dlib/dlib/float_details.h

// Copyright (C) 2013  Davis E. King (davis@dlib.net)
// License: Boost Software License   See LICENSE.txt for the full license.
#ifndef DLIB_FLOAT_DEtAILS_Hh_
#define DLIB_FLOAT_DEtAILS_Hh_

#include <cmath>
#include "algs.h"
#include <limits> 

namespace dlib
{
    struct float_details
    {
        /*!
            WHAT THIS OBJECT REPRESENTS
                This object is a tool for converting floating point numbers into an
                explicit integer representation and then also converting back.  In
                particular, a float_details object represents a floating point number with
                a 64 bit mantissa and 16 bit exponent.  These are stored in the public
                fields of the same names.

                The main use of this object is to convert floating point values into a
                known uniform representation so they can be serialized to an output stream.
                This allows dlib serialization code to work on any system, regardless of
                the floating point representation used by the hardware.  It also means
                that, for example, a double can be serialized and then deserialized into a
                float and it will perform the appropriate conversion.


                In more detail, this object represents a floating point value equal to
                mantissa*pow(2,exponent), except when exponent takes on any of the
                following special values: 
                    - is_inf
                    - is_ninf
                    - is_nan
                These values are used to indicate that the floating point value should be
                either infinity, negative infinity, or not-a-number respectively.
        !*/

        float_details(
            int64 man,
            int16 exp
        ) : mantissa(man), exponent(exp) {}
        /*!
            ensures
                - #mantissa == man
                - #exponent == exp
        !*/

        float_details() :
            mantissa(0), exponent(0)
        {}
        /*!
            ensures
                - this object represents a floating point value of 0
        !*/

        float_details ( const double&      val) { *this = val; }
        float_details ( const float&       val) { *this = val; }
        float_details ( const long double& val) { *this = val; }
        /*!
            ensures
                - converts the given value into a float_details representation.  This 
                  means that converting #*this back into a floating point number should
                  recover the input val.
        !*/

        float_details& operator= ( const double&      val) { convert_from_T(val); return *this; }
        float_details& operator= ( const float&       val) { convert_from_T(val); return *this; }
        float_details& operator= ( const long double& val) { convert_from_T(val); return *this; }
        /*!
            ensures
                - converts the given value into a float_details representation.  This 
                  means that converting #*this back into a floating point number should
                  recover the input val.
        !*/

        operator double      () const { return convert_to_T<double>(); }
        operator float       () const { return convert_to_T<float>(); }
        operator long double () const { return convert_to_T<long double>(); }
        /*!
            ensures
                - converts the contents of this float_details object into a floating point number.
        !*/

        const static int16 is_inf  = 32000;
        const static int16 is_ninf = 32001;
        const static int16 is_nan  = 32002;

        int64 mantissa;
        int16 exponent;


    private:


// ----------------------------------------------------------------------------------------
// ----------------------------------------------------------------------------------------
//                                  IMPLEMENTATION DETAILS 
// ----------------------------------------------------------------------------------------
// ----------------------------------------------------------------------------------------

        template <typename T>
        void convert_from_T (
            const T& val
        )
        {
            mantissa = 0;

            const int digits = dlib::tmin<std::numeric_limits<T>::digits, 63>::value;

            if (val == std::numeric_limits<T>::infinity())
            {
                exponent = is_inf;
            }
            else if (val == -std::numeric_limits<T>::infinity())
            {
                exponent = is_ninf;
            }
            else if (val < std::numeric_limits<T>::infinity())
            {
                int exp;
                mantissa = static_cast<int64>(std::frexp(val, &exp)*(((uint64)1)<<digits));
                exponent = exp - digits;

                // Compact the representation a bit by shifting off any low order bytes 
                // which are zero in the mantissa.  This makes the numbers in mantissa and
                // exponent generally smaller which can make serialization and other things
                // more efficient in some cases.
                for (int i = 0; i < 8 && ((mantissa&0xFF)==0); ++i)
                {
                    mantissa >>= 8;
                    exponent += 8;
                }
            }
            else
            {
                exponent = is_nan;
            }
        }

        template <typename T>
        T convert_to_T (
        ) const
        {
            if (exponent < is_inf)
                return std::ldexp((T)mantissa, exponent);
            else if (exponent == is_inf)
                return std::numeric_limits<T>::infinity();
            else if (exponent == is_ninf)
                return -std::numeric_limits<T>::infinity();
            else
                return std::numeric_limits<T>::quiet_NaN();
        }

    };

}

#endif // DLIB_FLOAT_DEtAILS_Hh_