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

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+// 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