//  (C) Copyright John Maddock 2005.

//  Use, modification and distribution are subject to 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)

#ifndef BOOST_MATH_EXPM1_INCLUDED

#define BOOST_MATH_EXPM1_INCLUDED

#include <cmath>

#include <math.h> // platform's ::expm1

#include <boost/limits.hpp>

#include <boost/math/special_functions/detail/series.hpp>

#ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS

#  include <boost/static_assert.hpp>

#else

#  include <boost/assert.hpp>

#endif

#ifdef BOOST_NO_STDC_NAMESPACE

namespace std{ using ::exp; using ::fabs; }

#endif

namespace boost{ namespace math{

namespace detail{

//

// Functor expm1_series returns the next term in the Taylor series

// x^k / k!

// each time that operator() is invoked.

//

template <class T>

struct expm1_series

{

   typedef T result_type;

   expm1_series(T x)

      : k(0), m_x(x), m_term(1) {}

   T operator()()

   {

      ++k;

      m_term *= m_x;

      m_term /= k;

      return m_term; 

   }

   int count()const

   {

      return k;

   }

private:

   int k;

   const T m_x;

   T m_term;

   expm1_series(const expm1_series&);

   expm1_series& operator=(const expm1_series&);

};

} // namespace

//

// Algorithm expm1 is part of C99, but is not yet provided by many compilers.

//

// This version uses a Taylor series expansion for 0.5 > |x| > epsilon.

//

template <class T>

T expm1(T x)

{

#ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS

   BOOST_STATIC_ASSERT(::std::numeric_limits<T>::is_specialized);

#else

   BOOST_ASSERT(std::numeric_limits<T>::is_specialized);

#endif

   T a = std::fabs(x);

   if(a > T(0.5L))

      return std::exp(x) - T(1);

   if(a < std::numeric_limits<T>::epsilon())

      return x;

   detail::expm1_series<T> s(x);

   T result = detail::kahan_sum_series(s, std::numeric_limits<T>::digits + 2);

   return result;

}

#if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x564))

inline float expm1(float z)

{

   return expm1<float>(z);

}

inline double expm1(double z)

{

   return expm1<double>(z);

}

inline long double expm1(long double z)

{

   return expm1<long double>(z);

}

#endif

#ifdef expm1

#  ifndef BOOST_HAS_expm1

#     define BOOST_HAS_expm1

#  endif

#  undef expm1

#endif

#ifdef BOOST_HAS_EXPM1

#  if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901)

inline float expm1(float x){ return ::expm1f(x); }

inline long double expm1(long double x){ return ::expm1l(x); }

#else

inline float expm1(float x){ return ::expm1(x); }

#endif

inline double expm1(double x){ return ::expm1(x); }

#endif

} } // namespaces

#endif // BOOST_MATH_HYPOT_INCLUDED