1 /* ====================================================================
2 * Copyright (c) 2006 J.T. Beetstra
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining
5 * a copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sublicense, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
11 *
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
18 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
19 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
20 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
21 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
22 * ====================================================================
23 */
24 package com.beetstra.jutf7;
25
26 import java.util.Arrays;
27
28 /**
29 * <p>Represent a base 64 mapping. The 64 characters used in the encoding can be specified,
30 * since modified-UTF-7 uses other characters than UTF-7 (',' instead of '/').</p>
31 *
32 * <p>The exact type of the arguments and result values is adapted to the needs of the
33 * encoder and decoder, as opposed to following a strict interpretation of base 64.</p>
34 * <p>Base 64, as specified in RFC 2045, is an encoding used to encode bytes as characters.
35 * In (modified-)UTF-7 however, it is used to encode characters as bytes, using some
36 * intermediate steps:</p>
37 * <ol>
38 * <li>Encode all characters as a 16-bit (UTF-16) integer value</li>
39 * <li>Write this as stream of bytes (most-significant first)</li>
40 * <li>Encode these bytes using (modified) base 64 encoding</li>
41 * <li>Write the thus formed stream of characters as a stream of bytes, using ASCII encoding</li>
42 * </ol>
43 *
44 * @author Jaap Beetstra
45 */
46 class Base64Util {
47 private static final int ALPHABET_LENGTH = 64;
48 private final char[] alphabet;
49 private final int[] inverseAlphabet;
50
51 /**
52 * Initializes the class with the specified encoding/decoding alphabet.
53 *
54 * @param alphabet
55 * @throws IllegalArgumentException if alphabet is not 64 characters long or
56 * contains characters which are not 7-bit ASCII
57 */
58 Base64Util(final String alphabet) {
59 this.alphabet = alphabet.toCharArray();
60 if (alphabet.length() != ALPHABET_LENGTH)
61 throw new IllegalArgumentException("alphabet has incorrect length (should be 64, not "
62 + alphabet.length() + ")");
63 inverseAlphabet = new int[128];
64 Arrays.fill(inverseAlphabet, -1);
65 for (int i = 0; i < this.alphabet.length; i++) {
66 final char ch = this.alphabet[i];
67 if (ch >= 128)
68 throw new IllegalArgumentException("invalid character in alphabet: " + ch);
69 inverseAlphabet[ch] = i;
70 }
71 }
72
73 /**
74 * Returns the integer value of the six bits represented by the specified character.
75 *
76 * @param ch The character, as a ASCII encoded byte
77 * @return The six bits, as an integer value, or -1 if the byte is not in the alphabet
78 */
79 int getSextet(final byte ch) {
80 if (ch >= 128)
81 return -1;
82 return inverseAlphabet[ch];
83 }
84
85 /**
86 * Tells whether the alphabet contains the specified character.
87 *
88 * @param ch The character
89 * @return true if the alphabet contains <code>ch</code>, false otherwise
90 */
91 boolean contains(final char ch) {
92 if (ch >= 128)
93 return false;
94 return inverseAlphabet[ch] >= 0;
95 }
96
97 /**
98 * Encodes the six bit group as a character.
99 *
100 * @param sextet The six bit group to be encoded
101 * @return The ASCII value of the character
102 */
103 byte getChar(final int sextet) {
104 return (byte) alphabet[sextet];
105 }
106 }