just like {@link #getYears()} except
	 * that this method works on the MINUTES field.
	 * 
	 * @return Minutes of this <code>Duration</code>.
	 * 
	 */
	public int getMinutes() {
		return getField(DatatypeConstants.MINUTES).intValue();
	}
    
	/**
	 * Obtains the value of the SECONDS field as an integer value,
	 * or 0 if not present.
	 * 
	 * This method works just like {@link #getYears()} except
	 * that this method works on the SECONDS field.
	 * 
	 * @return seconds in the integer value. The fraction of seconds
	 *   will be discarded (for example, if the actual value is 2.5,
	 *   this method returns 2)
	 */
	public int getSeconds() {
		return getField(DatatypeConstants.SECONDS).intValue();
	}
	
	/**
	 * <p>Returns the length of the duration in milli-seconds.</p>
	 * 
	 * <p>If the seconds field carries more digits than milli-second order,
	 * those will be simply discarded (or in other words, rounded to zero.)  
	 * For example, for any Calendar value <code>x<code>,</p>
	 * <pre>
	 * <code>new Duration("PT10.00099S").getTimeInMills(x) == 10000</code>.
	 * <code>new Duration("-PT10.00099S").getTimeInMills(x) == -10000</code>.
	 * </pre>
	 * 
	 * <p>
	 * Note that this method uses the {@link #addTo(Calendar)} method,
	 * which may work incorrectly with <code>Duration</code> objects with
	 * very large values in its fields. See the {@link #addTo(Calendar)}
	 * method for details.
	 * 
	 * @param startInstant
	 *      The length of a month/year varies. The <code>startInstant</code> is
	 *      used to disambiguate this variance. Specifically, this method
	 *      returns the difference between <code>startInstant</code> and
	 *      <code>startInstant+duration</code>
	 * 
	 * @return milliseconds between <code>startInstant</code> and
	 *   <code>startInstant</code> plus this <code>Duration</code>
	 *
	 * @throws NullPointerException if <code>startInstant</code> parameter 
	 * is null.
	 * 
	 */
	public long getTimeInMillis(final Calendar startInstant) {
		Calendar cal = (Calendar) startInstant.clone();
		addTo(cal);
		return getCalendarTimeInMillis(cal)
					- getCalendarTimeInMillis(startInstant);
	}
    
	/**
	 * <p>Returns the length of the duration in milli-seconds.</p>
	 * 
	 * <p>If the seconds field carries more digits than milli-second order,
	 * those will be simply discarded (or in other words, rounded to zero.)
	 * For example, for any <code>Date</code> value <code>x<code>,</p>   
	 * <pre>
	 * <code>new Duration("PT10.00099S").getTimeInMills(x) == 10000</code>.
	 * <code>new Duration("-PT10.00099S").getTimeInMills(x) == -10000</code>.
	 * </pre>
	 * 
	 * <p>
	 * Note that this method uses the {@link #addTo(Date)} method,
	 * which may work incorrectly with <code>Duration</code> objects with
	 * very large values in its fields. See the {@link #addTo(Date)}
	 * method for details.
	 * 
	 * @param startInstant
	 *      The length of a month/year varies. The <code>startInstant</code> is
	 *      used to disambiguate this variance. Specifically, this method
	 *      returns the difference between <code>startInstant</code> and
	 *      <code>startInstant+duration</code>.
	 * 
	 * @throws NullPointerException
	 *      If the startInstant parameter is null.
	 * 
	 * @return milliseconds between <code>startInstant</code> and
	 *   <code>startInstant</code> plus this <code>Duration</code>
	 *
	 * @see #getTimeInMillis(Calendar)
	 */
	public long getTimeInMillis(final Date startInstant) {
		Calendar cal = new GregorianCalendar();
		cal.setTime(startInstant);
		this.addTo(cal);
		return getCalendarTimeInMillis(cal) - startInstant.getTime();
	}
	
	/**
	 * Gets the value of a field. 
	 * 
	 * Fields of a duration object may contain arbitrary large value.
	 * Therefore this method is designed to return a {@link Number} object.
	 * 
	 * In case of YEARS, MONTHS, DAYS, HOURS, and MINUTES, the returned
	 * number will be a non-negative integer. In case of seconds,
	 * the returned number may be a non-negative decimal value.
	 * 
	 * @param field
	 *      one of the six Field constants (YEARS,MONTHS,DAYS,HOURS,
	 *      MINUTES, or SECONDS.)
	 * @return
	 *      If the specified field is present, this method returns
	 *      a non-null non-negative {@link Number} object that
	 *      represents its value. If it is not present, return null.
	 *      For YEARS, MONTHS, DAYS, HOURS, and MINUTES, this method
	 *      returns a {@link java.math.BigInteger} object. For SECONDS, this
	 *      method returns a {@link java.math.BigDecimal}. 
	 * 
	 * @throws NullPointerException If the <code>field</code> is <code>null</code>.
	 */
	public abstract Number getField(final DatatypeConstants.Field field);
	
	/**
	 * Checks if a field is set.
	 * 
	 * A field of a duration object may or may not be present.
	 * This method can be used to test if a field is present.
	 * 
	 * @param field
	 *      one of the six Field constants (YEARS,MONTHS,DAYS,HOURS,
	 *      MINUTES, or SECONDS.)
	 * @return
	 *      true if the field is present. false if not.
	 * 
	 * @throws NullPointerException
	 *      If the field parameter is null.
	 */
	public abstract boolean isSet(final DatatypeConstants.Field field);
	
	/**
	 * <p>Computes a new duration whose value is <code>this+rhs</code>.</p>
	 * 
	 * <p>For example,</p>
	 * <pre>
	 * "1 day" + "-3 days" = "-2 days"
	 * "1 year" + "1 day" = "1 year and 1 day"
	 * "-(1 hour,50 minutes)" + "-20 minutes" = "-(1 hours,70 minutes)"
	 * "15 hours" + "-3 days" = "-(2 days,9 hours)"
	 * "1 year" + "-1 day" = IllegalStateException
	 * </pre>
	 * 
	 * <p>Since there's no way to meaningfully subtract 1 day from 1 month,
	 * there are cases where the operation fails in
	 * {@link IllegalStateException}.</p> 
	 * 
	 * <p>
	 * Formally, the computation is defined as follows.</p>
	 * <p>
	 * Firstly, we can assume that two <code>Duration</code>s to be added
	 * are both positive without losing generality (i.e.,
	 * <code>(-X)+Y=Y-X</code>, <code>X+(-Y)=X-Y</code>,
	 * <code>(-X)+(-Y)=-(X+Y)</code>)
	 * 
	 * <p>
	 * Addition of two positive <code>Duration</code>s are simply defined as  
	 * field by field addition where missing fields are treated as 0.
	 * <p>
	 * A field of the resulting <code>Duration</code> will be unset if and
	 * only if respective fields of two input <code>Duration</code>s are unset. 
	 * <p>
	 * Note that <code>lhs.add(rhs)</code> will be always successful if
	 * <code>lhs.signum()*rhs.signum()!=-1</code> or both of them are
	 * normalized.</p>
	 * 
	 * @param rhs <code>Duration</code> to add to this <code>Duration</code>
	 * 
	 * @return
	 *      non-null valid Duration object.
	 * 
	 * @throws NullPointerException
	 *      If the rhs parameter is null.
	 * @throws IllegalStateException
	 *      If two durations cannot be meaningfully added. For
	 *      example, adding negative one day to one month causes
	 *      this exception.
	 * 
	 * 
	 * @see #subtract(Duration)
	 */
	public abstract Duration add(final Duration rhs);
	
	/**
	 * Adds this duration to a {@link Calendar} object.
	 * 
	 * <p>
	 * Calls {@link java.util.Calendar#add(int,int)} in the
	 * order of YEARS, MONTHS, DAYS, HOURS, MINUTES, SECONDS, and MILLISECONDS
	 * if those fields are present. Because the {@link Calendar} class
	 * uses int to hold values, there are cases where this method
	 * won't work correctly (for example if values of fields
	 * exceed the range of int.) 
	 * </p>
	 * 
	 * <p>
	 * Also, since this duration class is a Gregorian duration, this
	 * method will not work correctly if the given {@link Calendar}
	 * object is based on some other calendar systems. 
	 * </p>
	 * 
	 * <p>
	 * Any fractional parts of this <code>Duration</code> object
	 * beyond milliseconds will be simply ignored. For example, if
	 * this duration is "P1.23456S", then 1 is added to SECONDS,
	 * 234 is added to MILLISECONDS, and the rest will be unused. 
	 * </p>
	 * 
	 * <p>
	 * Note that because {@link Calendar#add(int, int)} is using
	 * <tt>int</tt>, <code>Duration</code> with values beyond the
	 * range of <tt>int</tt> in its fields
	 * will cause overflow/underflow to the given {@link Calendar}.
	 * {@link XMLGregorianCalendar#add(Duration)} provides the same
	 * basic operation as this method while avoiding
	 * the overflow/underflow issues.
	 * 
	 * @param calendar
	 *      A calendar object whose value will be modified.
	 * @throws NullPointerException
	 *      if the calendar parameter is null.
	 */
	public abstract void addTo(Calendar calendar);
    
	/**
	 * Adds this duration to a {@link Date} object.
	 * 
	 * <p>
	 * The given date is first converted into
	 * a {@link java.util.GregorianCalendar}, then the duration
	 * is added exactly like the {@link #addTo(Calendar)} method.
	 * 
	 * <p>
	 * The updated time instant is then converted back into a
	 * {@link Date} object and used to update the given {@link Date} object.
	 * 
	 * <p>
	 * This somewhat redundant computation is necessary to unambiguously
	 * determine the duration of months and years.
	 * 
	 * @param date
	 *      A date object whose value will be modified.
	 * @throws NullPointerException
	 *      if the date parameter is null.
	 */
	public void addTo(Date date) {
		
		// check data parameter
		if (date == null) {
			throw new NullPointerException(
				"Cannot call "
				+ this.getClass().getName()
				+ "#addTo(Date date) with date == null."
			);
		}
		
		Calendar cal = new GregorianCalendar();
		cal.setTime(date); 
		this.addTo(cal);
		date.setTime(getCalendarTimeInMillis(cal));
	}
	
	/**
	 * <p>Computes a new duration whose value is <code>this-rhs</code>.</p>
	 * 
	 * <p>For example:</p>
	 * <pre>
	 * "1 day" - "-3 days" = "4 days"
	 * "1 year" - "1 day" = IllegalStateException
	 * "-(1 hour,50 minutes)" - "-20 minutes" = "-(1hours,30 minutes)"
	 * "15 hours" - "-3 days" = "3 days and 15 hours"
	 * "1 year" - "-1 day" = "1 year and 1 day"
	 * </pre>
	 * 
	 * <p>Since there's no way to meaningfully subtract 1 day from 1 month,
	 * there are cases where the operation fails in {@link IllegalStateException}.</p> 
	 * 
	 * <p>Formally the computation is defined as follows.
	 * First, we can assume that two <code>Duration</code>s are both positive
	 * without losing generality.  (i.e.,
	 * <code>(-X)-Y=-(X+Y)</code>, <code>X-(-Y)=X+Y</code>,
	 * <code>(-X)-(-Y)=-(X-Y)</code>)</p>
	 *  
	 * <p>Then two durations are subtracted field by field.
	 * If the sign of any non-zero field <tt>F</tt> is different from
	 * the sign of the most significant field,
	 * 1 (if <tt>F</tt> is negative) or -1 (otherwise)
	 * will be borrowed from the next bigger unit of <tt>F</tt>.</p>
	 * 
	 * <p>This process is repeated until all the non-zero fields have
	 * the same sign.</p> 
	 * 
	 * <p>If a borrow occurs in the days field (in other words, if
	 * the computation needs to borrow 1 or -1 month to compensate
	 * days), then the computation fails by throwing an
	 * {@link IllegalStateException}.</p>
	 * 
	 * @param rhs <code>Duration</code> to subtract from this <code>Duration</code>.
	 *  
	 * @return New <code>Duration</code> created from subtracting <code>rhs</code> from this <code>Duration</code>.
	 * 
	 * @throws IllegalStateException
	 *      If two durations cannot be meaningfully subtracted. For
	 *      example, subtracting one day from one month causes
	 *      this exception.
	 * 
	 * @throws NullPointerException
	 *      If the rhs parameter is null.
	 * 
	 * @see #add(Duration)
	 */
	public Duration subtract(final Duration rhs) {
		return add(rhs.negate());
	}
	
	/**
	 * <p>Computes a new duration whose value is <code>factor</code> times
	 * longer than the value of this duration.</p>
	 * 
	 * <p>This method is provided for the convenience.
	 * It is functionally equivalent to the following code:</p>
	 * <pre>
	 * multiply(new BigDecimal(String.valueOf(factor)))
	 * </pre>
	 * 
	 * @param factor Factor times longer of new <code>Duration</code> to create.
	 * 
	 * @return New <code>Duration</code> that is <code>factor</code>times longer than this <code>Duration</code>.
	 * 
	 * @see #multiply(BigDecimal)
	 */
	public Duration multiply(int factor) {
		return multiply(new BigDecimal(String.valueOf(factor)));
	}
    
	/**
	 * Computes a new duration whose value is <code>factor</code> times
	 * longer than the value of this duration.
	 * 
	 * <p>
	 * For example,
	 * <pre>
	 * "P1M" (1 month) * "12" = "P12M" (12 months)
	 * "PT1M" (1 min) * "0.3" = "PT18S" (18 seconds)
	 * "P1M" (1 month) * "1.5" = IllegalStateException
	 * </pre>
	 *  
	 * <p>
	 * Since the <code>Duration</code> class is immutable, this method
	 * doesn't change the value of this object. It simply computes
	 * a new Duration object and returns it.
	 * 
	 * <p>
	 * The operation will be performed field by field with the precision
	 * of {@link BigDecimal}. Since all the fields except seconds are
	 * restricted to hold integers,
	 * any fraction produced by the computation will be
	 * carried down toward the next lower unit. For example,
	 * if you multiply "P1D" (1 day) with "0.5", then it will be 0.5 day,
	 * which will be carried down to "PT12H" (12 hours).
	 * When fractions of month cannot be meaningfully carried down
	 * to days, or year to months, this will cause an
	 * {@link IllegalStateException} to be thrown. 
	 * For example if you multiple one month by 0.5.</p>
	 * 
	 * <p>
	 * To avoid {@link IllegalStateException}, use
	 * the {@link #normalizeWith(Calendar)} method to remove the years
	 * and months fields.
	 * 
	 * @param factor to multiply by
	 * 
	 * @return
	 *      returns a non-null valid <code>Duration</code> object
	 *
	 * @throws IllegalStateException if operation produces fraction in 
	 * the months field.
	 *
	 * @throws NullPointerException if the <code>factor</code> parameter is 
	 * <code>null</code>.
	 *
	 */
	public abstract Duration multiply(final BigDecimal factor);
	
	/**
	 * Returns a new <code>Duration</code> object whose
	 * value is <code>-this</code>.
	 * 
	 * <p>
	 * Since the <code>Duration</code> class is immutable, this method
	 * doesn't change the value of this object. It simply computes
	 * a new Duration object and returns it.
	 * 
	 * @return
	 *      always return a non-null valid <code>Duration</code> object.
	 */
	public abstract Duration negate();	
	
	/**
	 * <p>Converts the years and months fields into the days field
	 * by using a specific time instant as the reference point.</p>
	 * 
	 * <p>For example, duration of one month normalizes to 31 days
	 * given the start time instance "July 8th 2003, 17:40:32".</p>
	 * 
	 * <p>Formally, the computation is done as follows:</p>
	 * <ol>
	 *  <li>the given Calendar object is cloned</li>
	 *  <li>the years, months and days fields will be added to the {@link Calendar} object
	 *      by using the {@link Calendar#add(int,int)} method</li> 
	 *  <li>the difference between the two Calendars in computed in milliseconds and converted to days,
	 *     if a remainder occurs due to Daylight Savings Time, it is discarded</li>
	 *  <li>the computed days, along with the hours, minutes and seconds
	 *      fields of this duration object is used to construct a new
	 *      Duration object.</li>
	 * </ol>
	 * 
	 * <p>Note that since the Calendar class uses <code>int</code> to
	 * hold the value of year and month, this method may produce
	 * an unexpected result if this duration object holds
	 * a very large value in the years or months fields.</p>
	 *
	 * @param startTimeInstant <code>Calendar</code> reference point.
	 *  
	 * @return <code>Duration</code> of years and months of this <code>Duration</code> as days.
	 * 
	 * @throws NullPointerException If the startTimeInstant parameter is null.
	 */
	public abstract Duration normalizeWith(final Calendar startTimeInstant);
	
	/**
	 * <p>Partial order relation comparison with this <code>Duration</code> instance.</p>
	 * 
	 * <p>Comparison result must be in accordance with
	 * <a href="http://www.w3.org/TR/xmlschema-2/#duration-order">W3C XML Schema 1.0 Part 2, Section 3.2.7.6.2,
	 * <i>Order relation on duration</i></a>.</p>
	 * 
	 * <p>Return:</p>
	 * <ul>
	 *   <li>{@link DatatypeConstants#LESSER} if this <code>Duration</code> is shorter than <code>duration</code> parameter</li>
	 *   <li>{@link DatatypeConstants#EQUAL} if this <code>Duration</code> is equal to <code>duration</code> parameter</li>
	 *   <li>{@link DatatypeConstants#GREATER} if this <code>Duration</code> is longer than <code>duration</code> parameter</li>
	 *   <li>{@link DatatypeConstants#INDETERMINATE} if a conclusive partial order relation cannot be determined</li>
	 * </ul>
	 *
	 * @param duration to compare
	 * 
	 * @return the relationship between <code>this</code> <code>Duration</code>and <code>duration</code> parameter as
	 *   {@link DatatypeConstants#LESSER}, {@link DatatypeConstants#EQUAL}, {@link DatatypeConstants#GREATER}
	 *   or {@link DatatypeConstants#INDETERMINATE}.
	 * 
	 * @throws UnsupportedOperationException If the underlying implementation
	 *   cannot reasonably process the request, e.g. W3C XML Schema allows for
	 *   arbitrarily large/small/precise values, the request may be beyond the
	 *   implementations capability.
	 * @throws NullPointerException if <code>duration</code> is <code>null</code>. 
	 *
	 * @see #isShorterThan(Duration)
	 * @see #isLongerThan(Duration)
	 */
	public abstract int compare(final Duration duration);
	
	/**
	 * <p>Checks if this duration object is strictly longer than
	 * another <code>Duration</code> object.</p>
	 * 
	 * <p>Duration X is "longer" than Y if and only if X>Y 
	 * as defined in the section 3.2.6.2 of the XML Schema 1.0
	 * specification.</p>
	 * 
	 * <p>For example, "P1D" (one day) > "PT12H" (12 hours) and
	 * "P2Y" (two years) > "P23M" (23 months).</p> 
	 * 
	 * @param duration <code>Duration</code> to test this <code>Duration</code> against.
	 * 
	 * @throws UnsupportedOperationException If the underlying implementation
	 *   cannot reasonably process the request, e.g. W3C XML Schema allows for
	 *   arbitrarily large/small/precise values, the request may be beyond the
	 *   implementations capability.
	 * @throws NullPointerException If <code>duration</code> is null.
	 * 
	 * @return
	 *      true if the duration represented by this object
	 *      is longer than the given duration. false otherwise.
	 * 
	 * @see #isShorterThan(Duration)
	 * @see #compare(Duration duration)
	 */
	public boolean isLongerThan(final Duration duration) {
		return compare(duration) == DatatypeConstants.GREATER;
	}
    
	/**
	 * <p>Checks if this duration object is strictly shorter than
	 * another <code>Duration</code> object.</p>
	 * 
	 * @param duration <code>Duration</code> to test this <code>Duration</code> against.
	 * 
	 * @return <code>true</code> if <code>duration</code> parameter is shorter than this <code>Duration</code>,
	 *   else <code>false</code>. 
	 * 
	 * @throws UnsupportedOperationException If the underlying implementation
	 *   cannot reasonably process the request, e.g. W3C XML Schema allows for
	 *   arbitrarily large/small/precise values, the request may be beyond the
	 *   implementations capability.
	 * @throws NullPointerException if <code>duration</code> is null.
	 *
	 * @see #isLongerThan(Duration duration)
	 * @see #compare(Duration duration)
	 */
	public boolean isShorterThan(final Duration duration) {
		return compare(duration) == DatatypeConstants.LESSER;
	}
	
	/**
	 * <p>Checks if this duration object has the same duration
	 * as another <code>Duration</code> object.</p>
	 * 
	 * <p>For example, "P1D" (1 day) is equal to "PT24H" (24 hours).</p>
	 * 
	 * <p>Duration X is equal to Y if and only if time instant
	 * t+X and t+Y are the same for all the test time instants
	 * specified in the section 3.2.6.2 of the XML Schema 1.0 
	 * specification.</p>
	 * 
	 * <p>Note that there are cases where two <code>Duration</code>s are
	 * "incomparable" to each other, like one month and 30 days.
	 * For example,</p>
	 * <pre>
	 * !new Duration("P1M").isShorterThan(new Duration("P30D"))
	 * !new Duration("P1M").isLongerThan(new Duration("P30D"))
	 * !new Duration("P1M").equals(new Duration("P30D"))
	 * </pre>
	 * 
	 * @param duration
	 *      A non-null valid <code>Duration</code> object.
	 * 
	 * @return
	 *      <code>true</code> if this duration is the same length as
	 *         <code>duration</code>.
	 *      <code>false</code> if <code>duration</code> is not a
	 *         <code>Duration</code> object
	 *         or its length is different from this duration.
	 * 
	 * @throws UnsupportedOperationException If the underlying implementation
	 *   cannot reasonably process the request, e.g. W3C XML Schema allows for
	 *   arbitrarily large/small/precise values, the request may be beyond the
	 *   implementations capability.
	 * @throws NullPointerException if parameter is null.
	 *
	 * @see #compare(Duration duration)
	 */
	public boolean equals(final Object duration) {
		if (duration == null) {
			throw new NullPointerException();
		}

		if (!(duration instanceof Duration)) {
			return false;
		}

		return compare((Duration) duration) == DatatypeConstants.EQUAL;
	}
    
	/**
	 * Returns a hash code consistent with the definition of the equals method.
	 * 
	 * @see Object#hashCode() 
	 */
	public abstract int hashCode();
    
	/**
	 * <p>Returns a <code>String</code> representation of this <code>Duration</code> <code>Object</code>.</p>
	 * 
	 * <p>The result is formatted according to the XML Schema 1.0 spec and can be always parsed back later into the
	 * equivalent <code>Duration</code> <code>Object</code> by {@link DatatypeFactory#newDuration(String  lexicalRepresentation)}.</p>
	 * 
	 * <p>Formally, the following holds for any <code>Duration</code>
	 * <code>Object</code> x:</p> 
	 * <pre>
	 * new Duration(x.toString()).equals(x)
	 * </pre>
	 * 
	 * @return A non-<code>null</code> valid <code>String</code> representation of this <code>Duration</code>.
	 */
	public String toString() {
		
        StringBuffer buf = new StringBuffer();
        
        if (getSign() < 0) {
            buf.append('-');
        }
        buf.append('P');
        
        BigInteger years = (BigInteger) getField(DatatypeConstants.YEARS);
        if (years != null) {
            buf.append(years + "Y");
        }
        
        BigInteger months = (BigInteger) getField(DatatypeConstants.MONTHS);
        if (months != null) {
            buf.append(months + "M");
        }
        
        BigInteger days = (BigInteger) getField(DatatypeConstants.DAYS);
        if (days != null) {
            buf.append(days + "D");
        }

        BigInteger hours = (BigInteger) getField(DatatypeConstants.HOURS);
        BigInteger minutes = (BigInteger) getField(DatatypeConstants.MINUTES);
        BigDecimal seconds = (BigDecimal) getField(DatatypeConstants.SECONDS);
        if (hours != null || minutes != null || seconds != null) {
            buf.append('T');
            if (hours != null) {
                buf.append(hours + "H");
            }
            if (minutes != null) {
                buf.append(minutes + "M");
            }
            if (seconds != null) {
                buf.append(toString(seconds) + "S");
            }
        }
        
        return buf.toString();
	}
	
    /**
     * <p>Turns {@link BigDecimal} to a string representation.</p>
     * 
     * <p>Due to a behavior change in the {@link BigDecimal#toString()}
     * method in JDK 5, this had to be implemented here.</p>
     * 
     * @param bd <code>BigDecimal</code> to format as a <code>String</code>
     * 
     * @return  <code>String</code> representation of <code>BigDecimal</code> 
     */
    private String toString(BigDecimal bd) {
        String intString = bd.unscaledValue().toString();
        int scale = bd.scale();

        if (scale == 0) {
            return intString;
        }

        /* Insert decimal point */
        StringBuffer buf;
        int insertionPoint = intString.length() - scale;
        if (insertionPoint == 0) { /* Point goes right before intVal */
            return "0." + intString;
        } else if (insertionPoint > 0) { /* Point goes inside intVal */
            buf = new StringBuffer(intString);
            buf.insert(insertionPoint, '.');
        } else { /* We must insert zeros between point and intVal */
            buf = new StringBuffer(3 - insertionPoint + intString.length());
            buf.append("0.");
            for (int i = 0; i < -insertionPoint; i++) {
                buf.append('0');
            }
            buf.append(intString);
        }
        return buf.toString();
    }
	
	    
    /**
     * <p>Calls the {@link Calendar#getTimeInMillis} method.
     * Prior to JDK1.4, this method was protected and therefore
     * cannot be invoked directly.</p>
     * 
     * <p>TODO: In future, this should be replaced by <code>cal.getTimeInMillis()</code>.</p>
     * 
     * @param cal <code>Calendar</code> to get time in milliseconds.
     * 
     * @return Milliseconds of <code>cal</code>.
     */
    private static long getCalendarTimeInMillis(final Calendar cal) {
        return cal.getTime().getTime();
    }
}

