#ifndef STAN_MATH_PRIM_SCAL_ERR_CHECK_2F1_CONVERGES_HPP
#define STAN_MATH_PRIM_SCAL_ERR_CHECK_2F1_CONVERGES_HPP

#include <stan/math/prim/scal/err/check_not_nan.hpp>
#include <stan/math/prim/scal/fun/is_nonpositive_integer.hpp>
#include <stan/math/prim/scal/fun/value_of_rec.hpp>
#include <cmath>
#include <stdexcept>
#include <sstream>
#include <limits>

namespace stan {
  namespace math {

    /**
     * Check if the hypergeometric function (2F1) called with
     * supplied arguments will converge, assuming arguments are
     * finite values.
     *
     * @tparam T_a1 Type of a1
     * @tparam T_a2 Type of a2
     * @tparam T_b1 Type of b1
     * @tparam T_z Type of z
     *
     * @param function Name of function ultimately relying on 2F1 (for error
     *   messages)
     * @param a1 Variable to check
     * @param a2 Variable to check
     * @param b1 Variable to check
     * @param z Variable to check
     *
     * @throw <code>domain_error</code> if 2F1(a1, a2, b1, z)
     *   does not meet convergence conditions, or if any coefficient is NaN.
     */
    template <typename T_a1, typename T_a2, typename T_b1, typename T_z>
    inline void check_2F1_converges(const char* function,
      const T_a1& a1, const T_a2& a2, const T_b1& b1, const T_z& z
    ) {
      using std::floor;
      using std::fabs;

      check_not_nan("check_3F2_converges", "a1", a1);
      check_not_nan("check_3F2_converges", "a2", a2);
      check_not_nan("check_3F2_converges", "b1", b1);
      check_not_nan("check_3F2_converges", "z", z);

      int num_terms = 0;
      bool is_polynomial = false;

      if (is_nonpositive_integer(a1) && fabs(a1) >= num_terms) {
        is_polynomial = true;
        num_terms = floor(fabs(value_of_rec(a1)));
      }
      if (is_nonpositive_integer(a2) && fabs(a2) >= num_terms) {
        is_polynomial = true;
        num_terms = floor(fabs(value_of_rec(a2)));
      }

      bool is_undefined = is_nonpositive_integer(b1) && fabs(b1) <= num_terms;

      if (is_polynomial && !is_undefined) return;
      if (fabs(z) < 1.0 && !is_undefined) return;
      if (fabs(z) == 1.0 && !is_undefined && b1 > a1 + a2) return;

      std::stringstream msg;
      msg << "called from function '" << function << "', "
          << "hypergeometric function 2F1 does not meet convergence "
          << "conditions with given arguments. "
          << "a1: " << a1 << ", a2: " << a2 << ", "
          << "b1: " << b1 << ", z: " << z;
      throw std::domain_error(msg.str());
    }

  }
}
#endif
