/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.lang3.time;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.text.DateFormat;
import java.text.DateFormatSymbols;
import java.text.FieldPosition;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.List;
import java.util.Locale;
import java.util.TimeZone;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

/**
 * <p>FastDatePrinter is a fast and thread-safe version of
 * {@link java.text.SimpleDateFormat}.</p>
 *
 * <p>To obtain a FastDatePrinter, use {@link FastDateFormat#getInstance(String, TimeZone, Locale)} 
 * or another variation of the factory methods of {@link FastDateFormat}.</p>
 * 
 * <p>Since FastDatePrinter is thread safe, you can use a static member instance:</p>
 * <code>
 *     private static final DatePrinter DATE_PRINTER = FastDateFormat.getInstance("yyyy-MM-dd");
 * </code>
 * 
 * <p>This class can be used as a direct replacement to
 * {@code SimpleDateFormat} in most formatting situations.
 * This class is especially useful in multi-threaded server environments.
 * {@code SimpleDateFormat} is not thread-safe in any JDK version,
 * nor will it be as Sun have closed the bug/RFE.
 * </p>
 *
 * <p>Only formatting is supported by this class, but all patterns are compatible with
 * SimpleDateFormat (except time zones and some year patterns - see below).</p>
 *
 * <p>Java 1.4 introduced a new pattern letter, {@code 'Z'}, to represent
 * time zones in RFC822 format (eg. {@code +0800} or {@code -1100}).
 * This pattern letter can be used here (on all JDK versions).</p>
 *
 * <p>In addition, the pattern {@code 'ZZ'} has been made to represent
 * ISO 8601 full format time zones (eg. {@code +08:00} or {@code -11:00}).
 * This introduces a minor incompatibility with Java 1.4, but at a gain of
 * useful functionality.</p>
 * 
 * <p>Starting with JDK7, ISO 8601 support was added using the pattern {@code 'X'}.
 * To maintain compatibility, {@code 'ZZ'} will continue to be supported, but using
 * one of the {@code 'X'} formats is recommended.
 *
 * <p>Javadoc cites for the year pattern: <i>For formatting, if the number of
 * pattern letters is 2, the year is truncated to 2 digits; otherwise it is
 * interpreted as a number.</i> Starting with Java 1.7 a pattern of 'Y' or
 * 'YYY' will be formatted as '2003', while it was '03' in former Java
 * versions. FastDatePrinter implements the behavior of Java 7.</p>
 *
 * @version $Id: FastDatePrinter.java 1669774 2015-03-28 13:39:38Z britter $
 * @since 3.2
 * @see FastDateParser
 */
public class FastDatePrinter implements DatePrinter, Serializable {
    // A lot of the speed in this class comes from caching, but some comes
    // from the special int to StringBuffer conversion.
    //
    // The following produces a padded 2 digit number:
    //   buffer.append((char)(value / 10 + '0'));
    //   buffer.append((char)(value % 10 + '0'));
    //
    // Note that the fastest append to StringBuffer is a single char (used here).
    // Note that Integer.toString() is not called, the conversion is simply
    // taking the value and adding (mathematically) the ASCII value for '0'.
    // So, don't change this code! It works and is very fast.

    /**
     * Required for serialization support.
     *
     * @see java.io.Serializable
     */
    private static final long serialVersionUID = 1L;

    /**
     * FULL locale dependent date or time style.
     */
    public static final int FULL = DateFormat.FULL;
    /**
     * LONG locale dependent date or time style.
     */
    public static final int LONG = DateFormat.LONG;
    /**
     * MEDIUM locale dependent date or time style.
     */
    public static final int MEDIUM = DateFormat.MEDIUM;
    /**
     * SHORT locale dependent date or time style.
     */
    public static final int SHORT = DateFormat.SHORT;

    /**
     * The pattern.
     */
    private final String mPattern;
    /**
     * The time zone.
     */
    private final TimeZone mTimeZone;
    /**
     * The locale.
     */
    private final Locale mLocale;
    /**
     * The parsed rules.
     */
    private transient Rule[] mRules;
    /**
     * The estimated maximum length.
     */
    private transient int mMaxLengthEstimate;

    // Constructor
    //-----------------------------------------------------------------------
    /**
     * <p>Constructs a new FastDatePrinter.</p>
     * Use {@link FastDateFormat#getInstance(String, TimeZone, Locale)}  or another variation of the 
     * factory methods of {@link FastDateFormat} to get a cached FastDatePrinter instance.
     *
     * @param pattern  {@link java.text.SimpleDateFormat} compatible pattern
     * @param timeZone  non-null time zone to use
     * @param locale  non-null locale to use
     * @throws NullPointerException if pattern, timeZone, or locale is null.
     */
    protected FastDatePrinter(final String pattern, final TimeZone timeZone, final Locale locale) {
        mPattern = pattern;
        mTimeZone = timeZone;
        mLocale = locale;

        init();
    }

    /**
     * <p>Initializes the instance for first use.</p>
     */
    private void init() {
        final List<Rule> rulesList = parsePattern();
        mRules = rulesList.toArray(new Rule[rulesList.size()]);

        int len = 0;
        for (int i=mRules.length; --i >= 0; ) {
            len += mRules[i].estimateLength();
        }

        mMaxLengthEstimate = len;
    }

    // Parse the pattern
    //-----------------------------------------------------------------------
    /**
     * <p>Returns a list of Rules given a pattern.</p>
     *
     * @return a {@code List} of Rule objects
     * @throws IllegalArgumentException if pattern is invalid
     */
    protected List<Rule> parsePattern() {
        final DateFormatSymbols symbols = new DateFormatSymbols(mLocale);
        final List<Rule> rules = new ArrayList<Rule>();

        final String[] ERAs = symbols.getEras();
        final String[] months = symbols.getMonths();
        final String[] shortMonths = symbols.getShortMonths();
        final String[] weekdays = symbols.getWeekdays();
        final String[] shortWeekdays = symbols.getShortWeekdays();
        final String[] AmPmStrings = symbols.getAmPmStrings();

        final int length = mPattern.length();
        final int[] indexRef = new int[1];

        for (int i = 0; i < length; i++) {
            indexRef[0] = i;
            final String token = parseToken(mPattern, indexRef);
            i = indexRef[0];

            final int tokenLen = token.length();
            if (tokenLen == 0) {
                break;
            }

            Rule rule;
            final char c = token.charAt(0);

            switch (c) {
            case 'G': // era designator (text)
                rule = new TextField(Calendar.ERA, ERAs);
                break;
            case 'y': // year (number)
                if (tokenLen == 2) {
                    rule = TwoDigitYearField.INSTANCE;
                } else {
                    rule = selectNumberRule(Calendar.YEAR, tokenLen < 4 ? 4 : tokenLen);
                }
                break;
            case 'M': // month in year (text and number)
                if (tokenLen >= 4) {
                    rule = new TextField(Calendar.MONTH, months);
                } else if (tokenLen == 3) {
                    rule = new TextField(Calendar.MONTH, shortMonths);
                } else if (tokenLen == 2) {
                    rule = TwoDigitMonthField.INSTANCE;
                } else {
                    rule = UnpaddedMonthField.INSTANCE;
                }
                break;
            case 'd': // day in month (number)
                rule = selectNumberRule(Calendar.DAY_OF_MONTH, tokenLen);
                break;
            case 'h': // hour in am/pm (number, 1..12)
                rule = new TwelveHourField(selectNumberRule(Calendar.HOUR, tokenLen));
                break;
            case 'H': // hour in day (number, 0..23)
                rule = selectNumberRule(Calendar.HOUR_OF_DAY, tokenLen);
                break;
            case 'm': // minute in hour (number)
                rule = selectNumberRule(Calendar.MINUTE, tokenLen);
                break;
            case 's': // second in minute (number)
                rule = selectNumberRule(Calendar.SECOND, tokenLen);
                break;
            case 'S': // millisecond (number)
                rule = selectNumberRule(Calendar.MILLISECOND, tokenLen);
                break;
            case 'E': // day in week (text)
                rule = new TextField(Calendar.DAY_OF_WEEK, tokenLen < 4 ? shortWeekdays : weekdays);
                break;
            case 'D': // day in year (number)
                rule = selectNumberRule(Calendar.DAY_OF_YEAR, tokenLen);
                break;
            case 'F': // day of week in month (number)
                rule = selectNumberRule(Calendar.DAY_OF_WEEK_IN_MONTH, tokenLen);
                break;
            case 'w': // week in year (number)
                rule = selectNumberRule(Calendar.WEEK_OF_YEAR, tokenLen);
                break;
            case 'W': // week in month (number)
                rule = selectNumberRule(Calendar.WEEK_OF_MONTH, tokenLen);
                break;
            case 'a': // am/pm marker (text)
                rule = new TextField(Calendar.AM_PM, AmPmStrings);
                break;
            case 'k': // hour in day (1..24)
                rule = new TwentyFourHourField(selectNumberRule(Calendar.HOUR_OF_DAY, tokenLen));
                break;
            case 'K': // hour in am/pm (0..11)
                rule = selectNumberRule(Calendar.HOUR, tokenLen);
                break;
            case 'X': // ISO 8601 
                rule = Iso8601_Rule.getRule(tokenLen);
                break;    
            case 'z': // time zone (text)
                if (tokenLen >= 4) {
                    rule = new TimeZoneNameRule(mTimeZone, mLocale, TimeZone.LONG);
                } else {
                    rule = new TimeZoneNameRule(mTimeZone, mLocale, TimeZone.SHORT);
                }
                break;
            case 'Z': // time zone (value)
                if (tokenLen == 1) {
                    rule = TimeZoneNumberRule.INSTANCE_NO_COLON;
                } else if (tokenLen == 2) {
                    rule = TimeZoneNumberRule.INSTANCE_ISO_8601;
                } else {
                    rule = TimeZoneNumberRule.INSTANCE_COLON;
                }
                break;
            case '\'': // literal text
                final String sub = token.substring(1);
                if (sub.length() == 1) {
                    rule = new CharacterLiteral(sub.charAt(0));
                } else {
                    rule = new StringLiteral(sub);
                }
                break;
            default:
                throw new IllegalArgumentException("Illegal pattern component: " + token);
            }

            rules.add(rule);
        }

        return rules;
    }

    /**
     * <p>Performs the parsing of tokens.</p>
     *
     * @param pattern  the pattern
     * @param indexRef  index references
     * @return parsed token
     */
    protected String parseToken(final String pattern, final int[] indexRef) {
        final StringBuilder buf = new StringBuilder();

        int i = indexRef[0];
        final int length = pattern.length();

        char c = pattern.charAt(i);
        if (c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z') {
            // Scan a run of the same character, which indicates a time
            // pattern.
            buf.append(c);

            while (i + 1 < length) {
                final char peek = pattern.charAt(i + 1);
                if (peek == c) {
                    buf.append(c);
                    i++;
                } else {
                    break;
                }
            }
        } else {
            // This will identify token as text.
            buf.append('\'');

            boolean inLiteral = false;

            for (; i < length; i++) {
                c = pattern.charAt(i);

                if (c == '\'') {
                    if (i + 1 < length && pattern.charAt(i + 1) == '\'') {
                        // '' is treated as escaped '
                        i++;
                        buf.append(c);
                    } else {
                        inLiteral = !inLiteral;
                    }
                } else if (!inLiteral &&
                         (c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z')) {
                    i--;
                    break;
                } else {
                    buf.append(c);
                }
            }
        }

        indexRef[0] = i;
        return buf.toString();
    }

    /**
     * <p>Gets an appropriate rule for the padding required.</p>
     *
     * @param field  the field to get a rule for
     * @param padding  the padding required
     * @return a new rule with the correct padding
     */
    protected NumberRule selectNumberRule(final int field, final int padding) {
        switch (padding) {
        case 1:
            return new UnpaddedNumberField(field);
        case 2:
            return new TwoDigitNumberField(field);
        default:
            return new PaddedNumberField(field, padding);
        }
    }

    // Format methods
    //-----------------------------------------------------------------------
    /**
     * <p>Formats a {@code Date}, {@code Calendar} or
     * {@code Long} (milliseconds) object.</p>
     *
     * @param obj  the object to format
     * @param toAppendTo  the buffer to append to
     * @param pos  the position - ignored
     * @return the buffer passed in
     */
    @Override
    public StringBuffer format(final Object obj, final StringBuffer toAppendTo, final FieldPosition pos) {
        if (obj instanceof Date) {
            return format((Date) obj, toAppendTo);
        } else if (obj instanceof Calendar) {
            return format((Calendar) obj, toAppendTo);
        } else if (obj instanceof Long) {
            return format(((Long) obj).longValue(), toAppendTo);
        } else {
            throw new IllegalArgumentException("Unknown class: " +
                (obj == null ? "<null>" : obj.getClass().getName()));
        }
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#format(long)
     */
    @Override
    public String format(final long millis) {
        final Calendar c = newCalendar();  // hard code GregorianCalendar
        c.setTimeInMillis(millis);
        return applyRulesToString(c);
    }

    /**
     * Creates a String representation of the given Calendar by applying the rules of this printer to it.
     * @param c the Calender to apply the rules to.
     * @return a String representation of the given Calendar.
     */
    private String applyRulesToString(final Calendar c) {
        return applyRules(c, new StringBuffer(mMaxLengthEstimate)).toString();
    }

    /**
     * Creation method for ne calender instances.
     * @return a new Calendar instance.
     */
    private GregorianCalendar newCalendar() {
        // hard code GregorianCalendar
        return new GregorianCalendar(mTimeZone, mLocale);
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#format(java.util.Date)
     */
    @Override
    public String format(final Date date) {
        final Calendar c = newCalendar();  // hard code GregorianCalendar
        c.setTime(date);
        return applyRulesToString(c);
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#format(java.util.Calendar)
     */
    @Override
    public String format(final Calendar calendar) {
        return format(calendar, new StringBuffer(mMaxLengthEstimate)).toString();
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#format(long, java.lang.StringBuffer)
     */
    @Override
    public StringBuffer format(final long millis, final StringBuffer buf) {
        return format(new Date(millis), buf);
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#format(java.util.Date, java.lang.StringBuffer)
     */
    @Override
    public StringBuffer format(final Date date, final StringBuffer buf) {
        final Calendar c = newCalendar();  // hard code GregorianCalendar
        c.setTime(date);
        return applyRules(c, buf);
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#format(java.util.Calendar, java.lang.StringBuffer)
     */
    @Override
    public StringBuffer format(final Calendar calendar, final StringBuffer buf) {
        return applyRules(calendar, buf);
    }

    /**
     * <p>Performs the formatting by applying the rules to the
     * specified calendar.</p>
     *
     * @param calendar  the calendar to format
     * @param buf  the buffer to format into
     * @return the specified string buffer
     */
    protected StringBuffer applyRules(final Calendar calendar, final StringBuffer buf) {
        for (final Rule rule : mRules) {
            rule.appendTo(buf, calendar);
        }
        return buf;
    }

    // Accessors
    //-----------------------------------------------------------------------
    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#getPattern()
     */
    @Override
    public String getPattern() {
        return mPattern;
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#getTimeZone()
     */
    @Override
    public TimeZone getTimeZone() {
        return mTimeZone;
    }

    /* (non-Javadoc)
     * @see org.apache.commons.lang3.time.DatePrinter#getLocale()
     */
    @Override
    public Locale getLocale() {
        return mLocale;
    }

    /**
     * <p>Gets an estimate for the maximum string length that the
     * formatter will produce.</p>
     *
     * <p>The actual formatted length will almost always be less than or
     * equal to this amount.</p>
     *
     * @return the maximum formatted length
     */
    public int getMaxLengthEstimate() {
        return mMaxLengthEstimate;
    }

    // Basics
    //-----------------------------------------------------------------------
    /**
     * <p>Compares two objects for equality.</p>
     *
     * @param obj  the object to compare to
     * @return {@code true} if equal
     */
    @Override
    public boolean equals(final Object obj) {
        if (obj instanceof FastDatePrinter == false) {
            return false;
        }
        final FastDatePrinter other = (FastDatePrinter) obj;
        return mPattern.equals(other.mPattern)
            && mTimeZone.equals(other.mTimeZone) 
            && mLocale.equals(other.mLocale);
    }

    /**
     * <p>Returns a hashcode compatible with equals.</p>
     *
     * @return a hashcode compatible with equals
     */
    @Override
    public int hashCode() {
        return mPattern.hashCode() + 13 * (mTimeZone.hashCode() + 13 * mLocale.hashCode());
    }

    /**
     * <p>Gets a debugging string version of this formatter.</p>
     *
     * @return a debugging string
     */
    @Override
    public String toString() {
        return "FastDatePrinter[" + mPattern + "," + mLocale + "," + mTimeZone.getID() + "]";
    }

    // Serializing
    //-----------------------------------------------------------------------
    /**
     * Create the object after serialization. This implementation reinitializes the
     * transient properties.
     *
     * @param in ObjectInputStream from which the object is being deserialized.
     * @throws IOException if there is an IO issue.
     * @throws ClassNotFoundException if a class cannot be found.
     */
    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        init();
    }

    /**
     * Appends digits to the given buffer.
     * 
     * @param buffer the buffer to append to.
     * @param value the value to append digits from.
     */
    private static void appendDigits(final StringBuffer buffer, final int value) {
        buffer.append((char)(value / 10 + '0'));
        buffer.append((char)(value % 10 + '0'));
    }

    // Rules
    //-----------------------------------------------------------------------
    /**
     * <p>Inner class defining a rule.</p>
     */
    private interface Rule {
        /**
         * Returns the estimated length of the result.
         *
         * @return the estimated length
         */
        int estimateLength();

        /**
         * Appends the value of the specified calendar to the output buffer based on the rule implementation.
         *
         * @param buffer the output buffer
         * @param calendar calendar to be appended
         */
        void appendTo(StringBuffer buffer, Calendar calendar);
    }

    /**
     * <p>Inner class defining a numeric rule.</p>
     */
    private interface NumberRule extends Rule {
        /**
         * Appends the specified value to the output buffer based on the rule implementation.
         *
         * @param buffer the output buffer
         * @param value the value to be appended
         */
        void appendTo(StringBuffer buffer, int value);
    }

    /**
     * <p>Inner class to output a constant single character.</p>
     */
    private static class CharacterLiteral implements Rule {
        private final char mValue;

        /**
         * Constructs a new instance of {@code CharacterLiteral}
         * to hold the specified value.
         *
         * @param value the character literal
         */
        CharacterLiteral(final char value) {
            mValue = value;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 1;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            buffer.append(mValue);
        }
    }

    /**
     * <p>Inner class to output a constant string.</p>
     */
    private static class StringLiteral implements Rule {
        private final String mValue;

        /**
         * Constructs a new instance of {@code StringLiteral}
         * to hold the specified value.
         *
         * @param value the string literal
         */
        StringLiteral(final String value) {
            mValue = value;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return mValue.length();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            buffer.append(mValue);
        }
    }

    /**
     * <p>Inner class to output one of a set of values.</p>
     */
    private static class TextField implements Rule {
        private final int mField;
        private final String[] mValues;

        /**
         * Constructs an instance of {@code TextField}
         * with the specified field and values.
         *
         * @param field the field
         * @param values the field values
         */
        TextField(final int field, final String[] values) {
            mField = field;
            mValues = values;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            int max = 0;
            for (int i=mValues.length; --i >= 0; ) {
                final int len = mValues[i].length();
                if (len > max) {
                    max = len;
                }
            }
            return max;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            buffer.append(mValues[calendar.get(mField)]);
        }
    }

    /**
     * <p>Inner class to output an unpadded number.</p>
     */
    private static class UnpaddedNumberField implements NumberRule {
        private final int mField;

        /**
         * Constructs an instance of {@code UnpadedNumberField} with the specified field.
         *
         * @param field the field
         */
        UnpaddedNumberField(final int field) {
            mField = field;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 4;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            appendTo(buffer, calendar.get(mField));
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public final void appendTo(final StringBuffer buffer, final int value) {
            if (value < 10) {
                buffer.append((char)(value + '0'));
            } else if (value < 100) {
                appendDigits(buffer, value);
            } else {
                buffer.append(value);
            }
        }
    }

    /**
     * <p>Inner class to output an unpadded month.</p>
     */
    private static class UnpaddedMonthField implements NumberRule {
        static final UnpaddedMonthField INSTANCE = new UnpaddedMonthField();

        /**
         * Constructs an instance of {@code UnpaddedMonthField}.
         *
         */
        UnpaddedMonthField() {
            super();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 2;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            appendTo(buffer, calendar.get(Calendar.MONTH) + 1);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public final void appendTo(final StringBuffer buffer, final int value) {
            if (value < 10) {
                buffer.append((char)(value + '0'));
            } else {
                appendDigits(buffer, value);
            }
        }
    }

    /**
     * <p>Inner class to output a padded number.</p>
     */
    private static class PaddedNumberField implements NumberRule {
        private final int mField;
        private final int mSize;

        /**
         * Constructs an instance of {@code PaddedNumberField}.
         *
         * @param field the field
         * @param size size of the output field
         */
        PaddedNumberField(final int field, final int size) {
            if (size < 3) {
                // Should use UnpaddedNumberField or TwoDigitNumberField.
                throw new IllegalArgumentException();
            }
            mField = field;
            mSize = size;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return mSize;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            appendTo(buffer, calendar.get(mField));
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public final void appendTo(final StringBuffer buffer, int value) {
            // pad the buffer with adequate zeros
            for(int digit = 0; digit<mSize; ++digit) {
                buffer.append('0');                
            }
            // backfill the buffer with non-zero digits
            int index = buffer.length();
            for( ; value>0; value /= 10) {
                buffer.setCharAt(--index, (char)('0' + value % 10));
            }
        }
    }

    /**
     * <p>Inner class to output a two digit number.</p>
     */
    private static class TwoDigitNumberField implements NumberRule {
        private final int mField;

        /**
         * Constructs an instance of {@code TwoDigitNumberField} with the specified field.
         *
         * @param field the field
         */
        TwoDigitNumberField(final int field) {
            mField = field;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 2;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            appendTo(buffer, calendar.get(mField));
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public final void appendTo(final StringBuffer buffer, final int value) {
            if (value < 100) {
                appendDigits(buffer, value);
            } else {
                buffer.append(value);
            }
        }
    }

    /**
     * <p>Inner class to output a two digit year.</p>
     */
    private static class TwoDigitYearField implements NumberRule {
        static final TwoDigitYearField INSTANCE = new TwoDigitYearField();

        /**
         * Constructs an instance of {@code TwoDigitYearField}.
         */
        TwoDigitYearField() {
            super();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 2;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            appendTo(buffer, calendar.get(Calendar.YEAR) % 100);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public final void appendTo(final StringBuffer buffer, final int value) {
            appendDigits(buffer, value);
        }
    }

    /**
     * <p>Inner class to output a two digit month.</p>
     */
    private static class TwoDigitMonthField implements NumberRule {
        static final TwoDigitMonthField INSTANCE = new TwoDigitMonthField();

        /**
         * Constructs an instance of {@code TwoDigitMonthField}.
         */
        TwoDigitMonthField() {
            super();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 2;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            appendTo(buffer, calendar.get(Calendar.MONTH) + 1);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public final void appendTo(final StringBuffer buffer, final int value) {
            appendDigits(buffer, value);
        }
    }

    /**
     * <p>Inner class to output the twelve hour field.</p>
     */
    private static class TwelveHourField implements NumberRule {
        private final NumberRule mRule;

        /**
         * Constructs an instance of {@code TwelveHourField} with the specified
         * {@code NumberRule}.
         *
         * @param rule the rule
         */
        TwelveHourField(final NumberRule rule) {
            mRule = rule;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return mRule.estimateLength();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            int value = calendar.get(Calendar.HOUR);
            if (value == 0) {
                value = calendar.getLeastMaximum(Calendar.HOUR) + 1;
            }
            mRule.appendTo(buffer, value);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final int value) {
            mRule.appendTo(buffer, value);
        }
    }

    /**
     * <p>Inner class to output the twenty four hour field.</p>
     */
    private static class TwentyFourHourField implements NumberRule {
        private final NumberRule mRule;

        /**
         * Constructs an instance of {@code TwentyFourHourField} with the specified
         * {@code NumberRule}.
         *
         * @param rule the rule
         */
        TwentyFourHourField(final NumberRule rule) {
            mRule = rule;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return mRule.estimateLength();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            int value = calendar.get(Calendar.HOUR_OF_DAY);
            if (value == 0) {
                value = calendar.getMaximum(Calendar.HOUR_OF_DAY) + 1;
            }
            mRule.appendTo(buffer, value);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final int value) {
            mRule.appendTo(buffer, value);
        }
    }

    //-----------------------------------------------------------------------

    private static final ConcurrentMap<TimeZoneDisplayKey, String> cTimeZoneDisplayCache =
        new ConcurrentHashMap<TimeZoneDisplayKey, String>(7);
    /**
     * <p>Gets the time zone display name, using a cache for performance.</p>
     *
     * @param tz  the zone to query
     * @param daylight  true if daylight savings
     * @param style  the style to use {@code TimeZone.LONG} or {@code TimeZone.SHORT}
     * @param locale  the locale to use
     * @return the textual name of the time zone
     */
    static String getTimeZoneDisplay(final TimeZone tz, final boolean daylight, final int style, final Locale locale) {
        final TimeZoneDisplayKey key = new TimeZoneDisplayKey(tz, daylight, style, locale);
        String value = cTimeZoneDisplayCache.get(key);
        if (value == null) {
            // This is a very slow call, so cache the results.
            value = tz.getDisplayName(daylight, style, locale);
            final String prior = cTimeZoneDisplayCache.putIfAbsent(key, value);
            if (prior != null) {
                value= prior;
            }
        }
        return value;
    }

    /**
     * <p>Inner class to output a time zone name.</p>
     */
    private static class TimeZoneNameRule implements Rule {
        private final Locale mLocale;
        private final int mStyle;
        private final String mStandard;
        private final String mDaylight;

        /**
         * Constructs an instance of {@code TimeZoneNameRule} with the specified properties.
         *
         * @param timeZone the time zone
         * @param locale the locale
         * @param style the style
         */
        TimeZoneNameRule(final TimeZone timeZone, final Locale locale, final int style) {
            mLocale = locale;
            mStyle = style;
            
            mStandard = getTimeZoneDisplay(timeZone, false, style, locale);
            mDaylight = getTimeZoneDisplay(timeZone, true, style, locale);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            // We have no access to the Calendar object that will be passed to
            // appendTo so base estimate on the TimeZone passed to the
            // constructor
            return Math.max(mStandard.length(), mDaylight.length());
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            final TimeZone zone = calendar.getTimeZone();
            if (calendar.get(Calendar.DST_OFFSET) != 0) {
                buffer.append(getTimeZoneDisplay(zone, true, mStyle, mLocale));
            } else {
                buffer.append(getTimeZoneDisplay(zone, false, mStyle, mLocale));
            }
        }
    }

    /**
     * <p>Inner class to output a time zone as a number {@code +/-HHMM}
     * or {@code +/-HH:MM}.</p>
     */
    private static class TimeZoneNumberRule implements Rule {
        static final TimeZoneNumberRule INSTANCE_COLON = new TimeZoneNumberRule(true, false);
        static final TimeZoneNumberRule INSTANCE_NO_COLON = new TimeZoneNumberRule(false, false);
        static final TimeZoneNumberRule INSTANCE_ISO_8601 = new TimeZoneNumberRule(true, true);
        
        final boolean mColon;
        final boolean mISO8601;

        /**
         * Constructs an instance of {@code TimeZoneNumberRule} with the specified properties.
         *
         * @param colon add colon between HH and MM in the output if {@code true}
         * @param iso8601 create an ISO 8601 format output
         */
        TimeZoneNumberRule(final boolean colon, final boolean iso8601) {
            mColon = colon;
            mISO8601 = iso8601;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return 5;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            if (mISO8601 && calendar.getTimeZone().getID().equals("UTC")) {
                buffer.append("Z");
                return;
            }
            
            int offset = calendar.get(Calendar.ZONE_OFFSET) + calendar.get(Calendar.DST_OFFSET);

            if (offset < 0) {
                buffer.append('-');
                offset = -offset;
            } else {
                buffer.append('+');
            }

            final int hours = offset / (60 * 60 * 1000);
            appendDigits(buffer, hours);

            if (mColon) {
                buffer.append(':');
            }

            final int minutes = offset / (60 * 1000) - 60 * hours;
            appendDigits(buffer, minutes);
        }
    }

    /**
     * <p>Inner class to output a time zone as a number {@code +/-HHMM}
     * or {@code +/-HH:MM}.</p>
     */
    private static class Iso8601_Rule implements Rule {
        
        // Sign TwoDigitHours or Z
        static final Iso8601_Rule ISO8601_HOURS = new Iso8601_Rule(3);       
        // Sign TwoDigitHours Minutes or Z
        static final Iso8601_Rule ISO8601_HOURS_MINUTES = new Iso8601_Rule(5);
        // Sign TwoDigitHours : Minutes or Z
        static final Iso8601_Rule ISO8601_HOURS_COLON_MINUTES = new Iso8601_Rule(6);

        /**
         * Factory method for Iso8601_Rules.
         *
         * @param tokenLen a token indicating the length of the TimeZone String to be formatted.
         * @return a Iso8601_Rule that can format TimeZone String of length {@code tokenLen}. If no such
         *          rule exists, an IllegalArgumentException will be thrown.
         */
        static Iso8601_Rule getRule(int tokenLen) {
            switch(tokenLen) {
            case 1:
                return Iso8601_Rule.ISO8601_HOURS;
            case 2:
                return Iso8601_Rule.ISO8601_HOURS_MINUTES;
            case 3:
                return Iso8601_Rule.ISO8601_HOURS_COLON_MINUTES;
            default:
                throw new IllegalArgumentException("invalid number of X");                    
            }
        }        
        
        final int length;

        /**
         * Constructs an instance of {@code Iso8601_Rule} with the specified properties.
         *
         * @param length The number of characters in output (unless Z is output)
         */
        Iso8601_Rule(final int length) {
            this.length = length;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int estimateLength() {
            return length;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public void appendTo(final StringBuffer buffer, final Calendar calendar) {
            int zoneOffset = calendar.get(Calendar.ZONE_OFFSET);
            if (zoneOffset == 0) {
                buffer.append("Z");
                return;
            }
            
            int offset = zoneOffset + calendar.get(Calendar.DST_OFFSET);

            if (offset < 0) {
                buffer.append('-');
                offset = -offset;
            } else {
                buffer.append('+');
            }

            final int hours = offset / (60 * 60 * 1000);
            appendDigits(buffer, hours);

            if (length<5) {
                return;
            }
            
            if (length==6) {
                buffer.append(':');
            }

            final int minutes = offset / (60 * 1000) - 60 * hours;
            appendDigits(buffer, minutes);
        }
    }

    // ----------------------------------------------------------------------
    /**
     * <p>Inner class that acts as a compound key for time zone names.</p>
     */
    private static class TimeZoneDisplayKey {
        private final TimeZone mTimeZone;
        private final int mStyle;
        private final Locale mLocale;

        /**
         * Constructs an instance of {@code TimeZoneDisplayKey} with the specified properties.
         *
         * @param timeZone the time zone
         * @param daylight adjust the style for daylight saving time if {@code true}
         * @param style the timezone style
         * @param locale the timezone locale
         */
        TimeZoneDisplayKey(final TimeZone timeZone,
                           final boolean daylight, final int style, final Locale locale) {
            mTimeZone = timeZone;
            if (daylight) {
                mStyle = style | 0x80000000;
            } else {
                mStyle = style;
            }
            mLocale = locale;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int hashCode() {
            return (mStyle * 31 + mLocale.hashCode() ) * 31 + mTimeZone.hashCode();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public boolean equals(final Object obj) {
            if (this == obj) {
                return true;
            }
            if (obj instanceof TimeZoneDisplayKey) {
                final TimeZoneDisplayKey other = (TimeZoneDisplayKey)obj;
                return
                    mTimeZone.equals(other.mTimeZone) &&
                    mStyle == other.mStyle &&
                    mLocale.equals(other.mLocale);
            }
            return false;
        }
    }
}