/* expfit.c -- model functions for exponential + background */

struct data {
  size_t n;
  double * t;
  double * y;
};

int
gaussian_f (const gsl_vector * x, void *data, gsl_vector * f)
{
  size_t n = ((struct data *)data)->n;
  double *t = ((struct data *)data)->t;
  double *y = ((struct data *)data)->y;

  double A1 = gsl_vector_get(x, 0);
  double mu1 = gsl_vector_get(x, 1);
  double sigma1 = gsl_vector_get(x, 2);
  double A2 = gsl_vector_get(x, 3);
  double mu2 = gsl_vector_get(x, 4);
  double sigma2 = gsl_vector_get(x, 5);
  double A3 = gsl_vector_get(x, 6);
  double mu3 = gsl_vector_get(x, 7);
  double sigma3 = gsl_vector_get(x, 8);
  size_t i;

  for (i = 0; i < n; ++i)
    {
      double ti = t[i];
      double yi = y[i];
      double term1 = (ti - mu1) / sigma1;
      double term2 = (ti - mu2) / sigma2;
      double term3 = (ti - mu3) / sigma3;
      double fi = 0.0;
      
      fi += A1 * exp(-0.5*term1*term1);
      fi += A2 * exp(-0.5*term2*term2);
      fi += A3 * exp(-0.5*term3*term3);

      gsl_vector_set(f, i, fi - yi);
    }

  return GSL_SUCCESS;
}