libbase/BitsReader.cpp

   1 // BitsReader.cpp:  bits reader, for Gnash.
   2 //
   3 //   Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Free Software
   4 //   Foundation, Inc
   5 //
   6 // This program is free software; you can redistribute it and/or modify
   7 // it under the terms of the GNU General Public License as published by
   8 // the Free Software Foundation; either version 3 of the License, or
   9 // (at your option) any later version.
  10 //
  11 // This program is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15 //
  16 // You should have received a copy of the GNU General Public License
  17 // along with this program; if not, write to the Free Software
  18 // Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  19 //
  20
  21 #include "BitsReader.h"
  22
  23 #include "log.h"
  24
  25 namespace gnash {
  26
  27 bool BitsReader::read_bit()
  28 {
  29         bool ret = (*ptr&(128>>usedBits));
  30         if ( ++usedBits == 8 ) advanceToNextByte();
  31         return ret;
  32 }
  33
  34 unsigned BitsReader::read_uint(unsigned short bitcount)
  35 {
  36         assert(bitcount <= 32);
  37
  38         boost::uint32_t value = 0;
  39
  40         unsigned short bits_needed = bitcount;
  41         do
  42         {
  43                 int unusedMask = 0xFF >> usedBits;
  44                 int unusedBits = 8-usedBits;
  45
  46                 if (bits_needed == unusedBits)
  47                 {
  48                         // Consume all the unused bits.
  49                         value |= (*ptr&unusedMask);
  50                         advanceToNextByte();
  51                         break;
  52
  53                 }
  54                 else if (bits_needed > unusedBits)
  55                 {
  56                         // Consume all the unused bits.
  57
  58                         bits_needed -= unusedBits; // assert(bits_needed>0)
  59
  60                         value |= ((*ptr&unusedMask) << bits_needed);
  61                         advanceToNextByte();
  62                 }
  63                 else
  64                 {
  65                         assert(bits_needed <= unusedBits);
  66
  67                         // Consume some of the unused bits.
  68
  69                         unusedBits -= bits_needed;
  70
  71                         value |= ((*ptr&unusedMask) >> unusedBits);
  72
  73                         usedBits += bits_needed;
  74                         if ( usedBits >= 8 ) advanceToNextByte();
  75
  76                         // We're done.
  77                         break;
  78                 }
  79         }
  80         while (bits_needed > 0);
  81
  82         //std::cerr << "Returning value: " << value << " unused bits: " << (int)m_unused_bits << std::endl;
  83         return value;
  84
  85 }
  86
  87
  88 boost::int32_t BitsReader::read_sint(unsigned short bitcount)
  89 {
  90         boost::int32_t  value = boost::int32_t(read_uint(bitcount));
  91
  92         // Sign extend...
  93         if (value & (1 << (bitcount - 1)))
  94                 value |= -1 << bitcount;
  95
  96         return value;
  97 }
  98
  99
 100 } // end namespace gnash
 101