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/*
* Copyright 2007 ZXing authors
*
* 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.google.zxing.common;
/**
* <p>This provides an easy abstraction to read bits at a time from a sequence of bytes, where the
* number of bits read is not often a multiple of 8.</p>
*
* <p>This class is thread-safe but not reentrant. Unless the caller modifies the bytes array
* it passed in, in which case all bets are off.</p>
*
* @author Sean Owen
*/
public final class BitSource {
private final byte[] bytes;
private int byteOffset;
private int bitOffset;
/**
* @param bytes bytes from which this will read bits. Bits will be read from the first byte first.
* Bits are read within a byte from most-significant to least-significant bit.
*/
public BitSource(byte[] bytes) {
this.bytes = bytes;
}
/**
* @param numBits number of bits to read
* @return int representing the bits read. The bits will appear as the least-significant
* bits of the int
* @throws IllegalArgumentException if numBits isn't in [1,32]
*/
public int readBits(int numBits) {
if (numBits < 1 || numBits > 32) {
throw new IllegalArgumentException();
}
int result = 0;
// First, read remainder from current byte
if (bitOffset > 0) {
int bitsLeft = 8 - bitOffset;
int toRead = numBits < bitsLeft ? numBits : bitsLeft;
int bitsToNotRead = bitsLeft - toRead;
int mask = (0xFF >> (8 - toRead)) << bitsToNotRead;
result = (bytes[byteOffset] & mask) >> bitsToNotRead;
numBits -= toRead;
bitOffset += toRead;
if (bitOffset == 8) {
bitOffset = 0;
byteOffset++;
}
}
// Next read whole bytes
if (numBits > 0) {
while (numBits >= 8) {
result = (result << 8) | (bytes[byteOffset] & 0xFF);
byteOffset++;
numBits -= 8;
}
// Finally read a partial byte
if (numBits > 0) {
int bitsToNotRead = 8 - numBits;
int mask = (0xFF >> bitsToNotRead) << bitsToNotRead;
result = (result << numBits) | ((bytes[byteOffset] & mask) >> bitsToNotRead);
bitOffset += numBits;
}
}
return result;
}
/**
* @return number of bits that can be read successfully
*/
public int available() {
return 8 * (bytes.length - byteOffset) - bitOffset;
}
}
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