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+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra. Eigen itself is part of the KDE project.
+//
+// Copyright (C) 2008 Gael Guennebaud <g.gael@free.fr>
+//
+// Eigen is free software; you can redistribute it and/or
+// modify it under the terms of the GNU Lesser General Public
+// License as published by the Free Software Foundation; either
+// version 3 of the License, or (at your option) any later version.
+//
+// Alternatively, you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation; either version 2 of
+// the License, or (at your option) any later version.
+//
+// Eigen is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License or the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License and a copy of the GNU General Public License along with
+// Eigen. If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef EIGEN_SELFADJOINTEIGENSOLVER_H
+#define EIGEN_SELFADJOINTEIGENSOLVER_H
+
+/** \class SelfAdjointEigenSolver
+  *
+  * \brief Eigen values/vectors solver for selfadjoint matrix
+  *
+  * \param MatrixType the type of the matrix of which we are computing the eigen decomposition
+  *
+  * \sa MatrixBase::eigenvalues(), class EigenSolver
+  */
+template<typename _MatrixType> class SelfAdjointEigenSolver
+{
+  public:
+
+    typedef _MatrixType MatrixType;
+    typedef typename MatrixType::Scalar Scalar;
+    typedef typename NumTraits<Scalar>::Real RealScalar;
+    typedef std::complex<RealScalar> Complex;
+//     typedef Matrix<RealScalar, MatrixType::ColsAtCompileTime, 1> EigenvalueType;
+    typedef Matrix<RealScalar, MatrixType::ColsAtCompileTime, 1> RealVectorType;
+    typedef Matrix<RealScalar, Dynamic, 1> RealVectorTypeX;
+
+    SelfAdjointEigenSolver(const MatrixType& matrix)
+      : m_eivec(matrix.rows(), matrix.cols()),
+        m_eivalues(matrix.cols())
+    {
+      compute(matrix);
+    }
+
+    void compute(const MatrixType& matrix);
+
+    MatrixType eigenvectors(void) const { return m_eivec; }
+
+    RealVectorType eigenvalues(void) const { return m_eivalues; }
+
+
+  protected:
+    MatrixType m_eivec;
+    RealVectorType m_eivalues;
+};
+
+// from Golub's "Matrix Computations", algorithm 5.1.3
+template<typename Scalar>
+static void ei_givens_rotation(Scalar a, Scalar b, Scalar& c, Scalar& s)
+{
+  if (b==0)
+  {
+    c = 1; s = 0;
+  }
+  else if (ei_abs(b)>ei_abs(a))
+  {
+    Scalar t = -a/b;
+    s = Scalar(1)/ei_sqrt(1+t*t);
+    c = s * t;
+  }
+  else
+  {
+    Scalar t = -b/a;
+    c = Scalar(1)/ei_sqrt(1+t*t);
+    s = c * t;
+  }
+}
+
+/** \internal
+  * Performs a QR step on a tridiagonal symmetric matrix represented as a
+  * pair of two vectors \a diag \a subdiag.
+  *
+  * \param matA the input selfadjoint matrix
+  * \param hCoeffs returned Householder coefficients
+  *
+  * For compilation efficiency reasons, this procedure does not use eigen expression
+  * for its arguments.
+  *
+  * Implemented from Golub's "Matrix Computations", algorithm 8.3.2:
+  * "implicit symmetric QR step with Wilkinson shift"
+  */
+template<typename Scalar>
+static void ei_tridiagonal_qr_step(Scalar* diag, Scalar* subdiag, int n)
+{
+  Scalar td = (diag[n-2] - diag[n-1])*0.5;
+  Scalar e2 = ei_abs2(subdiag[n-2]);
+  Scalar mu = diag[n-1] - e2 / (td + (td>0 ? 1 : -1) * ei_sqrt(td*td + e2));
+  Scalar x = diag[0] - mu;
+  Scalar z = subdiag[0];
+
+  for (int k = 0; k < n-1; ++k)
+  {
+    Scalar c, s;
+    ei_givens_rotation(x, z, c, s);
+
+    // do T = G' T G
+    Scalar sdk = s * diag[k] + c * subdiag[k];
+    Scalar dkp1 = s * subdiag[k] + c * diag[k+1];
+
+    diag[k] = c * (c * diag[k] - s * subdiag[k]) - s * (c * subdiag[k] - s * diag[k+1]);
+    diag[k+1] = s * sdk + c * dkp1;
+    subdiag[k] = c * sdk - s * dkp1;
+
+    if (k > 0)
+      subdiag[k - 1] = c * subdiag[k-1] - s * z;
+
+    x = subdiag[k];
+    z = -s * subdiag[k+1];
+
+    if (k < n - 2)
+      subdiag[k + 1] = c * subdiag[k+1];
+  }
+}
+
+template<typename MatrixType>
+void SelfAdjointEigenSolver<MatrixType>::compute(const MatrixType& matrix)
+{
+  assert(matrix.cols() == matrix.rows());
+  int n = matrix.cols();
+  m_eivalues.resize(n,1);
+  m_eivec = matrix;
+
+  Tridiagonalization<MatrixType> tridiag(m_eivec);
+  RealVectorType& diag = m_eivalues;
+  RealVectorType subdiag(n-1);
+  diag = tridiag.diagonal();
+  subdiag = tridiag.subDiagonal();
+
+  int end = n-1;
+  int start = 0;
+  while (end>0)
+  {
+    for (int i = start; i<end; ++i)
+      if (ei_isMuchSmallerThan(ei_abs(subdiag[i]),(ei_abs(diag[i])+ei_abs(diag[i+1]))))
+        subdiag[i] = 0;
+
+    // find the largest unreduced block
+    while (end>0 && subdiag[end-1]==0)
+      end--;
+    if (end<=0)
+      break;
+    start = end - 1;
+    while (start>0 && subdiag[start-1]!=0)
+      start--;
+
+    ei_tridiagonal_qr_step(&diag.coeffRef(start), &subdiag.coeffRef(start), end-start+1);
+  }
+
+  std::cout << "ei values = " << m_eivalues.transpose() << "\n\n";
+}
+
+#endif // EIGEN_SELFADJOINTEIGENSOLVER_H