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2025.09.22_cpp_with_eigen_package/Eigen/src/Householder/BlockHouseholder.h

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+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2010 Vincent Lejeune
+// Copyright (C) 2010 Gael Guennebaud <gael.guennebaud@inria.fr>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_BLOCK_HOUSEHOLDER_H
+#define EIGEN_BLOCK_HOUSEHOLDER_H
+
+// This file contains some helper function to deal with block householder reflectors
+
+// IWYU pragma: private
+#include "./InternalHeaderCheck.h"
+
+namespace Eigen {
+
+namespace internal {
+
+/** \internal */
+// template<typename TriangularFactorType,typename VectorsType,typename CoeffsType>
+// void make_block_householder_triangular_factor(TriangularFactorType& triFactor, const VectorsType& vectors, const
+// CoeffsType& hCoeffs)
+// {
+//   typedef typename VectorsType::Scalar Scalar;
+//   const Index nbVecs = vectors.cols();
+//   eigen_assert(triFactor.rows() == nbVecs && triFactor.cols() == nbVecs && vectors.rows()>=nbVecs);
+//
+//   for(Index i = 0; i < nbVecs; i++)
+//   {
+//     Index rs = vectors.rows() - i;
+//     // Warning, note that hCoeffs may alias with vectors.
+//     // It is then necessary to copy it before modifying vectors(i,i).
+//     typename CoeffsType::Scalar h = hCoeffs(i);
+//     // This hack permits to pass through nested Block<> and Transpose<> expressions.
+//     Scalar *Vii_ptr = const_cast<Scalar*>(vectors.data() + vectors.outerStride()*i + vectors.innerStride()*i);
+//     Scalar Vii = *Vii_ptr;
+//     *Vii_ptr = Scalar(1);
+//     triFactor.col(i).head(i).noalias() = -h * vectors.block(i, 0, rs, i).adjoint()
+//                                        * vectors.col(i).tail(rs);
+//     *Vii_ptr = Vii;
+//     // FIXME add .noalias() once the triangular product can work inplace
+//     triFactor.col(i).head(i) = triFactor.block(0,0,i,i).template triangularView<Upper>()
+//                              * triFactor.col(i).head(i);
+//     triFactor(i,i) = hCoeffs(i);
+//   }
+// }
+
+/** \internal */
+// This variant avoid modifications in vectors
+template <typename TriangularFactorType, typename VectorsType, typename CoeffsType>
+void make_block_householder_triangular_factor(TriangularFactorType& triFactor, const VectorsType& vectors,
+                                              const CoeffsType& hCoeffs) {
+  const Index nbVecs = vectors.cols();
+  eigen_assert(triFactor.rows() == nbVecs && triFactor.cols() == nbVecs && vectors.rows() >= nbVecs);
+
+  for (Index i = nbVecs - 1; i >= 0; --i) {
+    Index rs = vectors.rows() - i - 1;
+    Index rt = nbVecs - i - 1;
+
+    if (rt > 0) {
+      triFactor.row(i).tail(rt).noalias() = -hCoeffs(i) * vectors.col(i).tail(rs).adjoint() *
+                                            vectors.bottomRightCorner(rs, rt).template triangularView<UnitLower>();
+
+      // FIXME use the following line with .noalias() once the triangular product can work inplace
+      // triFactor.row(i).tail(rt) = triFactor.row(i).tail(rt) * triFactor.bottomRightCorner(rt,rt).template
+      // triangularView<Upper>();
+      for (Index j = nbVecs - 1; j > i; --j) {
+        typename TriangularFactorType::Scalar z = triFactor(i, j);
+        triFactor(i, j) = z * triFactor(j, j);
+        if (nbVecs - j - 1 > 0) triFactor.row(i).tail(nbVecs - j - 1) += z * triFactor.row(j).tail(nbVecs - j - 1);
+      }
+    }
+    triFactor(i, i) = hCoeffs(i);
+  }
+}
+
+/** \internal
+ * if forward then perform   mat = H0 * H1 * H2 * mat
+ * otherwise perform         mat = H2 * H1 * H0 * mat
+ */
+template <typename MatrixType, typename VectorsType, typename CoeffsType>
+void apply_block_householder_on_the_left(MatrixType& mat, const VectorsType& vectors, const CoeffsType& hCoeffs,
+                                         bool forward) {
+  enum { TFactorSize = VectorsType::ColsAtCompileTime };
+  Index nbVecs = vectors.cols();
+  Matrix<typename MatrixType::Scalar, TFactorSize, TFactorSize, RowMajor> T(nbVecs, nbVecs);
+
+  if (forward)
+    make_block_householder_triangular_factor(T, vectors, hCoeffs);
+  else
+    make_block_householder_triangular_factor(T, vectors, hCoeffs.conjugate());
+  const TriangularView<const VectorsType, UnitLower> V(vectors);
+
+  // A -= V T V^* A
+  Matrix<typename MatrixType::Scalar, VectorsType::ColsAtCompileTime, MatrixType::ColsAtCompileTime,
+         (VectorsType::MaxColsAtCompileTime == 1 && MatrixType::MaxColsAtCompileTime != 1) ? RowMajor : ColMajor,
+         VectorsType::MaxColsAtCompileTime, MatrixType::MaxColsAtCompileTime>
+      tmp = V.adjoint() * mat;
+  // FIXME add .noalias() once the triangular product can work inplace
+  if (forward)
+    tmp = T.template triangularView<Upper>() * tmp;
+  else
+    tmp = T.template triangularView<Upper>().adjoint() * tmp;
+  mat.noalias() -= V * tmp;
+}
+
+}  // end namespace internal
+
+}  // end namespace Eigen
+
+#endif  // EIGEN_BLOCK_HOUSEHOLDER_H

2025.09.22_cpp_with_eigen_package/Eigen/src/Householder/Householder.h

@@ -0,0 +1,155 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2010 Benoit Jacob <jacob.benoit.1@gmail.com>
+// Copyright (C) 2009 Gael Guennebaud <gael.guennebaud@inria.fr>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_HOUSEHOLDER_H
+#define EIGEN_HOUSEHOLDER_H
+
+// IWYU pragma: private
+#include "./InternalHeaderCheck.h"
+
+namespace Eigen {
+
+namespace internal {
+template <int n>
+struct decrement_size {
+  enum { ret = n == Dynamic ? n : n - 1 };
+};
+}  // namespace internal
+
+/** Computes the elementary reflector H such that:
+ * \f$ H *this = [ beta 0 ... 0]^T \f$
+ * where the transformation H is:
+ * \f$ H = I - tau v v^*\f$
+ * and the vector v is:
+ * \f$ v^T = [1 essential^T] \f$
+ *
+ * The essential part of the vector \c v is stored in *this.
+ *
+ * On output:
+ * \param tau the scaling factor of the Householder transformation
+ * \param beta the result of H * \c *this
+ *
+ * \sa MatrixBase::makeHouseholder(), MatrixBase::applyHouseholderOnTheLeft(),
+ *     MatrixBase::applyHouseholderOnTheRight()
+ */
+template <typename Derived>
+EIGEN_DEVICE_FUNC void MatrixBase<Derived>::makeHouseholderInPlace(Scalar& tau, RealScalar& beta) {
+  VectorBlock<Derived, internal::decrement_size<Base::SizeAtCompileTime>::ret> essentialPart(derived(), 1, size() - 1);
+  makeHouseholder(essentialPart, tau, beta);
+}
+
+/** Computes the elementary reflector H such that:
+ * \f$ H *this = [ beta 0 ... 0]^T \f$
+ * where the transformation H is:
+ * \f$ H = I - tau v v^*\f$
+ * and the vector v is:
+ * \f$ v^T = [1 essential^T] \f$
+ *
+ * On output:
+ * \param essential the essential part of the vector \c v
+ * \param tau the scaling factor of the Householder transformation
+ * \param beta the result of H * \c *this
+ *
+ * \sa MatrixBase::makeHouseholderInPlace(), MatrixBase::applyHouseholderOnTheLeft(),
+ *     MatrixBase::applyHouseholderOnTheRight()
+ */
+template <typename Derived>
+template <typename EssentialPart>
+EIGEN_DEVICE_FUNC void MatrixBase<Derived>::makeHouseholder(EssentialPart& essential, Scalar& tau,
+                                                            RealScalar& beta) const {
+  using numext::conj;
+  using numext::sqrt;
+
+  EIGEN_STATIC_ASSERT_VECTOR_ONLY(EssentialPart)
+  VectorBlock<const Derived, EssentialPart::SizeAtCompileTime> tail(derived(), 1, size() - 1);
+
+  RealScalar tailSqNorm = size() == 1 ? RealScalar(0) : tail.squaredNorm();
+  Scalar c0 = coeff(0);
+  const RealScalar tol = (std::numeric_limits<RealScalar>::min)();
+
+  if (tailSqNorm <= tol && numext::abs2(numext::imag(c0)) <= tol) {
+    tau = RealScalar(0);
+    beta = numext::real(c0);
+    essential.setZero();
+  } else {
+    beta = sqrt(numext::abs2(c0) + tailSqNorm);
+    if (numext::real(c0) >= RealScalar(0)) beta = -beta;
+    essential = tail / (c0 - beta);
+    tau = conj((beta - c0) / beta);
+  }
+}
+
+/** Apply the elementary reflector H given by
+ * \f$ H = I - tau v v^*\f$
+ * with
+ * \f$ v^T = [1 essential^T] \f$
+ * from the left to a vector or matrix.
+ *
+ * On input:
+ * \param essential the essential part of the vector \c v
+ * \param tau the scaling factor of the Householder transformation
+ * \param workspace a pointer to working space with at least
+ *                  this->cols() entries
+ *
+ * \sa MatrixBase::makeHouseholder(), MatrixBase::makeHouseholderInPlace(),
+ *     MatrixBase::applyHouseholderOnTheRight()
+ */
+template <typename Derived>
+template <typename EssentialPart>
+EIGEN_DEVICE_FUNC void MatrixBase<Derived>::applyHouseholderOnTheLeft(const EssentialPart& essential, const Scalar& tau,
+                                                                      Scalar* workspace) {
+  if (rows() == 1) {
+    *this *= Scalar(1) - tau;
+  } else if (!numext::is_exactly_zero(tau)) {
+    Map<typename internal::plain_row_type<PlainObject>::type> tmp(workspace, cols());
+    Block<Derived, EssentialPart::SizeAtCompileTime, Derived::ColsAtCompileTime> bottom(derived(), 1, 0, rows() - 1,
+                                                                                        cols());
+    tmp.noalias() = essential.adjoint() * bottom;
+    tmp += this->row(0);
+    this->row(0) -= tau * tmp;
+    bottom.noalias() -= tau * essential * tmp;
+  }
+}
+
+/** Apply the elementary reflector H given by
+ * \f$ H = I - tau v v^*\f$
+ * with
+ * \f$ v^T = [1 essential^T] \f$
+ * from the right to a vector or matrix.
+ *
+ * On input:
+ * \param essential the essential part of the vector \c v
+ * \param tau the scaling factor of the Householder transformation
+ * \param workspace a pointer to working space with at least
+ *                  this->rows() entries
+ *
+ * \sa MatrixBase::makeHouseholder(), MatrixBase::makeHouseholderInPlace(),
+ *     MatrixBase::applyHouseholderOnTheLeft()
+ */
+template <typename Derived>
+template <typename EssentialPart>
+EIGEN_DEVICE_FUNC void MatrixBase<Derived>::applyHouseholderOnTheRight(const EssentialPart& essential,
+                                                                       const Scalar& tau, Scalar* workspace) {
+  if (cols() == 1) {
+    *this *= Scalar(1) - tau;
+  } else if (!numext::is_exactly_zero(tau)) {
+    Map<typename internal::plain_col_type<PlainObject>::type> tmp(workspace, rows());
+    Block<Derived, Derived::RowsAtCompileTime, EssentialPart::SizeAtCompileTime> right(derived(), 0, 1, rows(),
+                                                                                       cols() - 1);
+    tmp.noalias() = right * essential;
+    tmp += this->col(0);
+    this->col(0) -= tau * tmp;
+    right.noalias() -= tau * tmp * essential.adjoint();
+  }
+}
+
+}  // end namespace Eigen
+
+#endif  // EIGEN_HOUSEHOLDER_H

2025.09.22_cpp_with_eigen_package/Eigen/src/Householder/HouseholderSequence.h

@@ -0,0 +1,504 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2009 Gael Guennebaud <gael.guennebaud@inria.fr>
+// Copyright (C) 2010 Benoit Jacob <jacob.benoit.1@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_HOUSEHOLDER_SEQUENCE_H
+#define EIGEN_HOUSEHOLDER_SEQUENCE_H
+
+// IWYU pragma: private
+#include "./InternalHeaderCheck.h"
+
+namespace Eigen {
+
+/** \ingroup Householder_Module
+ * \householder_module
+ * \class HouseholderSequence
+ * \brief Sequence of Householder reflections acting on subspaces with decreasing size
+ * \tparam VectorsType type of matrix containing the Householder vectors
+ * \tparam CoeffsType  type of vector containing the Householder coefficients
+ * \tparam Side        either OnTheLeft (the default) or OnTheRight
+ *
+ * This class represents a product sequence of Householder reflections where the first Householder reflection
+ * acts on the whole space, the second Householder reflection leaves the one-dimensional subspace spanned by
+ * the first unit vector invariant, the third Householder reflection leaves the two-dimensional subspace
+ * spanned by the first two unit vectors invariant, and so on up to the last reflection which leaves all but
+ * one dimensions invariant and acts only on the last dimension. Such sequences of Householder reflections
+ * are used in several algorithms to zero out certain parts of a matrix. Indeed, the methods
+ * HessenbergDecomposition::matrixQ(), Tridiagonalization::matrixQ(), HouseholderQR::householderQ(),
+ * and ColPivHouseholderQR::householderQ() all return a %HouseholderSequence.
+ *
+ * More precisely, the class %HouseholderSequence represents an \f$ n \times n \f$ matrix \f$ H \f$ of the
+ * form \f$ H = \prod_{i=0}^{n-1} H_i \f$ where the i-th Householder reflection is \f$ H_i = I - h_i v_i
+ * v_i^* \f$. The i-th Householder coefficient \f$ h_i \f$ is a scalar and the i-th Householder vector \f$
+ * v_i \f$ is a vector of the form
+ * \f[
+ * v_i = [\underbrace{0, \ldots, 0}_{i-1\mbox{ zeros}}, 1, \underbrace{*, \ldots,*}_{n-i\mbox{ arbitrary entries}} ].
+ * \f]
+ * The last \f$ n-i \f$ entries of \f$ v_i \f$ are called the essential part of the Householder vector.
+ *
+ * Typical usages are listed below, where H is a HouseholderSequence:
+ * \code
+ * A.applyOnTheRight(H);             // A = A * H
+ * A.applyOnTheLeft(H);              // A = H * A
+ * A.applyOnTheRight(H.adjoint());   // A = A * H^*
+ * A.applyOnTheLeft(H.adjoint());    // A = H^* * A
+ * MatrixXd Q = H;                   // conversion to a dense matrix
+ * \endcode
+ * In addition to the adjoint, you can also apply the inverse (=adjoint), the transpose, and the conjugate operators.
+ *
+ * See the documentation for HouseholderSequence(const VectorsType&, const CoeffsType&) for an example.
+ *
+ * \sa MatrixBase::applyOnTheLeft(), MatrixBase::applyOnTheRight()
+ */
+
+namespace internal {
+
+template <typename VectorsType, typename CoeffsType, int Side>
+struct traits<HouseholderSequence<VectorsType, CoeffsType, Side> > {
+  typedef typename VectorsType::Scalar Scalar;
+  typedef typename VectorsType::StorageIndex StorageIndex;
+  typedef typename VectorsType::StorageKind StorageKind;
+  enum {
+    RowsAtCompileTime =
+        Side == OnTheLeft ? traits<VectorsType>::RowsAtCompileTime : traits<VectorsType>::ColsAtCompileTime,
+    ColsAtCompileTime = RowsAtCompileTime,
+    MaxRowsAtCompileTime =
+        Side == OnTheLeft ? traits<VectorsType>::MaxRowsAtCompileTime : traits<VectorsType>::MaxColsAtCompileTime,
+    MaxColsAtCompileTime = MaxRowsAtCompileTime,
+    Flags = 0
+  };
+};
+
+struct HouseholderSequenceShape {};
+
+template <typename VectorsType, typename CoeffsType, int Side>
+struct evaluator_traits<HouseholderSequence<VectorsType, CoeffsType, Side> >
+    : public evaluator_traits_base<HouseholderSequence<VectorsType, CoeffsType, Side> > {
+  typedef HouseholderSequenceShape Shape;
+};
+
+template <typename VectorsType, typename CoeffsType, int Side>
+struct hseq_side_dependent_impl {
+  typedef Block<const VectorsType, Dynamic, 1> EssentialVectorType;
+  typedef HouseholderSequence<VectorsType, CoeffsType, OnTheLeft> HouseholderSequenceType;
+  static EIGEN_DEVICE_FUNC inline const EssentialVectorType essentialVector(const HouseholderSequenceType& h, Index k) {
+    Index start = k + 1 + h.m_shift;
+    return Block<const VectorsType, Dynamic, 1>(h.m_vectors, start, k, h.rows() - start, 1);
+  }
+};
+
+template <typename VectorsType, typename CoeffsType>
+struct hseq_side_dependent_impl<VectorsType, CoeffsType, OnTheRight> {
+  typedef Transpose<Block<const VectorsType, 1, Dynamic> > EssentialVectorType;
+  typedef HouseholderSequence<VectorsType, CoeffsType, OnTheRight> HouseholderSequenceType;
+  static inline const EssentialVectorType essentialVector(const HouseholderSequenceType& h, Index k) {
+    Index start = k + 1 + h.m_shift;
+    return Block<const VectorsType, 1, Dynamic>(h.m_vectors, k, start, 1, h.rows() - start).transpose();
+  }
+};
+
+template <typename OtherScalarType, typename MatrixType>
+struct matrix_type_times_scalar_type {
+  typedef typename ScalarBinaryOpTraits<OtherScalarType, typename MatrixType::Scalar>::ReturnType ResultScalar;
+  typedef Matrix<ResultScalar, MatrixType::RowsAtCompileTime, MatrixType::ColsAtCompileTime, 0,
+                 MatrixType::MaxRowsAtCompileTime, MatrixType::MaxColsAtCompileTime>
+      Type;
+};
+
+}  // end namespace internal
+
+template <typename VectorsType, typename CoeffsType, int Side>
+class HouseholderSequence : public EigenBase<HouseholderSequence<VectorsType, CoeffsType, Side> > {
+  typedef typename internal::hseq_side_dependent_impl<VectorsType, CoeffsType, Side>::EssentialVectorType
+      EssentialVectorType;
+
+ public:
+  enum {
+    RowsAtCompileTime = internal::traits<HouseholderSequence>::RowsAtCompileTime,
+    ColsAtCompileTime = internal::traits<HouseholderSequence>::ColsAtCompileTime,
+    MaxRowsAtCompileTime = internal::traits<HouseholderSequence>::MaxRowsAtCompileTime,
+    MaxColsAtCompileTime = internal::traits<HouseholderSequence>::MaxColsAtCompileTime
+  };
+  typedef typename internal::traits<HouseholderSequence>::Scalar Scalar;
+
+  typedef HouseholderSequence<
+      std::conditional_t<NumTraits<Scalar>::IsComplex,
+                         internal::remove_all_t<typename VectorsType::ConjugateReturnType>, VectorsType>,
+      std::conditional_t<NumTraits<Scalar>::IsComplex, internal::remove_all_t<typename CoeffsType::ConjugateReturnType>,
+                         CoeffsType>,
+      Side>
+      ConjugateReturnType;
+
+  typedef HouseholderSequence<
+      VectorsType,
+      std::conditional_t<NumTraits<Scalar>::IsComplex, internal::remove_all_t<typename CoeffsType::ConjugateReturnType>,
+                         CoeffsType>,
+      Side>
+      AdjointReturnType;
+
+  typedef HouseholderSequence<
+      std::conditional_t<NumTraits<Scalar>::IsComplex,
+                         internal::remove_all_t<typename VectorsType::ConjugateReturnType>, VectorsType>,
+      CoeffsType, Side>
+      TransposeReturnType;
+
+  typedef HouseholderSequence<std::add_const_t<VectorsType>, std::add_const_t<CoeffsType>, Side>
+      ConstHouseholderSequence;
+
+  /** \brief Constructor.
+   * \param[in]  v      %Matrix containing the essential parts of the Householder vectors
+   * \param[in]  h      Vector containing the Householder coefficients
+   *
+   * Constructs the Householder sequence with coefficients given by \p h and vectors given by \p v. The
+   * i-th Householder coefficient \f$ h_i \f$ is given by \p h(i) and the essential part of the i-th
+   * Householder vector \f$ v_i \f$ is given by \p v(k,i) with \p k > \p i (the subdiagonal part of the
+   * i-th column). If \p v has fewer columns than rows, then the Householder sequence contains as many
+   * Householder reflections as there are columns.
+   *
+   * \note The %HouseholderSequence object stores \p v and \p h by reference.
+   *
+   * Example: \include HouseholderSequence_HouseholderSequence.cpp
+   * Output: \verbinclude HouseholderSequence_HouseholderSequence.out
+   *
+   * \sa setLength(), setShift()
+   */
+  EIGEN_DEVICE_FUNC HouseholderSequence(const VectorsType& v, const CoeffsType& h)
+      : m_vectors(v), m_coeffs(h), m_reverse(false), m_length(v.diagonalSize()), m_shift(0) {}
+
+  /** \brief Copy constructor. */
+  EIGEN_DEVICE_FUNC HouseholderSequence(const HouseholderSequence& other)
+      : m_vectors(other.m_vectors),
+        m_coeffs(other.m_coeffs),
+        m_reverse(other.m_reverse),
+        m_length(other.m_length),
+        m_shift(other.m_shift) {}
+
+  /** \brief Number of rows of transformation viewed as a matrix.
+   * \returns Number of rows
+   * \details This equals the dimension of the space that the transformation acts on.
+   */
+  EIGEN_DEVICE_FUNC constexpr Index rows() const noexcept {
+    return Side == OnTheLeft ? m_vectors.rows() : m_vectors.cols();
+  }
+
+  /** \brief Number of columns of transformation viewed as a matrix.
+   * \returns Number of columns
+   * \details This equals the dimension of the space that the transformation acts on.
+   */
+  EIGEN_DEVICE_FUNC constexpr Index cols() const noexcept { return rows(); }
+
+  /** \brief Essential part of a Householder vector.
+   * \param[in]  k  Index of Householder reflection
+   * \returns    Vector containing non-trivial entries of k-th Householder vector
+   *
+   * This function returns the essential part of the Householder vector \f$ v_i \f$. This is a vector of
+   * length \f$ n-i \f$ containing the last \f$ n-i \f$ entries of the vector
+   * \f[
+   * v_i = [\underbrace{0, \ldots, 0}_{i-1\mbox{ zeros}}, 1, \underbrace{*, \ldots,*}_{n-i\mbox{ arbitrary entries}} ].
+   * \f]
+   * The index \f$ i \f$ equals \p k + shift(), corresponding to the k-th column of the matrix \p v
+   * passed to the constructor.
+   *
+   * \sa setShift(), shift()
+   */
+  EIGEN_DEVICE_FUNC const EssentialVectorType essentialVector(Index k) const {
+    eigen_assert(k >= 0 && k < m_length);
+    return internal::hseq_side_dependent_impl<VectorsType, CoeffsType, Side>::essentialVector(*this, k);
+  }
+
+  /** \brief %Transpose of the Householder sequence. */
+  TransposeReturnType transpose() const {
+    return TransposeReturnType(m_vectors.conjugate(), m_coeffs)
+        .setReverseFlag(!m_reverse)
+        .setLength(m_length)
+        .setShift(m_shift);
+  }
+
+  /** \brief Complex conjugate of the Householder sequence. */
+  ConjugateReturnType conjugate() const {
+    return ConjugateReturnType(m_vectors.conjugate(), m_coeffs.conjugate())
+        .setReverseFlag(m_reverse)
+        .setLength(m_length)
+        .setShift(m_shift);
+  }
+
+  /** \returns an expression of the complex conjugate of \c *this if Cond==true,
+   *           returns \c *this otherwise.
+   */
+  template <bool Cond>
+  EIGEN_DEVICE_FUNC inline std::conditional_t<Cond, ConjugateReturnType, ConstHouseholderSequence> conjugateIf() const {
+    typedef std::conditional_t<Cond, ConjugateReturnType, ConstHouseholderSequence> ReturnType;
+    return ReturnType(m_vectors.template conjugateIf<Cond>(), m_coeffs.template conjugateIf<Cond>());
+  }
+
+  /** \brief Adjoint (conjugate transpose) of the Householder sequence. */
+  AdjointReturnType adjoint() const {
+    return AdjointReturnType(m_vectors, m_coeffs.conjugate())
+        .setReverseFlag(!m_reverse)
+        .setLength(m_length)
+        .setShift(m_shift);
+  }
+
+  /** \brief Inverse of the Householder sequence (equals the adjoint). */
+  AdjointReturnType inverse() const { return adjoint(); }
+
+  /** \internal */
+  template <typename DestType>
+  inline EIGEN_DEVICE_FUNC void evalTo(DestType& dst) const {
+    Matrix<Scalar, DestType::RowsAtCompileTime, 1, AutoAlign | ColMajor, DestType::MaxRowsAtCompileTime, 1> workspace(
+        rows());
+    evalTo(dst, workspace);
+  }
+
+  /** \internal */
+  template <typename Dest, typename Workspace>
+  EIGEN_DEVICE_FUNC void evalTo(Dest& dst, Workspace& workspace) const {
+    workspace.resize(rows());
+    Index vecs = m_length;
+    if (internal::is_same_dense(dst, m_vectors)) {
+      // in-place
+      dst.diagonal().setOnes();
+      dst.template triangularView<StrictlyUpper>().setZero();
+      for (Index k = vecs - 1; k >= 0; --k) {
+        Index cornerSize = rows() - k - m_shift;
+        if (m_reverse)
+          dst.bottomRightCorner(cornerSize, cornerSize)
+              .applyHouseholderOnTheRight(essentialVector(k), m_coeffs.coeff(k), workspace.data());
+        else
+          dst.bottomRightCorner(cornerSize, cornerSize)
+              .applyHouseholderOnTheLeft(essentialVector(k), m_coeffs.coeff(k), workspace.data());
+
+        // clear the off diagonal vector
+        dst.col(k).tail(rows() - k - 1).setZero();
+      }
+      // clear the remaining columns if needed
+      for (Index k = 0; k < cols() - vecs; ++k) dst.col(k).tail(rows() - k - 1).setZero();
+    } else if (m_length > BlockSize) {
+      dst.setIdentity(rows(), rows());
+      if (m_reverse)
+        applyThisOnTheLeft(dst, workspace, true);
+      else
+        applyThisOnTheLeft(dst, workspace, true);
+    } else {
+      dst.setIdentity(rows(), rows());
+      for (Index k = vecs - 1; k >= 0; --k) {
+        Index cornerSize = rows() - k - m_shift;
+        if (m_reverse)
+          dst.bottomRightCorner(cornerSize, cornerSize)
+              .applyHouseholderOnTheRight(essentialVector(k), m_coeffs.coeff(k), workspace.data());
+        else
+          dst.bottomRightCorner(cornerSize, cornerSize)
+              .applyHouseholderOnTheLeft(essentialVector(k), m_coeffs.coeff(k), workspace.data());
+      }
+    }
+  }
+
+  /** \internal */
+  template <typename Dest>
+  inline void applyThisOnTheRight(Dest& dst) const {
+    Matrix<Scalar, 1, Dest::RowsAtCompileTime, RowMajor, 1, Dest::MaxRowsAtCompileTime> workspace(dst.rows());
+    applyThisOnTheRight(dst, workspace);
+  }
+
+  /** \internal */
+  template <typename Dest, typename Workspace>
+  inline void applyThisOnTheRight(Dest& dst, Workspace& workspace) const {
+    workspace.resize(dst.rows());
+    for (Index k = 0; k < m_length; ++k) {
+      Index actual_k = m_reverse ? m_length - k - 1 : k;
+      dst.rightCols(rows() - m_shift - actual_k)
+          .applyHouseholderOnTheRight(essentialVector(actual_k), m_coeffs.coeff(actual_k), workspace.data());
+    }
+  }
+
+  /** \internal */
+  template <typename Dest>
+  inline void applyThisOnTheLeft(Dest& dst, bool inputIsIdentity = false) const {
+    Matrix<Scalar, 1, Dest::ColsAtCompileTime, RowMajor, 1, Dest::MaxColsAtCompileTime> workspace;
+    applyThisOnTheLeft(dst, workspace, inputIsIdentity);
+  }
+
+  /** \internal */
+  template <typename Dest, typename Workspace>
+  inline void applyThisOnTheLeft(Dest& dst, Workspace& workspace, bool inputIsIdentity = false) const {
+    if (inputIsIdentity && m_reverse) inputIsIdentity = false;
+    // if the entries are large enough, then apply the reflectors by block
+    if (m_length >= BlockSize && dst.cols() > 1) {
+      // Make sure we have at least 2 useful blocks, otherwise it is point-less:
+      Index blockSize = m_length < Index(2 * BlockSize) ? (m_length + 1) / 2 : Index(BlockSize);
+      for (Index i = 0; i < m_length; i += blockSize) {
+        Index end = m_reverse ? (std::min)(m_length, i + blockSize) : m_length - i;
+        Index k = m_reverse ? i : (std::max)(Index(0), end - blockSize);
+        Index bs = end - k;
+        Index start = k + m_shift;
+
+        typedef Block<internal::remove_all_t<VectorsType>, Dynamic, Dynamic> SubVectorsType;
+        SubVectorsType sub_vecs1(m_vectors.const_cast_derived(), Side == OnTheRight ? k : start,
+                                 Side == OnTheRight ? start : k, Side == OnTheRight ? bs : m_vectors.rows() - start,
+                                 Side == OnTheRight ? m_vectors.cols() - start : bs);
+        std::conditional_t<Side == OnTheRight, Transpose<SubVectorsType>, SubVectorsType&> sub_vecs(sub_vecs1);
+
+        Index dstRows = rows() - m_shift - k;
+
+        if (inputIsIdentity) {
+          Block<Dest, Dynamic, Dynamic> sub_dst = dst.bottomRightCorner(dstRows, dstRows);
+          apply_block_householder_on_the_left(sub_dst, sub_vecs, m_coeffs.segment(k, bs), !m_reverse);
+        } else {
+          auto sub_dst = dst.bottomRows(dstRows);
+          apply_block_householder_on_the_left(sub_dst, sub_vecs, m_coeffs.segment(k, bs), !m_reverse);
+        }
+      }
+    } else {
+      workspace.resize(dst.cols());
+      for (Index k = 0; k < m_length; ++k) {
+        Index actual_k = m_reverse ? k : m_length - k - 1;
+        Index dstRows = rows() - m_shift - actual_k;
+
+        if (inputIsIdentity) {
+          Block<Dest, Dynamic, Dynamic> sub_dst = dst.bottomRightCorner(dstRows, dstRows);
+          sub_dst.applyHouseholderOnTheLeft(essentialVector(actual_k), m_coeffs.coeff(actual_k), workspace.data());
+        } else {
+          auto sub_dst = dst.bottomRows(dstRows);
+          sub_dst.applyHouseholderOnTheLeft(essentialVector(actual_k), m_coeffs.coeff(actual_k), workspace.data());
+        }
+      }
+    }
+  }
+
+  /** \brief Computes the product of a Householder sequence with a matrix.
+   * \param[in]  other  %Matrix being multiplied.
+   * \returns    Expression object representing the product.
+   *
+   * This function computes \f$ HM \f$ where \f$ H \f$ is the Householder sequence represented by \p *this
+   * and \f$ M \f$ is the matrix \p other.
+   */
+  template <typename OtherDerived>
+  typename internal::matrix_type_times_scalar_type<Scalar, OtherDerived>::Type operator*(
+      const MatrixBase<OtherDerived>& other) const {
+    typename internal::matrix_type_times_scalar_type<Scalar, OtherDerived>::Type res(
+        other.template cast<typename internal::matrix_type_times_scalar_type<Scalar, OtherDerived>::ResultScalar>());
+    applyThisOnTheLeft(res, internal::is_identity<OtherDerived>::value && res.rows() == res.cols());
+    return res;
+  }
+
+  template <typename VectorsType_, typename CoeffsType_, int Side_>
+  friend struct internal::hseq_side_dependent_impl;
+
+  /** \brief Sets the length of the Householder sequence.
+   * \param [in]  length  New value for the length.
+   *
+   * By default, the length \f$ n \f$ of the Householder sequence \f$ H = H_0 H_1 \ldots H_{n-1} \f$ is set
+   * to the number of columns of the matrix \p v passed to the constructor, or the number of rows if that
+   * is smaller. After this function is called, the length equals \p length.
+   *
+   * \sa length()
+   */
+  EIGEN_DEVICE_FUNC HouseholderSequence& setLength(Index length) {
+    m_length = length;
+    return *this;
+  }
+
+  /** \brief Sets the shift of the Householder sequence.
+   * \param [in]  shift  New value for the shift.
+   *
+   * By default, a %HouseholderSequence object represents \f$ H = H_0 H_1 \ldots H_{n-1} \f$ and the i-th
+   * column of the matrix \p v passed to the constructor corresponds to the i-th Householder
+   * reflection. After this function is called, the object represents \f$ H = H_{\mathrm{shift}}
+   * H_{\mathrm{shift}+1} \ldots H_{n-1} \f$ and the i-th column of \p v corresponds to the (shift+i)-th
+   * Householder reflection.
+   *
+   * \sa shift()
+   */
+  EIGEN_DEVICE_FUNC HouseholderSequence& setShift(Index shift) {
+    m_shift = shift;
+    return *this;
+  }
+
+  EIGEN_DEVICE_FUNC Index length() const {
+    return m_length;
+  } /**< \brief Returns the length of the Householder sequence. */
+
+  EIGEN_DEVICE_FUNC Index shift() const {
+    return m_shift;
+  } /**< \brief Returns the shift of the Householder sequence. */
+
+  /* Necessary for .adjoint() and .conjugate() */
+  template <typename VectorsType2, typename CoeffsType2, int Side2>
+  friend class HouseholderSequence;
+
+ protected:
+  /** \internal
+   * \brief Sets the reverse flag.
+   * \param [in]  reverse  New value of the reverse flag.
+   *
+   * By default, the reverse flag is not set. If the reverse flag is set, then this object represents
+   * \f$ H^r = H_{n-1} \ldots H_1 H_0 \f$ instead of \f$ H = H_0 H_1 \ldots H_{n-1} \f$.
+   * \note For real valued HouseholderSequence this is equivalent to transposing \f$ H \f$.
+   *
+   * \sa reverseFlag(), transpose(), adjoint()
+   */
+  HouseholderSequence& setReverseFlag(bool reverse) {
+    m_reverse = reverse;
+    return *this;
+  }
+
+  bool reverseFlag() const { return m_reverse; } /**< \internal \brief Returns the reverse flag. */
+
+  typename VectorsType::Nested m_vectors;
+  typename CoeffsType::Nested m_coeffs;
+  bool m_reverse;
+  Index m_length;
+  Index m_shift;
+  enum { BlockSize = 48 };
+};
+
+/** \brief Computes the product of a matrix with a Householder sequence.
+ * \param[in]  other  %Matrix being multiplied.
+ * \param[in]  h      %HouseholderSequence being multiplied.
+ * \returns    Expression object representing the product.
+ *
+ * This function computes \f$ MH \f$ where \f$ M \f$ is the matrix \p other and \f$ H \f$ is the
+ * Householder sequence represented by \p h.
+ */
+template <typename OtherDerived, typename VectorsType, typename CoeffsType, int Side>
+typename internal::matrix_type_times_scalar_type<typename VectorsType::Scalar, OtherDerived>::Type operator*(
+    const MatrixBase<OtherDerived>& other, const HouseholderSequence<VectorsType, CoeffsType, Side>& h) {
+  typename internal::matrix_type_times_scalar_type<typename VectorsType::Scalar, OtherDerived>::Type res(
+      other.template cast<typename internal::matrix_type_times_scalar_type<typename VectorsType::Scalar,
+                                                                           OtherDerived>::ResultScalar>());
+  h.applyThisOnTheRight(res);
+  return res;
+}
+
+/** \ingroup Householder_Module
+ * \householder_module
+ * \brief Convenience function for constructing a Householder sequence.
+ * \returns A HouseholderSequence constructed from the specified arguments.
+ */
+template <typename VectorsType, typename CoeffsType>
+HouseholderSequence<VectorsType, CoeffsType> householderSequence(const VectorsType& v, const CoeffsType& h) {
+  return HouseholderSequence<VectorsType, CoeffsType, OnTheLeft>(v, h);
+}
+
+/** \ingroup Householder_Module
+ * \householder_module
+ * \brief Convenience function for constructing a Householder sequence.
+ * \returns A HouseholderSequence constructed from the specified arguments.
+ * \details This function differs from householderSequence() in that the template argument \p OnTheSide of
+ * the constructed HouseholderSequence is set to OnTheRight, instead of the default OnTheLeft.
+ */
+template <typename VectorsType, typename CoeffsType>
+HouseholderSequence<VectorsType, CoeffsType, OnTheRight> rightHouseholderSequence(const VectorsType& v,
+                                                                                  const CoeffsType& h) {
+  return HouseholderSequence<VectorsType, CoeffsType, OnTheRight>(v, h);
+}
+
+}  // end namespace Eigen
+
+#endif  // EIGEN_HOUSEHOLDER_SEQUENCE_H

2025.09.22_cpp_with_eigen_package/Eigen/src/Householder/InternalHeaderCheck.h

@@ -0,0 +1,3 @@
+#ifndef EIGEN_HOUSEHOLDER_MODULE_H
+#error "Please include Eigen/Householder instead of including headers inside the src directory directly."
+#endif