#ifndef STAN_MATH_PRIM_SCAL_PROB_RAYLEIGH_CCDF_LOG_HPP
#define STAN_MATH_PRIM_SCAL_PROB_RAYLEIGH_CCDF_LOG_HPP

#include <stan/math/prim/scal/meta/OperandsAndPartials.hpp>
#include <stan/math/prim/scal/err/check_consistent_sizes.hpp>
#include <stan/math/prim/scal/err/check_nonnegative.hpp>
#include <stan/math/prim/scal/err/check_not_nan.hpp>
#include <stan/math/prim/scal/err/check_positive.hpp>
#include <stan/math/prim/scal/fun/log1m.hpp>
#include <stan/math/prim/scal/fun/square.hpp>
#include <stan/math/prim/scal/fun/value_of.hpp>
#include <stan/math/prim/scal/meta/length.hpp>
#include <stan/math/prim/scal/meta/is_constant_struct.hpp>
#include <stan/math/prim/scal/meta/partials_return_type.hpp>
#include <stan/math/prim/scal/fun/constants.hpp>
#include <stan/math/prim/scal/meta/include_summand.hpp>
#include <stan/math/prim/scal/meta/VectorBuilder.hpp>
#include <boost/random/uniform_real_distribution.hpp>
#include <boost/random/variate_generator.hpp>

namespace stan {

  namespace math {

    template <typename T_y, typename T_scale>
    typename return_type<T_y, T_scale>::type
    rayleigh_ccdf_log(const T_y& y, const T_scale& sigma) {
      static const char* function("stan::math::rayleigh_ccdf_log");
      typedef typename stan::partials_return_type<T_y, T_scale>::type
        T_partials_return;

      using stan::math::check_nonnegative;
      using stan::math::check_positive;
      using stan::math::check_not_nan;
      using stan::math::check_consistent_sizes;
      using stan::math::include_summand;
      using stan::is_constant_struct;
      using stan::math::square;
      using stan::math::value_of;

      T_partials_return ccdf_log(0.0);

      // check if any vectors are zero length
      if (!(stan::length(y) && stan::length(sigma)))
        return ccdf_log;

      check_not_nan(function, "Random variable", y);
      check_nonnegative(function, "Random variable", y);
      check_not_nan(function, "Scale parameter", sigma);
      check_positive(function, "Scale parameter", sigma);
      check_consistent_sizes(function,
                             "Random variable", y,
                             "Scale parameter", sigma);


      // set up template expressions wrapping scalars into vector views
      OperandsAndPartials<T_y, T_scale> operands_and_partials(y, sigma);

      VectorView<const T_y> y_vec(y);
      VectorView<const T_scale> sigma_vec(sigma);
      size_t N = max_size(y, sigma);

      VectorBuilder<true, T_partials_return, T_scale> inv_sigma(length(sigma));
      for (size_t i = 0; i < length(sigma); i++) {
        inv_sigma[i] = 1.0 / value_of(sigma_vec[i]);
      }

      for (size_t n = 0; n < N; n++) {
        const T_partials_return y_dbl = value_of(y_vec[n]);
        const T_partials_return y_sqr = y_dbl * y_dbl;
        const T_partials_return inv_sigma_sqr = inv_sigma[n] * inv_sigma[n];

        if (include_summand<false, T_y, T_scale>::value)
          ccdf_log += -0.5 * y_sqr * inv_sigma_sqr;

        if (!is_constant_struct<T_y>::value)
          operands_and_partials.d_x1[n] -= y_dbl * inv_sigma_sqr;
        if (!is_constant_struct<T_scale>::value)
          operands_and_partials.d_x2[n] += y_sqr * inv_sigma_sqr
            * inv_sigma[n];
      }

      return operands_and_partials.value(ccdf_log);
    }
  }
}
#endif
