# include <cppad/cppad.hpp>
# include <limits>

bool CondExp(void)
{     bool ok = true;

     using CppAD::isnan;
     using CppAD::AD;
     using CppAD::NearEqual;
     using CppAD::log;
     double eps  = 100. * CppAD::numeric_limits<double>::epsilon();

     // domain space vector
     size_t n = 5;
     CPPAD_TESTVECTOR(AD<double>) ax(n);
     size_t j;
     for(j = 0; j < n; j++)
          ax[j] = 1.;

     // declare independent variables and start tape recording
     CppAD::Independent(ax);

     AD<double> asum  = 0.;
     AD<double> azero = 0.;
     for(j = 0; j < n; j++)
     {     // if x_j > 0, add x_j * log( x_j ) to the sum
          asum += CppAD::CondExpGt(ax[j], azero, ax[j] * log(ax[j]), azero);
     }

     // range space vector
     size_t m = 1;
     CPPAD_TESTVECTOR(AD<double>) ay(m);
     ay[0] = asum;

     // create f: x -> ay and stop tape recording
     CppAD::ADFun<double> f(ax, ay);

     // vectors for arguments to the function object f
     CPPAD_TESTVECTOR(double) x(n);   // argument values
     CPPAD_TESTVECTOR(double) y(m);   // function values
     CPPAD_TESTVECTOR(double) w(m);   // function weights
     CPPAD_TESTVECTOR(double) dw(n);  // derivative of weighted function

     // a case where x[j] > 0 for all j
     double check  = 0.;
     for(j = 0; j < n; j++)
     {     x[j]   = double(j + 1);
          check += x[j] * log( x[j] );
     }

     // function value
     y  = f.Forward(0, x);
     ok &= NearEqual(y[0], check, eps, eps);

     // compute derivative of y[0]
     w[0] = 1.;
     dw   = f.Reverse(1, w);
     for(j = 0; j < n; j++)
          ok &= NearEqual(dw[j], log(x[j]) + 1., eps, eps);

     // a case where x[3] is equal to zero
     check -= x[3] * log( x[3] );
     x[3]   = 0.;

     // function value
     y   = f.Forward(0, x);
     ok &= NearEqual(y[0], check, eps, eps);

     // check derivative of y[0]
     f.check_for_nan(false);
     w[0] = 1.;
     dw   = f.Reverse(1, w);
     for(j = 0; j < n; j++)
     {     if( x[j] > 0 )
               ok &= NearEqual(dw[j], log(x[j]) + 1., eps, eps);
          else
          {     // Note that in case where dw has type AD<double> and is a variable
               // this dw[j] can be nan (zero times nan is not zero).
               ok &= NearEqual(dw[j], 0.0, eps, eps);
          }
     }

     return ok;
}