SundanceFeketeQuadQuadrature.cpp

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#include "SundanceFeketeQuadQuadrature.hpp"
#include "SundanceOut.hpp"
#include "SundancePoint.hpp"
#include "SundanceGaussLobatto1D.hpp"
#include "PlayaTabs.hpp"

using namespace Sundance;
using namespace Teuchos;

extern "C"
{

/* LAPACK factorization */
void dgetrf_(const int* M, const int* N, double* A, const int* lda,
    const int* iPiv, int* info);

/* LAPACK inversion of factorized matrix */
void dgetri_(const int* n, double* a, const int* lda, const int* iPiv,
    double* work, const int* lwork, int* info);
}

void FeketeQuadQuadrature::getPoints(int order, Array<double>& wgt, Array<
    double>& x, Array<double>& y)
{
  int p = order + 3;
  p = p + (p % 2);
  int nNodes = p / 2;
  GaussLobatto1D rule(nNodes, 0.0, 1.0);
  Array<double> s = rule.nodes();
  Array<double> t = s;
  Array<double> w = rule.weights();
  int n = rule.nPoints();

  wgt.resize(n * n);
  x.resize(n * n);
  y.resize(n * n);

  int k = 0;
  for (int i = 0; i < n; i++)
  {
    double p = s[i];
    for (int j = 0; j < n; j++, k++)
    {
      double q = t[j]; //similar to the p value, caz we have quad
      x[k] = p;
      y[k] = q;
      wgt[k] = w[i] * w[j];
    }
  }
}

void FeketeQuadQuadrature::computeBasisCoeffs(const int order, Array<double>& basisCoeffs)
{
  // Get Fekete points of chosen order
  Array<Point> feketePts;

  Array<double> x;
  Array<double> y;
  Array<double> w;
  getPoints(order, w, x, y);
  int nFeketePts = w.length();
  feketePts.resize(nFeketePts);
  for (int i = 0; i < nFeketePts; i++)
    feketePts[i] = Point(x[i], y[i]);

  // We construct a Lagrange basis at feketePts (there are nFeketePts basis functions)
  basisCoeffs.resize(nFeketePts * nFeketePts);

  // Let's compute the coefficients:
  // Build Vandermonde matrix at Fekete points
  for (int n = 0; n < nFeketePts; n++)
  {
    // Set pointer to beginning of n-th row and determine values of
    // polynomials at n-th Fekete point
    double* start = &(basisCoeffs[n * nFeketePts]);
    evalPolynomials(nFeketePts, feketePts[n][0], feketePts[n][1], start);
  }

  // Invert Vandermonde matrix to obtain basis coefficients:
  // LAPACK error flag, array for switched rows, work array
  int lapack_err = 0;
  Array<int> pivot;
  pivot.resize(nFeketePts);
  Array<double> work;
  work.resize(1);
  int lwork = -1;

  // LU factorization
  ::dgetrf_(&nFeketePts, &nFeketePts, &(basisCoeffs[0]), &nFeketePts,
      &(pivot[0]), &lapack_err);

  TEUCHOS_TEST_FOR_EXCEPTION(
      lapack_err != 0,
      std::runtime_error,
      "FeketeQuadQuadrature::computeBasisCoeffs(): factorization of generalized Vandermonde matrix failed");

  // Determine work array size and invert factorized matrix
  ::dgetri_(&nFeketePts, &(basisCoeffs[0]), &nFeketePts, &(pivot[0]),
      &(work[0]), &lwork, &lapack_err);
  lwork = (int) work[0];
  work.resize(lwork);
  ::dgetri_(&nFeketePts, &(basisCoeffs[0]), &nFeketePts, &(pivot[0]),
      &(work[0]), &lwork, &lapack_err);

  TEUCHOS_TEST_FOR_EXCEPTION(
      lapack_err != 0,
      std::runtime_error,
      "FeketeQuadQuadrature::computeBasisCoeffs(): inversion of generalized Vandermonde matrix failed");
}

void FeketeQuadQuadrature::evalPolynomials(int nPts, double x,
    double y, double* resultPtr)
{
  // Calculate all (normalized and shifted) Legendre polynomials < order
  int i = 0;
  int order = (int) sqrt((double) nPts);

  double xLegendre  = 1.;
  double xLegendre1 = 0.;

  for (int k = 0; k < order; k++)
  {
    double yLegendre  = 1.;
    double yLegendre1 = 0.;
    double normxLegendre = sqrt( 2. * k + 1 ) * xLegendre;

    // 'i' implements a bijection polynomial index (k,l) onto basis index (i)
    for (int l = 0; l < order ; l++)
    {
      double normyLegendre = sqrt( 2. * l + 1 ) * yLegendre;

      // We will write order*order values
      resultPtr[i++] = normxLegendre * normyLegendre;

      double yLegendre2 = yLegendre1;
      yLegendre1 = yLegendre;
      yLegendre = ( (2 * l + 1) * (2 * y - 1) * yLegendre1 - l * yLegendre2 ) / ( l + 1 );
    }

    double xLegendre2 = xLegendre1;
    xLegendre1 = xLegendre;
    xLegendre = ( (2 * k + 1) * (2 * x - 1) * xLegendre1 - k * xLegendre2 ) / ( k + 1 );
  }
}

bool FeketeQuadQuadrature::test(int p)
{
  Array<double> w;
  Array<double> x;
  Array<double> y;

  getPoints(p, w, x, y);
  bool pass = true;
  for (int a = 0; a <= p; a++)
  {
    int bMax = p - a;
    for (int b = 0; b <= bMax; b++)
    {
      double sum = 0.0;
      for (int q = 0; q < w.length(); q++)
      {
        sum += w[q] * pow(x[q], (double) a) * pow(y[q], (double) b);
      }
      double err = fabs(sum - exact(a, b));
      bool localPass = err < 1.0e-14;
      pass = pass && localPass;
      if (!localPass)
      {
        fprintf(stderr,
            "order=%d m (%d, %d) q=%22.15g exact=%22.15g\n", p, a,
            b, sum, exact(a, b));
        std::cerr << "error = " << err << std::endl;
      }
    }
  }
  return pass;
}

double FeketeQuadQuadrature::exact(int a, int b)
{
  return 1.0 / (a + 1) / (b + 1);
}