dlib C++ Library - entropy_decoder_kernel_2.h

    // 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_
    #define DLIB_ENTROPY_DECODER_KERNEl_2_

    #include "../algs.h"
    #include "entropy_decoder_kernel_abstract.h"
    #include <iosfwd>
    #include "../uintn.h"

    namespace dlib
    {

        class entropy_decoder_kernel_2
        {
            /*!
                GENERAL NOTES
                    this decoder is implemented using "range" coding

                INITIAL VALUE
                    in       == 0
                    initial_low      == 0x00000001  (slightly more than zero)
                    initial_high     == 0xffffffff  (slightly less than one, 0.99999999976717)
                    target   == 0x00000000  (zero)
                    low      == initial_low
                    high     == initial_high
                    r        == 0

                CONVENTION
                    if (in != 0)
                        *in       == get_stream()
                        true      == stream_is_set()
                        streambuf == in->rdbuf()
                    else
                        false   == stream_is_set()

                    low      == the low end of the range used for arithmetic encoding.
                                this number is used as a 32bit fixed point real number.
                                the point is fixed just before the first bit, so it is
                                always in the range [0,1)

                                low is also never allowed to be zero to avoid overflow
                                in the calculation (high-low+1)/total.

                    high     == the high end of the range - 1 used for arithmetic encoding.
                                this number is used as a 32bit fixed point real number.
                                the point is fixed just before the first bit, so when we
                                interpret high as a real number then it is always in the
                                range [0,1)

                                the range for arithmetic encoding is always
                                [low,high + 0.9999999...)   the 0.9999999... is why
                                high == real upper range - 1

                    target  ==  32 bits of the fraction produced from an arithmetic encoder.
                                this number is used as a 32bit fixed point real number.
                                the point is fixed just before the first bit, so it is
                                always in the range [0,1)

                    r       ==  the value (high-low+1)/total from the last call to
                                get_target() or 0 if get_target_called() should be false

                    get_target_called() == (r != 0)

            !*/

        public:

            entropy_decoder_kernel_2 (
            );

            virtual ~entropy_decoder_kernel_2 (
            );

            void clear(
            );

            void set_stream (
                std::istream& in
            );

            bool stream_is_set (
            ) const;

            std::istream& get_stream (
            ) const;

            void decode (
                uint32 low_count,
                uint32 high_count
            );

            bool get_target_called (
            ) const;

            uint32 get_target (
                uint32 total
            );

        private:

            // restricted functions
            entropy_decoder_kernel_2(entropy_decoder_kernel_2&);        // copy constructor
            entropy_decoder_kernel_2& operator=(entropy_decoder_kernel_2&);    // assignment operator

            // data members
            const uint32 initial_low;
            const uint32 initial_high;
            std::istream* in;
            uint32 low;
            uint32 high;
            uint32 target;
            uint32 r;
            std::streambuf* streambuf;

        };

    }

    #ifdef NO_MAKEFILE
    #include "entropy_decoder_kernel_2.cpp"
    #endif

    #endif // DLIB_ENTROPY_DECODER_KERNEl_2_