bitstream.c

nds上大名鼎鼎的DSOrganize 2.8最新源代码。很值得研究。

/* ***** BEGIN LICENSE BLOCK *****  
 * Source last modified: $Id: bitstream.c,v 1.1.2.1.6.1 2005/10/19 00:18:49 gwright Exp $ 
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 * Portions Copyright (c) 1995-2005 RealNetworks, Inc. All Rights Reserved.  
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 * will apply. You may also obtain the license terms directly from 
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 * to this file, the RCSL.  Please see the applicable RPSL or RCSL for 
 * the rights, obligations and limitations governing use of the 
 * contents of the file. 
 *   
 * This file is part of the Helix DNA Technology. RealNetworks is the 
 * developer of the Original Code and owns the copyrights in the 
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 * ***** END LICENSE BLOCK ***** */  

/**************************************************************************************
 * Fixed-point HE-AAC decoder
 * Jon Recker ([e-mail])
 * February 2005
 *
 * bitstream.c - bitstream parsing functions
 **************************************************************************************/

#include "bitstream.h"

/**************************************************************************************
 * Function:    SetBitstreamPointer
 *
 * Description: initialize bitstream reader
 *
 * Inputs:      pointer to BitStreamInfo struct
 *              number of bytes in bitstream
 *              pointer to byte-aligned buffer of data to read from
 *
 * Outputs:     initialized bitstream info struct
 *
 * Return:      none
 **************************************************************************************/
void SetBitstreamPointer(BitStreamInfo *bsi, int nBytes, unsigned char *buf)
{
	/* init bitstream */
	bsi->bytePtr = buf;
	bsi->iCache = 0;		/* 4-byte unsigned int */
	bsi->cachedBits = 0;	/* i.e. zero bits in cache */
	bsi->nBytes = nBytes;
}

/**************************************************************************************
 * Function:    RefillBitstreamCache
 *
 * Description: read new data from bitstream buffer into 32-bit cache
 *
 * Inputs:      pointer to initialized BitStreamInfo struct
 *
 * Outputs:     updated bitstream info struct
 *
 * Return:      none
 *
 * Notes:       only call when iCache is completely drained (resets bitOffset to 0)
 *              always loads 4 new bytes except when bsi->nBytes < 4 (end of buffer)
 *              stores data as big-endian in cache, regardless of machine endian-ness
 **************************************************************************************/
static __inline void RefillBitstreamCache(BitStreamInfo *bsi)
{
	int nBytes = bsi->nBytes;

	/* optimize for common case, independent of machine endian-ness */
	if (nBytes >= 4) {
		bsi->iCache  = (*bsi->bytePtr++) << 24;
		bsi->iCache |= (*bsi->bytePtr++) << 16;
		bsi->iCache |= (*bsi->bytePtr++) <<  8;
		bsi->iCache |= (*bsi->bytePtr++);
		bsi->cachedBits = 32;
		bsi->nBytes -= 4;
	} else {
		bsi->iCache = 0;
		while (nBytes--) {
			bsi->iCache |= (*bsi->bytePtr++);
			bsi->iCache <<= 8;
		}
		bsi->iCache <<= ((3 - bsi->nBytes)*8);
		bsi->cachedBits = 8*bsi->nBytes;
		bsi->nBytes = 0;
	}
}

/**************************************************************************************
 * Function:    GetBits
 *
 * Description: get bits from bitstream, advance bitstream pointer
 *
 * Inputs:      pointer to initialized BitStreamInfo struct
 *              number of bits to get from bitstream
 *
 * Outputs:     updated bitstream info struct
 *
 * Return:      the next nBits bits of data from bitstream buffer
 *
 * Notes:       nBits must be in range [0, 31], nBits outside this range masked by 0x1f
 *              for speed, does not indicate error if you overrun bit buffer 
 *              if nBits == 0, returns 0
 **************************************************************************************/
unsigned int GetBits(BitStreamInfo *bsi, int nBits)
{
	unsigned int data, lowBits;

	nBits &= 0x1f;							/* nBits mod 32 to avoid unpredictable results like >> by negative amount */
	data = bsi->iCache >> (31 - nBits);		/* unsigned >> so zero-extend */
	data >>= 1;								/* do as >> 31, >> 1 so that nBits = 0 works okay (returns 0) */
	bsi->iCache <<= nBits;					/* left-justify cache */
	bsi->cachedBits -= nBits;				/* how many bits have we drawn from the cache so far */

	/* if we cross an int boundary, refill the cache */
	if (bsi->cachedBits < 0) {
		lowBits = -bsi->cachedBits;
		RefillBitstreamCache(bsi);
		data |= bsi->iCache >> (32 - lowBits);		/* get the low-order bits */
	
		bsi->cachedBits -= lowBits;			/* how many bits have we drawn from the cache so far */
		bsi->iCache <<= lowBits;			/* left-justify cache */
	}

	return data;
}

/**************************************************************************************
 * Function:    GetBitsNoAdvance
 *
 * Description: get bits from bitstream, do not advance bitstream pointer
 *
 * Inputs:      pointer to initialized BitStreamInfo struct
 *              number of bits to get from bitstream
 *
 * Outputs:     none (state of BitStreamInfo struct left unchanged) 
 *
 * Return:      the next nBits bits of data from bitstream buffer
 *
 * Notes:       nBits must be in range [0, 31], nBits outside this range masked by 0x1f
 *              for speed, does not indicate error if you overrun bit buffer 
 *              if nBits == 0, returns 0
 **************************************************************************************/
unsigned int GetBitsNoAdvance(BitStreamInfo *bsi, int nBits)
{
	unsigned char *buf;
	unsigned int data, iCache;
	signed int lowBits;

	nBits &= 0x1f;							/* nBits mod 32 to avoid unpredictable results like >> by negative amount */
	data = bsi->iCache >> (31 - nBits);		/* unsigned >> so zero-extend */
	data >>= 1;								/* do as >> 31, >> 1 so that nBits = 0 works okay (returns 0) */
	lowBits = nBits - bsi->cachedBits;		/* how many bits do we have left to read */

	/* if we cross an int boundary, read next bytes in buffer */
	if (lowBits > 0) {
		iCache = 0;
		buf = bsi->bytePtr;
		while (lowBits > 0) {
			iCache <<= 8;
			if (buf < bsi->bytePtr + bsi->nBytes)
				iCache |= (unsigned int)*buf++;
			lowBits -= 8;
		}
		lowBits = -lowBits;
		data |= iCache >> lowBits;
	}

	return data;
}

/**************************************************************************************
 * Function:    AdvanceBitstream
 *
 * Description: move bitstream pointer ahead
 *
 * Inputs:      pointer to initialized BitStreamInfo struct
 *              number of bits to advance bitstream
 *
 * Outputs:     updated bitstream info struct
 *
 * Return:      none
 *
 * Notes:       generally used following GetBitsNoAdvance(bsi, maxBits)
 **************************************************************************************/
void AdvanceBitstream(BitStreamInfo *bsi, int nBits)
{
	nBits &= 0x1f;
	if (nBits > bsi->cachedBits) {
		nBits -= bsi->cachedBits;
		RefillBitstreamCache(bsi);
	}
	bsi->iCache <<= nBits;
	bsi->cachedBits -= nBits;
}

/**************************************************************************************
 * Function:    CalcBitsUsed
 *
 * Description: calculate how many bits have been read from bitstream
 *
 * Inputs:      pointer to initialized BitStreamInfo struct
 *              pointer to start of bitstream buffer
 *              bit offset into first byte of startBuf (0-7) 
 *
 * Outputs:     none
 *
 * Return:      number of bits read from bitstream, as offset from startBuf:startOffset
 **************************************************************************************/
int CalcBitsUsed(BitStreamInfo *bsi, unsigned char *startBuf, int startOffset)
{
	int bitsUsed;

	bitsUsed  = (bsi->bytePtr - startBuf) * 8;
	bitsUsed -= bsi->cachedBits;
	bitsUsed -= startOffset;

	return bitsUsed;
}

/**************************************************************************************
 * Function:    ByteAlignBitstream
 *
 * Description: bump bitstream pointer to start of next byte
 *
 * Inputs:      pointer to initialized BitStreamInfo struct
 *
 * Outputs:     byte-aligned bitstream BitStreamInfo struct
 *
 * Return:      none
 *
 * Notes:       if bitstream is already byte-aligned, do nothing
 **************************************************************************************/
void ByteAlignBitstream(BitStreamInfo *bsi)
{
	int offset;

	offset = bsi->cachedBits & 0x07;
	AdvanceBitstream(bsi, offset);
}