src/com/beetstra/jutf7/Base64Util.java

/* ====================================================================
 * Copyright (c) 2006 J.T. Beetstra
 *
 * Permission is hereby granted, free of charge, to any person obtaining 
 * a copy of this software and associated documentation files (the 
 * "Software"), to deal in the Software without restriction, including 
 * without limitation the rights to use, copy, modify, merge, publish, 
 * distribute, sublicense, and/or sell copies of the Software, and to 
 * permit persons to whom the Software is furnished to do so, subject to 
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be 
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 * ====================================================================
 */

package com.beetstra.jutf7;

import java.util.Arrays;

/**
 * <p>
 * Represent a base 64 mapping. The 64 characters used in the encoding can be
 * specified, since modified-UTF-7 uses other characters than UTF-7 (',' instead
 * of '/').
 * </p>
 * <p>
 * The exact type of the arguments and result values is adapted to the needs of
 * the encoder and decoder, as opposed to following a strict interpretation of
 * base 64.
 * </p>
 * <p>
 * Base 64, as specified in RFC 2045, is an encoding used to encode bytes as
 * characters. In (modified-)UTF-7 however, it is used to encode characters as
 * bytes, using some intermediate steps:
 * </p>
 * <ol>
 * <li>Encode all characters as a 16-bit (UTF-16) integer value</li>
 * <li>Write this as stream of bytes (most-significant first)</li>
 * <li>Encode these bytes using (modified) base 64 encoding</li>
 * <li>Write the thus formed stream of characters as a stream of bytes, using
 * ASCII encoding</li>
 * </ol>
 * 
 * @author Jaap Beetstra
 */
class Base64Util {
    private static final int ALPHABET_LENGTH = 64;
    private final char[] alphabet;
    private final int[] inverseAlphabet;

    /**
     * Initializes the class with the specified encoding/decoding alphabet.
     * 
     * @param alphabet
     * @throws IllegalArgumentException if alphabet is not 64 characters long or
     *             contains characters which are not 7-bit ASCII
     */
    Base64Util(final String alphabet) {
        this.alphabet = alphabet.toCharArray();
        if (alphabet.length() != ALPHABET_LENGTH)
            throw new IllegalArgumentException("alphabet has incorrect length (should be 64, not "
                    + alphabet.length() + ")");
        inverseAlphabet = new int[128];
        Arrays.fill(inverseAlphabet, -1);
        for (int i = 0; i < this.alphabet.length; i++) {
            final char ch = this.alphabet[i];
            if (ch >= 128)
                throw new IllegalArgumentException("invalid character in alphabet: " + ch);
            inverseAlphabet[ch] = i;
        }
    }

    /**
     * Returns the integer value of the six bits represented by the specified
     * character.
     * 
     * @param ch The character, as a ASCII encoded byte
     * @return The six bits, as an integer value, or -1 if the byte is not in
     *         the alphabet
     */
    int getSextet(final byte ch) {
        if (ch >= 128)
            return -1;
        return inverseAlphabet[ch];
    }

    /**
     * Tells whether the alphabet contains the specified character.
     * 
     * @param ch The character
     * @return true if the alphabet contains <code>ch</code>, false otherwise
     */
    boolean contains(final char ch) {
        if (ch >= 128)
            return false;
        return inverseAlphabet[ch] >= 0;
    }

    /**
     * Encodes the six bit group as a character.
     * 
     * @param sextet The six bit group to be encoded
     * @return The ASCII value of the character
     */
    byte getChar(final int sextet) {
        return (byte)alphabet[sextet];
    }
}