root/libbase/BitsReader.cpp

DEFINITIONS

1. read_bit
2. read_uint
3. read_sint

// BitsReader.cpp:  bits reader, for Gnash.
// 
//   Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010,
//   2011 Free Software Foundation, Inc
// 
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3 of the License, or
// (at your option) any later version.
// 
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
// 
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
//

#include "BitsReader.h"

#include "log.h"

namespace gnash {
        
bool BitsReader::read_bit()
{
        bool ret = (*ptr&(128>>usedBits));
        if ( ++usedBits == 8 ) advanceToNextByte();
        return ret;
}

unsigned BitsReader::read_uint(unsigned short bitcount)
{
        assert(bitcount <= 32);

        boost::uint32_t value = 0;

        unsigned short bits_needed = bitcount;
        do
        {
                int unusedMask = 0xFF >> usedBits;
                int unusedBits = 8-usedBits;

                if (bits_needed == unusedBits)
                {
                        // Consume all the unused bits.
                        value |= (*ptr&unusedMask);
                        advanceToNextByte();
                        break;

                }
                else if (bits_needed > unusedBits)
                {
                        // Consume all the unused bits.

                        bits_needed -= unusedBits; // assert(bits_needed>0)

                        value |= ((*ptr&unusedMask) << bits_needed);
                        advanceToNextByte();
                }
                else
                {
                        assert(bits_needed <= unusedBits);

                        // Consume some of the unused bits.

                        unusedBits -= bits_needed;

                        value |= ((*ptr&unusedMask) >> unusedBits);

                        usedBits += bits_needed;
                        if ( usedBits >= 8 ) advanceToNextByte();

                        // We're done.
                        break;
                }
        }
        while (bits_needed > 0);

        //std::cerr << "Returning value: " << value << " unused bits: " << (int)m_unused_bits << std::endl;
        return value;

}


boost::int32_t BitsReader::read_sint(unsigned short bitcount)
{
        boost::int32_t  value = boost::int32_t(read_uint(bitcount));

        // Sign extend...
        if (value & (1 << (bitcount - 1))) 
                value |= -1 << bitcount;

        return value;
}


} // end namespace gnash