1 /*
2  * Copyright (C) 2008 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 
17 package com.android.internal.util;
18 
19 import android.compat.annotation.UnsupportedAppUsage;
20 
21 /**
22  * An object that provides bitwise incremental read access to a byte array.
23  *
24  * This is useful, for example, when accessing a series of fields that
25  * may not be aligned on byte boundaries.
26  *
27  * NOTE -- This class is not threadsafe.
28  */
29 public class BitwiseInputStream {
30 
31     // The byte array being read from.
32     private byte[] mBuf;
33 
34     // The current position offset, in bits, from the msb in byte 0.
35     private int mPos;
36 
37     // The last valid bit offset.
38     private int mEnd;
39 
40     /**
41      * An exception to report access problems.
42      */
43     public static class AccessException extends Exception {
AccessException(String s)44         public AccessException(String s) {
45             super("BitwiseInputStream access failed: " + s);
46         }
47     }
48 
49     /**
50      * Create object from byte array.
51      *
52      * @param buf a byte array containing data
53      */
54     @UnsupportedAppUsage
BitwiseInputStream(byte buf[])55     public BitwiseInputStream(byte buf[]) {
56         mBuf = buf;
57         mEnd = buf.length << 3;
58         mPos = 0;
59     }
60 
61     /**
62      * Return the number of bit still available for reading.
63      */
64     @UnsupportedAppUsage
available()65     public int available() {
66         return mEnd - mPos;
67     }
68 
69     /**
70      * Read some data and increment the current position.
71      *
72      * The 8-bit limit on access to bitwise streams is intentional to
73      * avoid endianness issues.
74      *
75      * @param bits the amount of data to read (gte 0, lte 8)
76      * @return byte of read data (possibly partially filled, from lsb)
77      */
78     @UnsupportedAppUsage
read(int bits)79     public int read(int bits) throws AccessException {
80         int index = mPos >>> 3;
81         int offset = 16 - (mPos & 0x07) - bits;  // &7==%8
82         if ((bits < 0) || (bits > 8) || ((mPos + bits) > mEnd)) {
83             throw new AccessException("illegal read " +
84                 "(pos " + mPos + ", end " + mEnd + ", bits " + bits + ")");
85         }
86         int data = (mBuf[index] & 0xFF) << 8;
87         if (offset < 8) data |= mBuf[index + 1] & 0xFF;
88         data >>>= offset;
89         data &= (-1 >>> (32 - bits));
90         mPos += bits;
91         return data;
92     }
93 
94     /**
95      * Read data in bulk into a byte array and increment the current position.
96      *
97      * @param bits the amount of data to read
98      * @return newly allocated byte array of read data
99      */
100     @UnsupportedAppUsage
readByteArray(int bits)101     public byte[] readByteArray(int bits) throws AccessException {
102         int bytes = (bits >>> 3) + ((bits & 0x07) > 0 ? 1 : 0);  // &7==%8
103         byte[] arr = new byte[bytes];
104         for (int i = 0; i < bytes; i++) {
105             int increment = Math.min(8, bits - (i << 3));
106             arr[i] = (byte)(read(increment) << (8 - increment));
107         }
108         return arr;
109     }
110 
111     /**
112      * Increment the current position and ignore contained data.
113      *
114      * @param bits the amount by which to increment the position
115      */
116     @UnsupportedAppUsage
skip(int bits)117     public void skip(int bits) throws AccessException {
118         if ((mPos + bits) > mEnd) {
119             throw new AccessException("illegal skip " +
120                 "(pos " + mPos + ", end " + mEnd + ", bits " + bits + ")");
121         }
122         mPos += bits;
123     }
124 }
125