1/*
2 * Copyright (C) 2014 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package libcore.util;
18
19/**
20 * Hexadecimal encoding where each byte is represented by two hexadecimal digits.
21 */
22public class HexEncoding {
23
24    /** Hidden constructor to prevent instantiation. */
25    private HexEncoding() {}
26
27    private static final char[] HEX_DIGITS = "0123456789ABCDEF".toCharArray();
28
29    /**
30     * Encodes the provided data as a sequence of hexadecimal characters.
31     */
32    public static char[] encode(byte[] data) {
33        return encode(data, 0, data.length);
34    }
35
36    /**
37     * Encodes the provided data as a sequence of hexadecimal characters.
38     */
39    public static char[] encode(byte[] data, int offset, int len) {
40        char[] result = new char[len * 2];
41        for (int i = 0; i < len; i++) {
42            byte b = data[offset + i];
43            int resultIndex = 2 * i;
44            result[resultIndex] = (HEX_DIGITS[(b >>> 4) & 0x0f]);
45            result[resultIndex + 1] = (HEX_DIGITS[b & 0x0f]);
46        }
47
48        return result;
49    }
50
51    /**
52     * Decodes the provided hexadecimal string into a byte array. If {@code allowSingleChar}
53     * is {@code true} odd-length inputs are allowed and the first character is interpreted
54     * as the lower bits of the first result byte.
55     *
56     * Throws an {@code IllegalArgumentException} if the input is malformed.
57     */
58    public static byte[] decode(char[] encoded, boolean allowSingleChar) throws IllegalArgumentException {
59        int resultLengthBytes = (encoded.length + 1) / 2;
60        byte[] result = new byte[resultLengthBytes];
61
62        int resultOffset = 0;
63        int i = 0;
64        if (allowSingleChar) {
65            if ((encoded.length % 2) != 0) {
66                // Odd number of digits -- the first digit is the lower 4 bits of the first result byte.
67                result[resultOffset++] = (byte) toDigit(encoded, i);
68                i++;
69            }
70        } else {
71            if ((encoded.length % 2) != 0) {
72                throw new IllegalArgumentException("Invalid input length: " + encoded.length);
73            }
74        }
75
76        for (int len = encoded.length; i < len; i += 2) {
77            result[resultOffset++] = (byte) ((toDigit(encoded, i) << 4) | toDigit(encoded, i + 1));
78        }
79
80        return result;
81    }
82
83    private static int toDigit(char[] str, int offset) throws IllegalArgumentException {
84        // NOTE: that this isn't really a code point in the traditional sense, since we're
85        // just rejecting surrogate pairs outright.
86        int pseudoCodePoint = str[offset];
87
88        if ('0' <= pseudoCodePoint && pseudoCodePoint <= '9') {
89            return pseudoCodePoint - '0';
90        } else if ('a' <= pseudoCodePoint && pseudoCodePoint <= 'f') {
91            return 10 + (pseudoCodePoint - 'a');
92        } else if ('A' <= pseudoCodePoint && pseudoCodePoint <= 'F') {
93            return 10 + (pseudoCodePoint - 'A');
94        }
95
96        throw new IllegalArgumentException("Illegal char: " + str[offset] +
97                " at offset " + offset);
98    }
99}
100