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The Apache Commons Codec package contains simple encoder and decoders for various formats such as Base64 and Hexadecimal. In addition to these widely used encoders and decoders, the codec package also maintains a collection of phonetic encoding utilities.

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/*
 *  ====================================================================
 *    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.poi.hwpf.converter;

import java.util.Locale;

import org.apache.poi.util.Beta;

/**
 * Utility class to translate numbers in letters, usually for lists.
 * 
 * @author Sergey Vladimirov (vlsergey {at} gmail {dot} com)
 */
@Beta
public final class NumberFormatter {
    private static final String[] ROMAN_LETTERS = { "m", "cm", "d", "cd", "c",
            "xc", "l", "xl", "x", "ix", "v", "iv", "i" };

    private static final int[] ROMAN_VALUES = { 1000, 900, 500, 400, 100, 90,
            50, 40, 10, 9, 5, 4, 1 };

    private static final int T_ARABIC = 0;
    private static final int T_LOWER_LETTER = 4;
    private static final int T_LOWER_ROMAN = 2;
    private static final int T_ORDINAL = 5;
    private static final int T_UPPER_LETTER = 3;
    private static final int T_UPPER_ROMAN = 1;

    public static String getNumber( int num, int style )
    {
        switch ( style )
        {
        case T_UPPER_ROMAN:
            return toRoman( num ).toUpperCase(Locale.ROOT);
        case T_LOWER_ROMAN:
            return toRoman( num );
        case T_UPPER_LETTER:
            return toLetters( num ).toUpperCase(Locale.ROOT);
        case T_LOWER_LETTER:
            return toLetters( num );
        case T_ARABIC:
        case T_ORDINAL:
        default:
            return String.valueOf( num );
        }
    }
    
    private static String toLetters(int number) {
        if ( number <= 0 ) {
            throw new IllegalArgumentException( "Unsupported number: " + number );
        }

        int num = number;
        final int radix = 26;

        char[] buf = new char[33];
        int charPos = buf.length;

        while (num > 0) {
            num--; // 1 => a, not 0 => a
            int remainder = num % radix;
            buf[--charPos] = (char)('a'+remainder);
            num = (num - remainder) / radix;
        }

        return new String(buf, charPos, (buf.length - charPos));
    }

    private static String toRoman( int number )
    {
        if ( number <= 0 )
            throw new IllegalArgumentException( "Unsupported number: " + number );

        StringBuilder result = new StringBuilder();

        for ( int i = 0; i < ROMAN_LETTERS.length; i++ )
        {
            String letter = ROMAN_LETTERS[i];
            int value = ROMAN_VALUES[i];
            while ( number >= value )
            {
                number -= value;
                result.append( letter );
            }
        }
        return result.toString();
    }
}