/* ====================================================================
* 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];
}
}