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/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.internal.util;
/**
* An object that provides bitwise incremental read access to a byte array.
*
* This is useful, for example, when accessing a series of fields that
* may not be aligned on byte boundaries.
*
* NOTE -- This class is not threadsafe.
*/
public class BitwiseInputStream {
// The byte array being read from.
private byte[] mBuf;
// The current position offset, in bits, from the msb in byte 0.
private int mPos;
// The last valid bit offset.
private int mEnd;
/**
* An exception to report access problems.
*/
public static class AccessException extends Exception {
public AccessException(String s) {
super("BitwiseInputStream access failed: " + s);
}
}
/**
* Create object from byte array.
*
* @param buf a byte array containing data
*/
public BitwiseInputStream(byte buf[]) {
mBuf = buf;
mEnd = buf.length << 3;
mPos = 0;
}
/**
* Return the number of bit still available for reading.
*/
public int available() {
return mEnd - mPos;
}
/**
* Read some data and increment the current position.
*
* The 8-bit limit on access to bitwise streams is intentional to
* avoid endianness issues.
*
* @param bits the amount of data to read (gte 0, lte 8)
* @return byte of read data (possibly partially filled, from lsb)
*/
public int read(int bits) throws AccessException {
int index = mPos >>> 3;
int offset = 16 - (mPos & 0x07) - bits; // &7==%8
if ((bits < 0) || (bits > 8) || ((mPos + bits) > mEnd)) {
throw new AccessException("illegal read " +
"(pos " + mPos + ", end " + mEnd + ", bits " + bits + ")");
}
int data = (mBuf[index] & 0xFF) << 8;
if (offset < 8) data |= mBuf[index + 1] & 0xFF;
data >>>= offset;
data &= (-1 >>> (32 - bits));
mPos += bits;
return data;
}
/**
* Read data in bulk into a byte array and increment the current position.
*
* @param bits the amount of data to read
* @return newly allocated byte array of read data
*/
public byte[] readByteArray(int bits) throws AccessException {
int bytes = (bits >>> 3) + ((bits & 0x07) > 0 ? 1 : 0); // &7==%8
byte[] arr = new byte[bytes];
for (int i = 0; i < bytes; i++) {
int increment = Math.min(8, bits - (i << 3));
arr[i] = (byte)(read(increment) << (8 - increment));
}
return arr;
}
/**
* Increment the current position and ignore contained data.
*
* @param bits the amount by which to increment the position
*/
public void skip(int bits) throws AccessException {
if ((mPos + bits) > mEnd) {
throw new AccessException("illegal skip " +
"(pos " + mPos + ", end " + mEnd + ", bits " + bits + ")");
}
mPos += bits;
}
}
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