COMMON: Rework the BitStream class to improve its performance
* Fixed peekBits not to seek the underlying stream. Seeking can be slow when the stream is a file. * Changed multi-bit operations to work on multiple bits at once rather than iterating over single-bit operations. This is an almost direct port of a patch for xoreos provided by DrMcCoy.
This commit is contained in:
Bastien Bouclet
Filippos Karapetis
parent
2af325f60a
commit
ae9eeb731f
2 changed files with 149 additions and 135 deletions
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@ -20,7 +20,7 @@
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*
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*/
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// Based on eos' BitStream implementation
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// Based on xoreos' BitStream implementation
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#ifndef COMMON_BITSTREAM_H
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#define COMMON_BITSTREAM_H
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@ -43,14 +43,14 @@ namespace Common {
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* for valueBits, isLE and isMSB2LSB, reads 32bit little-endian values
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* from the data stream and hands out the bits in the order of LSB to MSB.
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*/
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template<class STREAM, int valueBits, bool isLE, bool isMSB2LSB>
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template<class STREAM, int valueBits, bool isLE, bool MSB2LSB>
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class BitStreamImpl {
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private:
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STREAM *_stream; ///< The input stream.
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DisposeAfterUse::Flag _disposeAfterUse; ///< Should we delete the stream on destruction?
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uint32 _value; ///< Current value.
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uint8 _inValue; ///< Position within the current value.
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uint64 _bitContainer; ///< The currently available bits.
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uint8 _bitsLeft; ///< Number of bits currently left in the bit container.
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uint32 _size; ///< Total bitstream size (in bits)
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uint32 _pos; ///< Current bitstream position (in bits)
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@ -76,37 +76,76 @@ private:
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return 0;
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}
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/** Read the next data value. */
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inline void readValue() {
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if (_size - _pos < valueBits)
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error("BitStreamImpl::readValue(): End of bit stream reached");
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/** Fill the container with at least min bits. */
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inline void fillContainer(size_t min) {
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while (_bitsLeft < min) {
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_value = readData();
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if (_stream->err() || _stream->eos())
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error("BitStreamImpl::readValue(): Read error");
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uint64 data;
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if (_pos + _bitsLeft + valueBits <= _size) {
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data = readData();
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} else {
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// Peeking data out of bounds is well defined and returns 0 bits.
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// This is for convenience when using speed-up techniques reading
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// more bits than actually available. Users should call eos() to
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// check if data was actually read out of bounds. Peeking out of
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// bounds does not set the eos flag.
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data = 0;
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}
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// If we're reading the bits MSB first, we need to shift the value to that position
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if (isMSB2LSB)
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_value <<= 32 - valueBits;
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// Move the data value to the right position in the bit container
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if (MSB2LSB)
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_bitContainer |= data << (64 - valueBits - _bitsLeft);
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else
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_bitContainer |= data << _bitsLeft;
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_bitsLeft += valueBits;
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}
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}
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/** Get n bits from the bit container. */
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inline static uint32 getNBits(uint64 value, size_t n) {
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if (n == 0)
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return 0;
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const size_t toShift = 64 - n;
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if (MSB2LSB)
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return value >> toShift;
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else
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return (value << toShift) >> toShift;
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}
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/** Skip already read bits. */
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inline void skipBits(size_t n) {
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assert(n <= _bitsLeft);
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// Shift to the next bit
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if (MSB2LSB)
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_bitContainer <<= n;
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else
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_bitContainer >>= n;
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_bitsLeft -= n;
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_pos += n;
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}
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public:
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/** Create a bit stream using this input data stream and optionally delete it on destruction. */
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BitStreamImpl(STREAM *stream, DisposeAfterUse::Flag disposeAfterUse = DisposeAfterUse::NO) :
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_stream(stream), _disposeAfterUse(disposeAfterUse), _value(0), _inValue(0), _pos(0) {
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_stream(stream), _disposeAfterUse(disposeAfterUse), _bitContainer(0), _bitsLeft(0), _pos(0) {
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if ((valueBits != 8) && (valueBits != 16) && (valueBits != 32))
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error("BitStreamImpl: Invalid memory layout %d, %d, %d", valueBits, isLE, isMSB2LSB);
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error("BitStreamImpl: Invalid memory layout %d, %d, %d", valueBits, isLE, MSB2LSB);
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_size = (_stream->size() & ~((uint32) ((valueBits >> 3) - 1))) * 8;
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}
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/** Create a bit stream using this input data stream. */
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BitStreamImpl(STREAM &stream) :
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_stream(&stream), _disposeAfterUse(DisposeAfterUse::NO), _value(0), _inValue(0), _pos(0) {
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_stream(&stream), _disposeAfterUse(DisposeAfterUse::NO), _bitContainer(0), _bitsLeft(0), _pos(0) {
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if ((valueBits != 8) && (valueBits != 16) && (valueBits != 32))
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error("BitStreamImpl: Invalid memory layout %d, %d, %d", valueBits, isLE, isMSB2LSB);
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error("BitStreamImpl: Invalid memory layout %d, %d, %d", valueBits, isLE, MSB2LSB);
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_size = (_stream->size() & ~((uint32) ((valueBits >> 3) - 1))) * 8;
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}
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@ -116,38 +155,33 @@ public:
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delete _stream;
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}
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private:
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uint32 getBit_internal() {
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// Get the current bit
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uint32 b = 0;
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if (isMSB2LSB)
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b = ((_value & 0x80000000) == 0) ? 0 : 1;
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else
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b = ((_value & 1) == 0) ? 0 : 1;
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/** Read a bit from the bit stream, without changing the stream's position. */
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uint peekBit() {
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fillContainer(1);
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// Shift to the next bit
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if (isMSB2LSB)
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_value <<= 1;
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else
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_value >>= 1;
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return getNBits(_bitContainer, 1);
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}
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/** Read a bit from the bit stream. */
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uint getBit() {
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const uint b = peekBit();
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skipBits(1);
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return b;
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}
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public:
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/** Read a bit from the bit stream. */
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uint32 getBit() {
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// Check if we need the next value
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if (_inValue == 0)
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readValue();
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/**
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* Read a multi-bit value from the bit stream, without changing the stream's position.
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*
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* The bit order is the same as in getBits().
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*/
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uint32 peekBits(size_t n) {
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if (n > 32)
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error("BitStreamImpl::peekBits(): Too many bits requested to be peeked");
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uint32 b = getBit_internal();
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// Increase the position within the current value
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_inValue = (_inValue + 1) % valueBits;
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_pos++;
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return b;
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fillContainer(n);
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return getNBits(_bitContainer, n);
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}
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/**
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@ -160,91 +194,15 @@ public:
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* If the bitstream is MSB2LSB, the 4-bit value would be 0101.
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* If the bitstream is LSB2MSB, the 4-bit value would be 0011.
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*/
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uint32 getBits(uint8 n) {
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if (n == 0)
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return 0;
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uint32 getBits(size_t n) {
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if (n > 32)
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error("BitStreamImpl::getBits(): Too many bits requested to be read");
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// Read the number of bits
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uint32 v = 0;
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const uint32 b = peekBits(n);
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uint8 nOrig = n;
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if (_inValue) {
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int count = MIN((int)n, valueBits - _inValue);
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for (int i = 0; i < count; ++i) {
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if (isMSB2LSB) {
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v = (v << 1) | getBit_internal();
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} else {
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v = (v >> 1) | (getBit_internal() << 31);
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}
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}
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skipBits(n);
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n -= count;
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}
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while (n > 0) {
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// NB: readValue doesn't care that _inValue is incorrect here
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readValue();
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int count = MIN((int)n, valueBits);
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for (int i = 0; i < count; ++i) {
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if (isMSB2LSB) {
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v = (v << 1) | getBit_internal();
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} else {
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v = (v >> 1) | (getBit_internal() << 31);
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}
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}
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n -= count;
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}
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_inValue = (_inValue + nOrig) % valueBits;
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_pos += nOrig;
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if (!isMSB2LSB)
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v >>= (32 - nOrig);
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return v;
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}
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/** Read a bit from the bit stream, without changing the stream's position. */
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uint32 peekBit() {
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uint32 value = _value;
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uint8 inValue = _inValue;
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uint32 curStreamPos = _stream->pos();
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uint32 curPos = _pos;
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uint32 v = getBit();
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_pos = curPos;
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_stream->seek(curStreamPos);
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_inValue = inValue;
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_value = value;
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return v;
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}
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/**
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* Read a multi-bit value from the bit stream, without changing the stream's position.
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*
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* The bit order is the same as in getBits().
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*/
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uint32 peekBits(uint8 n) {
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uint32 value = _value;
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uint8 inValue = _inValue;
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uint32 curStreamPos = _stream->pos();
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uint32 curPos = _pos;
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uint32 v = getBits(n);
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_pos = curPos;
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_stream->seek(curStreamPos);
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_inValue = inValue;
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_value = value;
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return v;
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return b;
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}
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/**
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@ -262,7 +220,7 @@ public:
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if (n >= 32)
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error("BitStreamImpl::addBit(): Too many bits requested to be read");
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if (isMSB2LSB)
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if (MSB2LSB)
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x = (x << 1) | getBit();
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else
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x = (x & ~(1 << n)) | (getBit() << n);
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@ -272,21 +230,29 @@ public:
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void rewind() {
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_stream->seek(0);
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_value = 0;
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_inValue = 0;
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_pos = 0;
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_bitContainer = 0;
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_bitsLeft = 0;
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_pos = 0;
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}
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/** Skip the specified amount of bits. */
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void skip(uint32 n) {
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while (n-- > 0)
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getBit();
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while (n > 32) {
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fillContainer(32);
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skipBits(32);
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n -= 32;
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}
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fillContainer(n);
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skipBits(n);
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}
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/** Skip the bits to closest data value border. */
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void align() {
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while (_inValue)
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getBit();
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uint32 bitsAfterBoundary = _pos % valueBits;
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if (bitsAfterBoundary) {
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skip(valueBits - bitsAfterBoundary);
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}
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}
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/** Return the stream position in bits. */
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@ -302,6 +268,10 @@ public:
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bool eos() const {
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return _stream->eos() || (_pos >= _size);
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}
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static bool isMSB2LSB() {
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return MSB2LSB;
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}
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};
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@ -50,7 +50,7 @@ public:
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private:
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template<class MS, class BS>
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void tmpl_skip() {
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byte contents[] = { 'a', 'b' };
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byte contents[] = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j' };
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MS ms(contents, sizeof(contents));
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@ -61,6 +61,8 @@ private:
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bs.skip(4);
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TS_ASSERT_EQUALS(bs.pos(), 9u);
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TS_ASSERT_EQUALS(bs.getBits(3), 6u);
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bs.skip(65);
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TS_ASSERT_EQUALS(bs.pos(), 77u);
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TS_ASSERT(!bs.eos());
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}
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public:
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@ -133,7 +135,7 @@ private:
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TS_ASSERT_EQUALS(bs.pos(), 3u);
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bs.skip(8);
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TS_ASSERT_EQUALS(bs.pos(), 11u);
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TS_ASSERT_EQUALS(bs.peekBits(5), 2u);
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TS_ASSERT_EQUALS(bs.peekBits(6), 4u);
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TS_ASSERT(!bs.eos());
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}
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public:
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@ -203,7 +205,7 @@ private:
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TS_ASSERT_EQUALS(bs.pos(), 3u);
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bs.skip(8);
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TS_ASSERT_EQUALS(bs.pos(), 11u);
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TS_ASSERT_EQUALS(bs.peekBits(5), 12u);
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TS_ASSERT_EQUALS(bs.peekBits(20), 12u);
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TS_ASSERT(!bs.eos());
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}
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public:
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@ -211,4 +213,46 @@ public:
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tmpl_peek_bits_lsb<Common::MemoryReadStream, Common::BitStream8LSB>();
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tmpl_peek_bits_lsb<Common::BitStreamMemoryStream, Common::BitStreamMemory8LSB>();
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}
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private:
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template<class MS, class BS>
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void tmpl_align() {
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byte contents[] = { 'a', 'b' };
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MS ms(contents, sizeof(contents));
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BS bs(ms);
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TS_ASSERT_EQUALS(bs.pos(), 0u);
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bs.align();
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TS_ASSERT_EQUALS(bs.pos(), 0u);
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bs.skip(3);
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bs.align();
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TS_ASSERT_EQUALS(bs.pos(), 8u);
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}
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public:
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void test_align() {
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tmpl_align<Common::MemoryReadStream, Common::BitStream8LSB>();
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tmpl_align<Common::BitStreamMemoryStream, Common::BitStreamMemory8LSB>();
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}
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private:
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template<class MS, class BS>
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void tmpl_align_16() {
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byte contents[] = { 'a', 'b' };
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MS ms(contents, sizeof(contents));
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BS bs(ms);
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TS_ASSERT_EQUALS(bs.pos(), 0u);
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bs.align();
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TS_ASSERT_EQUALS(bs.pos(), 0u);
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bs.skip(3);
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bs.align();
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TS_ASSERT_EQUALS(bs.pos(), 16u);
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}
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public:
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void test_align_16() {
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tmpl_align_16<Common::MemoryReadStream, Common::BitStream16BELSB>();
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tmpl_align_16<Common::BitStreamMemoryStream, Common::BitStreamMemory16BELSB>();
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}
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};
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