c_ghk.cpp

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#include "fvar.hpp"

double ghk(const dvector& lower,const dvector& upper,const dmatrix& Sigma,
  const dmatrix& eps)
{
  int m=eps.indexmax();
  int n=lower.indexmax();
  double ssum=0.0;
  dmatrix ch=choleski_decomp(Sigma);
  dvector l(1,n);
  dvector u(1,n);

  for (int k=1;k<=m;k++)
  {
    double weight=1.0;
    l=lower;
    u=upper;
    for (int j=1;j<=n;j++)
    {
      l(j)/=ch(j,j);
      u(j)/=ch(j,j);
      double Phiu=cumd_norm(u(j));
      double Phil=cumd_norm(l(j));
      weight*=Phiu-Phil;
      double eta=inv_cumd_norm((Phiu-Phil)*eps(k,j)+Phil);
      for (int i=j+1;i<=n;i++)
      {
        double tmp=ch(i,j)*eta;
        l(i)-=tmp;
        u(i)-=tmp;
      }
    }
    ssum+=weight;
  }
  return ssum/m;
}

void ghk_test(const dmatrix& eps,int i)
{
  if (i<eps.indexmin())
  {
    cerr << "Index too low in function ghk -- min is "
         << eps.indexmin() << " you have " << i << endl;
    ad_exit(21);
  }
  else  if (i>eps.indexmax())
  {
    cerr << "Index too high in function ghk -- max is "
         << eps.indexmax() << " you have " << i << endl;
    ad_exit(21);
  }
}

double ghk(const dvector& lower,const dvector& upper,const dmatrix& Sigma,
  const dmatrix& eps,int _i)
{
  int n=lower.indexmax();
  dmatrix ch=choleski_decomp(Sigma);
  dvector l(1,n);
  dvector u(1,n);

  ghk_test(eps,_i); // test for valid i range
  double weight=1.0;
  int k=_i;
  {
    l=lower;
    u=upper;
    for (int j=1;j<=n;j++)
    {
      l(j)/=ch(j,j);
      u(j)/=ch(j,j);
      double Phiu=cumd_norm(u(j));
      double Phil=cumd_norm(l(j));
      weight*=Phiu-Phil;
      double eta=inv_cumd_norm((Phiu-Phil)*eps(k,j)+Phil);
      for (int i=j+1;i<=n;i++)
      {
        double tmp=ch(i,j)*eta;
        l(i)-=tmp;
        u(i)-=tmp;
      }
    }
  }
  return weight;
}