dlib C++ Library - entropy_decoder_kernel_1.cpp

    // Copyright (C) 2003  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_ENTROPY_DECODER_KERNEL_1_CPp_
    #define DLIB_ENTROPY_DECODER_KERNEL_1_CPp_
    #include "entropy_decoder_kernel_1.h"
    #include <iostream>
    #include <streambuf>
    #include <sstream>

    #include "../assert.h"

    namespace dlib
    {

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

        entropy_decoder_kernel_1::
        entropy_decoder_kernel_1(
        ) :
            initial_low(0x00000001),
            initial_high(0xffffffff),
            in(0),
            low(initial_low),
            high(initial_high),
            buf(0),
            buf_used(0),
            target(0x00000000),
            r(0)
        {
        }

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

        entropy_decoder_kernel_1::
        ~entropy_decoder_kernel_1 (
        )
        {
        }

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

        void entropy_decoder_kernel_1::
        clear(
        )
        {
            in       = 0;
            buf_used = 0;
            buf      = 0;
            r        = 0;
            low      = initial_low;
            high     = initial_high;
            target   = 0x00000000;
        }

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

        void entropy_decoder_kernel_1::
        set_stream (
            std::istream& in_
        )
        {
            buf_used = 0;
            buf      = 0;
            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_1::
        stream_is_set (
        ) const
        {
            if (in != 0)
                return true;
            else
                return false;
        }

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

        std::istream& entropy_decoder_kernel_1::
        get_stream (
        ) const
        {
            return *in;
        }

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

        void entropy_decoder_kernel_1::
        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 the highest order bit in high and low is the same
                if ( low >= 0x80000000 || high < 0x80000000)
                {
                    // make sure buf isn't empty
                    if (buf_used == 0)
                    {
                        buf_used = 8;
                        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;
                        }
                    }

                    // we will be taking one bit from buf to replace the one we threw away
                    --buf_used;

                    // roll off the bit in target
                    target <<= 1;

                    // roll off the bit
                    high <<= 1;
                    low <<= 1;
                    high |= 1;  // note that it is ok to add one 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;

                      // take a bit from buf to fill in the one we threw away
                    target += (buf>>buf_used)&0x01;
                }
                // if the distance between high and low is small and there aren't
                // any bits we can roll off then round low up or high down.
                else if (high-low < 0x10000)
                {
                    if (high == 0x80000000)
                        high = 0x7fffffff;
                    else
                        low = 0x80000000;
                }
                else
                {
                    break;
                }
            } // while (true)

        }

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

        bool entropy_decoder_kernel_1::
        get_target_called (
        ) const
        {
            return (r != 0);
        }

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

        uint32 entropy_decoder_kernel_1::
        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_1_CPp_