#include "StatTools/Funct/Math.h"
#include <cmath>

namespace Funct {

double factorial( unsigned int n )
{
  if ( n <= 1 ) return 1;
  double x = 1;
  for ( unsigned int i = 2; i < n; i++ ) x *= i;
    return x;
}

#ifdef __STRICT_ANSI__
double erf( double x )
{
  static const double epsilon = 1.e-06;
  double sgn = 1;
  double fac = 1;
  double xx = x;
  int k = 1;
  double sum = 0;
  double delta = 100;
  for( int n = 0; fabs( delta ) > epsilon; n++ )
  {
    delta = sgn * xx / fac / k;
    sum += delta;
    k += 2;
    sgn = - sgn;
    fac *= ( n + 1 );
    xx *= ( x*x );
  }
  return sum * M_2_SQRTPI;      
}
#endif

double erfi( double x )
{
  static const double epsilon = 1.e-06;
  double fac = 1;
  double xx = x;
  int k = 1;
  double sum = 0;
  double delta = 100;
  for( int n = 0; fabs( delta ) > epsilon; n++ )
  {
    delta = xx / fac / k;
    sum += delta;
    k += 2;
    fac *= ( n + 1 );
    xx *= ( x*x );
  }
  return sum * M_2_SQRTPI;
}

}