118 lines
4.3 KiB
Java
118 lines
4.3 KiB
Java
/* ====================================================================
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* Copyright (c) 2006 J.T. Beetstra
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*
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* Permission is hereby granted, free of charge, to any person obtaining
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* a copy of this software and associated documentation files (the
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* "Software"), to deal in the Software without restriction, including
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* without limitation the rights to use, copy, modify, merge, publish,
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* distribute, sublicense, and/or sell copies of the Software, and to
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* permit persons to whom the Software is furnished to do so, subject to
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* the following conditions:
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*
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* The above copyright notice and this permission notice shall be
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* included in all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
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* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
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* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
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* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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* ====================================================================
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*/
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package com.beetstra.jutf7;
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import java.util.Arrays;
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/**
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* <p>
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* Represent a base 64 mapping. The 64 characters used in the encoding can be
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* specified, since modified-UTF-7 uses other characters than UTF-7 (',' instead
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* of '/').
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* </p>
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* <p>
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* The exact type of the arguments and result values is adapted to the needs of
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* the encoder and decoder, as opposed to following a strict interpretation of
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* base 64.
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* </p>
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* <p>
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* Base 64, as specified in RFC 2045, is an encoding used to encode bytes as
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* characters. In (modified-)UTF-7 however, it is used to encode characters as
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* bytes, using some intermediate steps:
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* </p>
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* <ol>
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* <li>Encode all characters as a 16-bit (UTF-16) integer value</li>
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* <li>Write this as stream of bytes (most-significant first)</li>
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* <li>Encode these bytes using (modified) base 64 encoding</li>
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* <li>Write the thus formed stream of characters as a stream of bytes, using
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* ASCII encoding</li>
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* </ol>
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*
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* @author Jaap Beetstra
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*/
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class Base64Util {
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private static final int ALPHABET_LENGTH = 64;
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private final char[] alphabet;
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private final int[] inverseAlphabet;
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/**
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* Initializes the class with the specified encoding/decoding alphabet.
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*
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* @param alphabet
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* @throws IllegalArgumentException if alphabet is not 64 characters long or
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* contains characters which are not 7-bit ASCII
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*/
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Base64Util(final String alphabet) {
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this.alphabet = alphabet.toCharArray();
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if (alphabet.length() != ALPHABET_LENGTH)
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throw new IllegalArgumentException("alphabet has incorrect length (should be 64, not "
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+ alphabet.length() + ")");
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inverseAlphabet = new int[128];
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Arrays.fill(inverseAlphabet, -1);
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for (int i = 0; i < this.alphabet.length; i++) {
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final char ch = this.alphabet[i];
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if (ch >= 128)
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throw new IllegalArgumentException("invalid character in alphabet: " + ch);
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inverseAlphabet[ch] = i;
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}
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}
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/**
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* Returns the integer value of the six bits represented by the specified
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* character.
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*
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* @param ch The character, as a ASCII encoded byte
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* @return The six bits, as an integer value, or -1 if the byte is not in
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* the alphabet
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*/
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int getSextet(final byte ch) {
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if (ch >= 128)
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return -1;
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return inverseAlphabet[ch];
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}
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/**
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* Tells whether the alphabet contains the specified character.
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*
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* @param ch The character
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* @return true if the alphabet contains <code>ch</code>, false otherwise
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*/
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boolean contains(final char ch) {
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if (ch >= 128)
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return false;
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return inverseAlphabet[ch] >= 0;
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}
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/**
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* Encodes the six bit group as a character.
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*
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* @param sextet The six bit group to be encoded
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* @return The ASCII value of the character
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*/
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byte getChar(final int sextet) {
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return (byte)alphabet[sextet];
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}
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}
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