dlib C++ Library - entropy_decoder_kernel_2.cpp

    // Copyright (C) 2004  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_ENTROPY_DECODER_KERNEL_2_CPp_
    #define DLIB_ENTROPY_DECODER_KERNEL_2_CPp_
    #include "entropy_decoder_kernel_2.h"
    #include <iostream>
    #include <streambuf>
    #include <sstream>

    #include "../assert.h"

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        entropy_decoder_kernel_2::
        entropy_decoder_kernel_2(
        ) :
            initial_low(0x00000001),
            initial_high(0xffffffff),
            in(0),
            low(initial_low),
            high(initial_high),
            target(0x00000000),
            r(0)
        {
        }

    // ----------------------------------------------------------------------------------------

        entropy_decoder_kernel_2::
        ~entropy_decoder_kernel_2 (
        )
        {
        }

    // ----------------------------------------------------------------------------------------

        void entropy_decoder_kernel_2::
        clear(
        )
        {
            in       = 0;
            r        = 0;
            low      = initial_low;
            high     = initial_high;
            target   = 0x00000000;
        }

    // ----------------------------------------------------------------------------------------

        void entropy_decoder_kernel_2::
        set_stream (
            std::istream& in_
        )
        {
            r        = 0;
            low      = initial_low;
            high     = initial_high;
            target   = 0x00000000;

            in = &in_;
            streambuf = in_.rdbuf();

            unsigned char ch;

            streambuf->sgetn((char*)&ch,1);
            target = ch;

            target <<= 8;
            if (streambuf->sgetn((char*)&ch,1))
                target += ch;

            target <<= 8;
            if (streambuf->sgetn((char*)&ch,1))
                target += ch;

            target <<= 8;
            if (streambuf->sgetn((char*)&ch,1))
                target += ch;
        }

    // ----------------------------------------------------------------------------------------

        bool entropy_decoder_kernel_2::
        stream_is_set (
        ) const
        {
            if (in != 0)
                return true;
            else
                return false;
        }

    // ----------------------------------------------------------------------------------------

        std::istream& entropy_decoder_kernel_2::
        get_stream (
        ) const
        {
            return *in;
        }

    // ----------------------------------------------------------------------------------------

        void entropy_decoder_kernel_2::
        decode (
            uint32 low_count,
            uint32 high_count
        )
        {
            // note that we must subtract 1 to preserve the convention that
            // high == the real upper range - 1
            high = low + r*high_count - 1;
            low = low + r*low_count;
            r = 0;

            while (true )
            {

                // if high and low don't have the same 8 high order bits
                if ((high&0xFF000000) != (low&0xFF000000))
                {
                    // if the distance between high and low is small and there aren't
                    // any bits we can roll off then force high and low to have common high
                    // order bits.
                    if ((high-low < 0x10000))
                    {
                        if (high-low > 0x1000)
                        {
                            high>>=1;
                            low>>=1;
                            high = low = high+low;
                            high += 0xFF;
                            low -= 0xFF;
                        }
                        else /**/
                        {
                            high>>=1;
                            low>>=1;
                            high = low = high+low;
                        }
                    }
                    else
                    {
                        // there are no bits to roll off and high and low are not
                        // too close so just quit the loop
                        break;
                    }

                }
                // else if there are 8 bits we can roll off
                else
                {
                    unsigned char buf;
                    if (streambuf->sgetn(reinterpret_cast<char*>(&buf),1)==0)
                    {
                        // if there isn't anything else in the streambuffer then just
                        // make buf zero.
                        buf = 0;
                    }

                    // also roll off the bits in target
                    target <<= 8;

                    // roll off the bits
                    high <<= 8;
                    low <<= 8;
                    high |= 0xFF;  // note that it is ok to add 0xFF to high here because
                                // of the convention that high == real upper range - 1.
                                // so that means that if we want to shift the upper range
                                // left by one then we must shift a one into high also
                                // since real upper range == high + 0.999999999...

                    // make sure low is never zero
                    if (low == 0)
                        low = 1;

                    // put the new bits into target
                    target |= static_cast<uint32>(buf);
                }

            } // while (true)
        }

    // ----------------------------------------------------------------------------------------

        bool entropy_decoder_kernel_2::
        get_target_called (
        ) const
        {
            return (r != 0);
        }

    // ----------------------------------------------------------------------------------------

        uint32 entropy_decoder_kernel_2::
        get_target (
            uint32 total
        )
        {
            // note that we must add one because of the convention that
            // high == the real upper range minus 1
            r = (high-low+1)/total;
            uint32 temp = (target-low)/r;
            if (temp < total)
                return temp;
            else
                return total-1;
        }

    // ----------------------------------------------------------------------------------------

    }
    #endif // DLIB_ENTROPY_DECODER_KERNEL_2_CPp_