bitpool.cpp

完整的MP3播放器源码

///////////////////////////////////////////////////////////////////////////////

#include "stdafx.h"
#include "bitpool.h"


///////////////////////////////////////////////////////////////////////////////
//
CBitPool::CBitPool():m_offset(0),m_PlayedBits(0),m_playedOctets(0),
					m_restBits(8),m_InWordAcumulated(0)
{
	//pretty bad to alloc in the constructor
	VERIFY(m_pDwBuff = new DWORD[BUFSIZE]);
}

///////////////////////////////////////////////////////////////////////////////
//
CBitPool::~CBitPool ()
{
    delete[] m_pDwBuff;
	m_pDwBuff = 0;	// for debiger purpose
}

///////////////////////////////////////////////////////////////////////////////
// add 32 bits at the end

///////////////////////////////////////////////////////////////////////////////
//
void CBitPool::GoBackBits(int nBits)
{
    m_PlayedBits -= nBits;
    m_restBits   += nBits;
    while(m_restBits >= 8) 
	{
        m_restBits -= 8;
        m_playedOctets--;
    }
}

///////////////////////////////////////////////////////////////////////////////
//
void CBitPool::GoBackOctets(int nOctets)
{
	m_PlayedBits		 -= (nOctets*8);
    m_playedOctets		 -= nOctets;
}


///////////////////////////////////////////////////////////////////////////////
//
//
DWORD CBitPool::GetBits(DWORD nBits)
{

	m_PlayedBits += nBits;
   	DWORD	dwRetVal = 0;
	int		contor = nBits;
	while (contor > 0) 
	{
		if (m_restBits == 0) 
		{
			m_restBits = 8;
			m_playedOctets++;
		}
		int minBidx = min((int)m_restBits,contor);
		DWORD xmask = 0xFFFFFFFF >> (32 - m_restBits);
		int tempVal = m_pDwBuff[m_playedOctets & 0xfff];
		tempVal&= xmask;		
		m_restBits -= minBidx;
		tempVal = tempVal >> m_restBits;
		contor -= minBidx;
		dwRetVal |= tempVal << contor;
	}
	return dwRetVal;
}