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Filename : bessel_jy_derivatives_series.hpp
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//  Copyright (c) 2013 Anton Bikineev
//  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_BESSEL_JY_DERIVATIVES_SERIES_HPP
#define BOOST_MATH_BESSEL_JY_DERIVATIVES_SERIES_HPP

#ifdef _MSC_VER
#pragma once
#endif

namespace boost{ namespace math{ namespace detail{

template <class T, class Policy>
struct bessel_j_derivative_small_z_series_term
{
   typedef T result_type;

   bessel_j_derivative_small_z_series_term(T v_, T x)
      : N(0), v(v_), term(1), mult(x / 2)
   {
      mult *= -mult;
      // iterate if v == 0; otherwise result of
      // first term is 0 and tools::sum_series stops
      if (v == 0)
         iterate();
   }
   T operator()()
   {
      T r = term * (v + 2 * N);
      iterate();
      return r;
   }
private:
   void iterate()
   {
      ++N;
      term *= mult / (N * (N + v));
   }
   unsigned N;
   T v;
   T term;
   T mult;
};
//
// Series evaluation for BesselJ'(v, z) as z -> 0.
// It's derivative of http://functions.wolfram.com/Bessel-TypeFunctions/BesselJ/06/01/04/01/01/0003/
// Converges rapidly for all z << v.
//
template <class T, class Policy>
inline T bessel_j_derivative_small_z_series(T v, T x, const Policy& pol)
{
   BOOST_MATH_STD_USING
   T prefix;
   if (v < boost::math::max_factorial<T>::value)
   {
      prefix = pow(x / 2, v - 1) / 2 / boost::math::tgamma(v + 1, pol);
   }
   else
   {
      prefix = (v - 1) * log(x / 2) - constants::ln_two<T>() - boost::math::lgamma(v + 1, pol);
      prefix = exp(prefix);
   }
   if (0 == prefix)
      return prefix;

   bessel_j_derivative_small_z_series_term<T, Policy> s(v, x);
   boost::uintmax_t max_iter = boost::math::policies::get_max_series_iterations<Policy>();
#if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x582))
   T zero = 0;
   T result = boost::math::tools::sum_series(s, boost::math::policies::get_epsilon<T, Policy>(), max_iter, zero);
#else
   T result = boost::math::tools::sum_series(s, boost::math::policies::get_epsilon<T, Policy>(), max_iter);
#endif
   boost::math::policies::check_series_iterations<T>("boost::math::bessel_j_derivative_small_z_series<%1%>(%1%,%1%)", max_iter, pol);
   return prefix * result;
}

template <class T, class Policy>
struct bessel_y_derivative_small_z_series_term_a
{
   typedef T result_type;

   bessel_y_derivative_small_z_series_term_a(T v_, T x)
      : N(0), v(v_)
   {
      mult = x / 2;
      mult *= -mult;
      term = 1;
   }
   T operator()()
   {
      T r = term * (-v + 2 * N);
      ++N;
      term *= mult / (N * (N - v));
      return r;
   }
private:
   unsigned N;
   T v;
   T mult;
   T term;
};

template <class T, class Policy>
struct bessel_y_derivative_small_z_series_term_b
{
   typedef T result_type;

   bessel_y_derivative_small_z_series_term_b(T v_, T x)
      : N(0), v(v_)
   {
      mult = x / 2;
      mult *= -mult;
      term = 1;
   }
   T operator()()
   {
      T r = term * (v + 2 * N);
      ++N;
      term *= mult / (N * (N + v));
      return r;
   }
private:
   unsigned N;
   T v;
   T mult;
   T term;
};
//
// Series form for BesselY' as z -> 0,
// It's derivative of http://functions.wolfram.com/Bessel-TypeFunctions/BesselY/06/01/04/01/01/0003/
// This series is only useful when the second term is small compared to the first
// otherwise we get catastrophic cancellation errors.
//
// Approximating tgamma(v) by v^v, and assuming |tgamma(-z)| < eps we end up requiring:
// eps/2 * v^v(x/2)^-v > (x/2)^v or log(eps/2) > v log((x/2)^2/v)
//
template <class T, class Policy>
inline T bessel_y_derivative_small_z_series(T v, T x, const Policy& pol)
{
   BOOST_MATH_STD_USING
   static const char* function = "bessel_y_derivative_small_z_series<%1%>(%1%,%1%)";
   T prefix;
   T gam;
   T p = log(x / 2);
   T scale = 1;
   bool need_logs = (v >= boost::math::max_factorial<T>::value) || (boost::math::tools::log_max_value<T>() / v < fabs(p));
   if (!need_logs)
   {
      gam = boost::math::tgamma(v, pol);
      p = pow(x / 2, v + 1) * 2;
      if (boost::math::tools::max_value<T>() * p < gam)
      {
         scale /= gam;
         gam = 1;
         if (boost::math::tools::max_value<T>() * p < gam)
         {
            // This term will overflow to -INF, when combined with the series below it becomes +INF:
            return boost::math::policies::raise_overflow_error<T>(function, 0, pol);
         }
      }
      prefix = -gam / (boost::math::constants::pi<T>() * p);
   }
   else
   {
      gam = boost::math::lgamma(v, pol);
      p = (v + 1) * p + constants::ln_two<T>();
      prefix = gam - log(boost::math::constants::pi<T>()) - p;
      if (boost::math::tools::log_max_value<T>() < prefix)
      {
         prefix -= log(boost::math::tools::max_value<T>() / 4);
         scale /= (boost::math::tools::max_value<T>() / 4);
         if (boost::math::tools::log_max_value<T>() < prefix)
         {
            return boost::math::policies::raise_overflow_error<T>(function, 0, pol);
         }
      }
      prefix = -exp(prefix);
   }
   bessel_y_derivative_small_z_series_term_a<T, Policy> s(v, x);
   boost::uintmax_t max_iter = boost::math::policies::get_max_series_iterations<Policy>();
#if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x582))
   T zero = 0;
   T result = boost::math::tools::sum_series(s, boost::math::policies::get_epsilon<T, Policy>(), max_iter, zero);
#else
   T result = boost::math::tools::sum_series(s, boost::math::policies::get_epsilon<T, Policy>(), max_iter);
#endif
   boost::math::policies::check_series_iterations<T>("boost::math::bessel_y_derivative_small_z_series<%1%>(%1%,%1%)", max_iter, pol);
   result *= prefix;

   p = pow(x / 2, v - 1) / 2;
   if (!need_logs)
   {
      prefix = boost::math::tgamma(-v, pol) * boost::math::cos_pi(v) * p / boost::math::constants::pi<T>();
   }
   else
   {
      int sgn;
      prefix = boost::math::lgamma(-v, &sgn, pol) + (v - 1) * log(x / 2) - constants::ln_two<T>();
      prefix = exp(prefix) * sgn / boost::math::constants::pi<T>();
   }
   bessel_y_derivative_small_z_series_term_b<T, Policy> s2(v, x);
   max_iter = boost::math::policies::get_max_series_iterations<Policy>();
#if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x582))
   T b = boost::math::tools::sum_series(s2, boost::math::policies::get_epsilon<T, Policy>(), max_iter, zero);
#else
   T b = boost::math::tools::sum_series(s2, boost::math::policies::get_epsilon<T, Policy>(), max_iter);
#endif
   result += scale * prefix * b;
   return result;
}

// Calculating of BesselY'(v,x) with small x (x < epsilon) and integer x using derivatives
// of formulas in http://functions.wolfram.com/Bessel-TypeFunctions/BesselY/06/01/04/01/02/
// seems to lose precision. Instead using linear combination of regular Bessel is preferred.

}}} // namespaces

#endif // BOOST_MATH_BESSEL_JY_DERIVATVIES_SERIES_HPP