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/*
* Copyright (C) 2016 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 org.mozilla.thirdparty.com.google.android.exoplayer2.util;
/**
* Wraps a byte array, providing methods that allow it to be read as a NAL unit bitstream.
* <p>
* Whenever the byte sequence [0, 0, 3] appears in the wrapped byte array, it is treated as [0, 0]
* for all reading/skipping operations, which makes the bitstream appear to be unescaped.
*/
public final class ParsableNalUnitBitArray {
private byte[] data;
private int byteLimit;
// The byte offset is never equal to the offset of the 3rd byte in a subsequence [0, 0, 3].
private int byteOffset;
private int bitOffset;
/**
* @param data The data to wrap.
* @param offset The byte offset in {@code data} to start reading from.
* @param limit The byte offset of the end of the bitstream in {@code data}.
*/
@SuppressWarnings({"initialization.fields.uninitialized", "method.invocation.invalid"})
public ParsableNalUnitBitArray(byte[] data, int offset, int limit) {
reset(data, offset, limit);
}
/**
* Resets the wrapped data, limit and offset.
*
* @param data The data to wrap.
* @param offset The byte offset in {@code data} to start reading from.
* @param limit The byte offset of the end of the bitstream in {@code data}.
*/
public void reset(byte[] data, int offset, int limit) {
this.data = data;
byteOffset = offset;
byteLimit = limit;
bitOffset = 0;
assertValidOffset();
}
/**
* Skips a single bit.
*/
public void skipBit() {
if (++bitOffset == 8) {
bitOffset = 0;
byteOffset += shouldSkipByte(byteOffset + 1) ? 2 : 1;
}
assertValidOffset();
}
/**
* Skips bits and moves current reading position forward.
*
* @param numBits The number of bits to skip.
*/
public void skipBits(int numBits) {
int oldByteOffset = byteOffset;
int numBytes = numBits / 8;
byteOffset += numBytes;
bitOffset += numBits - (numBytes * 8);
if (bitOffset > 7) {
byteOffset++;
bitOffset -= 8;
}
for (int i = oldByteOffset + 1; i <= byteOffset; i++) {
if (shouldSkipByte(i)) {
// Skip the byte and move forward to check three bytes ahead.
byteOffset++;
i += 2;
}
}
assertValidOffset();
}
/**
* Returns whether it's possible to read {@code n} bits starting from the current offset. The
* offset is not modified.
*
* @param numBits The number of bits.
* @return Whether it is possible to read {@code n} bits.
*/
public boolean canReadBits(int numBits) {
int oldByteOffset = byteOffset;
int numBytes = numBits / 8;
int newByteOffset = byteOffset + numBytes;
int newBitOffset = bitOffset + numBits - (numBytes * 8);
if (newBitOffset > 7) {
newByteOffset++;
newBitOffset -= 8;
}
for (int i = oldByteOffset + 1; i <= newByteOffset && newByteOffset < byteLimit; i++) {
if (shouldSkipByte(i)) {
// Skip the byte and move forward to check three bytes ahead.
newByteOffset++;
i += 2;
}
}
return newByteOffset < byteLimit || (newByteOffset == byteLimit && newBitOffset == 0);
}
/**
* Reads a single bit.
*
* @return Whether the bit is set.
*/
public boolean readBit() {
boolean returnValue = (data[byteOffset] & (0x80 >> bitOffset)) != 0;
skipBit();
return returnValue;
}
/**
* Reads up to 32 bits.
*
* @param numBits The number of bits to read.
* @return An integer whose bottom n bits hold the read data.
*/
public int readBits(int numBits) {
int returnValue = 0;
bitOffset += numBits;
while (bitOffset > 8) {
bitOffset -= 8;
returnValue |= (data[byteOffset] & 0xFF) << bitOffset;
byteOffset += shouldSkipByte(byteOffset + 1) ? 2 : 1;
}
returnValue |= (data[byteOffset] & 0xFF) >> (8 - bitOffset);
returnValue &= 0xFFFFFFFF >>> (32 - numBits);
if (bitOffset == 8) {
bitOffset = 0;
byteOffset += shouldSkipByte(byteOffset + 1) ? 2 : 1;
}
assertValidOffset();
return returnValue;
}
/**
* Returns whether it is possible to read an Exp-Golomb-coded integer starting from the current
* offset. The offset is not modified.
*
* @return Whether it is possible to read an Exp-Golomb-coded integer.
*/
public boolean canReadExpGolombCodedNum() {
int initialByteOffset = byteOffset;
int initialBitOffset = bitOffset;
int leadingZeros = 0;
while (byteOffset < byteLimit && !readBit()) {
leadingZeros++;
}
boolean hitLimit = byteOffset == byteLimit;
byteOffset = initialByteOffset;
bitOffset = initialBitOffset;
return !hitLimit && canReadBits(leadingZeros * 2 + 1);
}
/**
* Reads an unsigned Exp-Golomb-coded format integer.
*
* @return The value of the parsed Exp-Golomb-coded integer.
*/
public int readUnsignedExpGolombCodedInt() {
return readExpGolombCodeNum();
}
/**
* Reads an signed Exp-Golomb-coded format integer.
*
* @return The value of the parsed Exp-Golomb-coded integer.
*/
public int readSignedExpGolombCodedInt() {
int codeNum = readExpGolombCodeNum();
return ((codeNum % 2) == 0 ? -1 : 1) * ((codeNum + 1) / 2);
}
private int readExpGolombCodeNum() {
int leadingZeros = 0;
while (!readBit()) {
leadingZeros++;
}
return (1 << leadingZeros) - 1 + (leadingZeros > 0 ? readBits(leadingZeros) : 0);
}
private boolean shouldSkipByte(int offset) {
return 2 <= offset && offset < byteLimit && data[offset] == (byte) 0x03
&& data[offset - 2] == (byte) 0x00 && data[offset - 1] == (byte) 0x00;
}
private void assertValidOffset() {
// It is fine for position to be at the end of the array, but no further.
Assertions.checkState(byteOffset >= 0
&& (byteOffset < byteLimit || (byteOffset == byteLimit && bitOffset == 0)));
}
}
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