//    boost asinh.hpp header file

//  (C) Copyright Eric Ford & Hubert Holin 2001.

//  Distributed under the Boost Software License, Version 1.0. (See

//  accompanying file LICENSE_1_0.txt or copy at

//  http://www.boost.org/LICENSE_1_0.txt)

// See http://www.boost.org for updates, documentation, and revision history.

#ifndef BOOST_ASINH_HPP

#define BOOST_ASINH_HPP

#include <cmath>

#include <limits>

#include <string>

#include <stdexcept>

#include <boost/config.hpp>

// This is the inverse of the hyperbolic sine function.

namespace boost

{

    namespace math

    {

#if defined(__GNUC__) && (__GNUC__ < 3)

        // gcc 2.x ignores function scope using declarations,

        // put them in the scope of the enclosing namespace instead:

        using    ::std::abs;

        using    ::std::sqrt;

        using    ::std::log;

        using    ::std::numeric_limits;

#endif

        template<typename T>

        inline T    asinh(const T x)

        {

            using    ::std::abs;

            using    ::std::sqrt;

            using    ::std::log;

            using    ::std::numeric_limits;

            T const            one = static_cast<T>(1);

            T const            two = static_cast<T>(2);

            static T const    taylor_2_bound = sqrt(numeric_limits<T>::epsilon());

            static T const    taylor_n_bound = sqrt(taylor_2_bound);

            static T const    upper_taylor_2_bound = one/taylor_2_bound;

            static T const    upper_taylor_n_bound = one/taylor_n_bound;

            if        (x >= +taylor_n_bound)

            {

                if        (x > upper_taylor_n_bound)

                {

                    if        (x > upper_taylor_2_bound)

                    {

                        // approximation by laurent series in 1/x at 0+ order from -1 to 0

                        return( log( x * two) );

                    }

                    else

                    {

                        // approximation by laurent series in 1/x at 0+ order from -1 to 1

                        return( log( x*two + (one/(x*two)) ) );

                    }

                }

                else

                {

                    return( log( x + sqrt(x*x+one) ) );

                }

            }

            else if    (x <= -taylor_n_bound)

            {

                return(-asinh(-x));

            }

            else

            {

                // approximation by taylor series in x at 0 up to order 2

                T    result = x;

                if    (abs(x) >= taylor_2_bound)

                {

                    T    x3 = x*x*x;

                    // approximation by taylor series in x at 0 up to order 4

                    result -= x3/static_cast<T>(6);

                }

                return(result);

            }

        }

    }

}

#endif /* BOOST_ASINH_HPP */