bitstream.c

著名的 helix realplayer 基于手机 symbian 系统的 播放器全套源代码

/* ***** BEGIN LICENSE BLOCK ***** 
 * Version: RCSL 1.0/RPSL 1.0 
 *  
 * Portions Copyright (c) 1995-2002 RealNetworks, Inc. All Rights Reserved. 
 *      
 * The contents of this file, and the files included with this file, are 
 * subject to the current version of the RealNetworks Public Source License 
 * Version 1.0 (the "RPSL") available at 
 * http://www.helixcommunity.org/content/rpsl unless you have licensed 
 * the file under the RealNetworks Community Source License Version 1.0 
 * (the "RCSL") available at http://www.helixcommunity.org/content/rcsl, 
 * in which case the RCSL will apply. You may also obtain the license terms 
 * directly from RealNetworks.  You may not use this file except in 
 * compliance with the RPSL or, if you have a valid RCSL with RealNetworks 
 * applicable 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 portions 
 * it created. 
 *  
 * This file, and the files included with this file, is distributed and made 
 * available on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER 
 * EXPRESS OR IMPLIED, AND REALNETWORKS HEREBY DISCLAIMS ALL SUCH WARRANTIES, 
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY, FITNESS 
 * FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT. 
 * 
 * Technology Compatibility Kit Test Suite(s) Location: 
 *    http://www.helixcommunity.org/content/tck 
 * 
 * Contributor(s): 
 *  
 * ***** END LICENSE BLOCK ***** */ 

/* bitstream reader functions. No checking for end-of-bits included! */

#include "bitstream.h"
#include <stdlib.h>

#define ASSERT(a)

/** The bitstream structure.
 *  The idea of the bitstream reader is to keep a cache word that has the machine's
 *  largest native size. This word keeps the next-to-read bits left-aligned so that
 *  on a read, one shift suffices.
 *  The cache word is only refilled if it does not contain enough bits to satisy a
 *  a read request. Because the refill only happens in multiple of 8 bits, the maximum
 *  read size that is guaranteed to be always fulfilled is the number of bits in a long
 *  minus 8 (or whatever the number of bits in a byte is).
 */

struct BITSTREAM
{
    const unsigned char *buffer ; /**< points to the buffer holding the bits */
    const unsigned char *pkptr ;  /**< read pointer */
    unsigned long cache ;   /**< cache, always holds next bits left-aligned. */
    int cacheBitsLeft ;     /**< number of bits left in cache */
    int nBits ;    /**< the number of bits in the buffer */
    int inc ;      /**< read direction (forward/backward) */
} ;

enum {
    CACHEBITS = 8*sizeof(unsigned long)
} ;

__inline static void fillUp(struct BITSTREAM *pBitstream)
{
    unsigned long k = 0 ;

    while (CACHEBITS - pBitstream->cacheBitsLeft >= 8)
    {
        k = (k<<8) + *(pBitstream->pkptr) ;
        (pBitstream->pkptr)+= pBitstream->inc ;
        pBitstream->cacheBitsLeft += 8 ;
    }
    pBitstream->cache |= (k << (CACHEBITS - pBitstream->cacheBitsLeft)) ;
}

/** read nBits bits from bitstream
  * @param pBitstream the bitstream to read from
  * @param nBits the number of bits to read. nBits must be <= 32, currently.
  * @return the bits read, right-justified
  */
unsigned int readBits(struct BITSTREAM *pBitstream, int nBits)
{
    unsigned int data ;

    if (pBitstream->cacheBitsLeft < nBits)
    {
        fillUp(pBitstream) ;
    }

    data = pBitstream->cache >> (CACHEBITS - nBits) ;
    pBitstream->cache <<= nBits ;
    pBitstream->cacheBitsLeft -= nBits ;

    return data ;
}

/** push bits back into the bitstream.
  * This call is here to make look-ahead possible, where after reading the client
  * may realize it has read too far ahead. It is guaranteed to succeed as long as
  * you don't push more bits back than have been read in the last readBits() call.
  * @param pBitstream the bitstream to push back into
  * @param bits the bits to push back
  * @param nBits the number of bits to push back.
  * @return an error code, signalling success or failure.
  */
int unreadBits(struct BITSTREAM *pBitstream, int bits, int nBits)
{
    pBitstream->cache = (pBitstream->cache>>nBits) | (bits << (CACHEBITS - nBits)) ;
    pBitstream->cacheBitsLeft += nBits ;
    return pBitstream->cacheBitsLeft > (signed)CACHEBITS ? -1 : 0 ;
}

/** byte-align the bitstream read pointer. */
void byteAlign(struct BITSTREAM *pBitstream)
{
    int adjust = (pBitstream->cacheBitsLeft & 7) ;
    pBitstream->cache <<= adjust ;
    pBitstream->cacheBitsLeft -= adjust ;
}

/** allocate memory for a new bitstream structure.
  * @param ppBitstream a pointer to a bitstream handle, to be initialized on
  *        successfull return
  * @param nBits the maximum number of bits this bitstream must be able to hold.
  *        nBits must be divisible by 32.
  * @return an error code, signalling success or failure.
  * @see reverseBitstream
  */
int newBitstream(struct BITSTREAM **ppBitstream, int nBits)
{
    struct BITSTREAM *pBitstream ;

    pBitstream = (struct BITSTREAM *)malloc(sizeof(struct BITSTREAM)) ;
    if (!pBitstream || !ppBitstream)
        return -1 ;

    pBitstream->cacheBitsLeft = 0 ;
    *ppBitstream = pBitstream ;

    return 0 ;
}

/** free memory associated with a bitstream structure.
  * @param pBitstream a bitstream handle
  */
void deleteBitstream(struct BITSTREAM *pBitstream)
{
	if (pBitstream) free (pBitstream) ;
}

/** feed nbits bits to the bitstream, byte-wise.
  * @param pBitstream the bitstream into which to feed the bytes
  * @param input the input from which to read the bytes
  * @param nbits the number of bits in the input. nbits must be divisible by 32
  *  for reverseBitstream() to work.
  * @return an error code, signalling success or failure.
  * @see reverseBitstream
  */

int feedBitstream(struct BITSTREAM *pBitstream, const unsigned char *input, int nbits)
{
    pBitstream->buffer = input ;
    pBitstream->nBits  = nbits ;

    return 0 ;
}

/** return the number of bits left until end-of-stream.
  * @param pBitstream the bitstream
  * @return the number of bits left
  */

int bitsLeftInBitstream(struct BITSTREAM *pBitstream)
{
    return pBitstream->nBits - (pBitstream->pkptr - pBitstream->buffer) * 8 + pBitstream->cacheBitsLeft ;
}

/** set bitstream position, relative to origin defined through feedBitstream().
  * @param pBitstream the bitstream
  * @param position the position in bits (must be multiple of 8, currently).
  * Always measured from beginning, regardless of direction.
  * @param direction the direction of reading (+1/-1)
  */

int setAtBitstream(struct BITSTREAM *pBitstream, int position, int direction)
{
    ASSERT( position % 8 == 0 ) ;

    pBitstream->pkptr = pBitstream->buffer + (position>>3) ;
    pBitstream->cacheBitsLeft = 0 ;
    pBitstream->cache = 0 ;
    pBitstream->inc = direction ;
    return 0 ;
}