UpperBidiagonalization.h

// 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) 2013-2014 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_BIDIAGONALIZATION_H
#define EIGEN_BIDIAGONALIZATION_H

// IWYU pragma: private
#include "./InternalHeaderCheck.h"

namespace Eigen {

namespace internal {
// UpperBidiagonalization will probably be replaced by a Bidiagonalization class, don't want to make it stable API.
// At the same time, it's useful to keep for now as it's about the only thing that is testing the BandMatrix class.

template <typename MatrixType_>
class UpperBidiagonalization {
 public:
  typedef MatrixType_ MatrixType;
  enum {
    RowsAtCompileTime = MatrixType::RowsAtCompileTime,
    ColsAtCompileTime = MatrixType::ColsAtCompileTime,
    ColsAtCompileTimeMinusOne = internal::decrement_size<ColsAtCompileTime>::ret
  };
  typedef typename MatrixType::Scalar Scalar;
  typedef typename MatrixType::RealScalar RealScalar;
  typedef Eigen::Index Index;  
  typedef Matrix<Scalar, 1, ColsAtCompileTime> RowVectorType;
  typedef Matrix<Scalar, RowsAtCompileTime, 1> ColVectorType;
  typedef BandMatrix<RealScalar, ColsAtCompileTime, ColsAtCompileTime, 1, 0, RowMajor> BidiagonalType;
  typedef Matrix<Scalar, ColsAtCompileTime, 1> DiagVectorType;
  typedef Matrix<Scalar, ColsAtCompileTimeMinusOne, 1> SuperDiagVectorType;
  typedef HouseholderSequence<
      const MatrixType, const internal::remove_all_t<typename Diagonal<const MatrixType, 0>::ConjugateReturnType> >
      HouseholderUSequenceType;
  typedef HouseholderSequence<const internal::remove_all_t<typename MatrixType::ConjugateReturnType>,
                              Diagonal<const MatrixType, 1>, OnTheRight>
      HouseholderVSequenceType;

  UpperBidiagonalization() : m_householder(), m_bidiagonal(0, 0), m_isInitialized(false) {}

  explicit UpperBidiagonalization(const MatrixType& matrix)
      : m_householder(matrix.rows(), matrix.cols()),
        m_bidiagonal(matrix.cols(), matrix.cols()),
        m_isInitialized(false) {
    compute(matrix);
  }

  UpperBidiagonalization(Index rows, Index cols)
      : m_householder(rows, cols), m_bidiagonal(cols, cols), m_isInitialized(false) {}

  UpperBidiagonalization& compute(const MatrixType& matrix);
  UpperBidiagonalization& computeUnblocked(const MatrixType& matrix);

  const MatrixType& householder() const { return m_householder; }
  const BidiagonalType& bidiagonal() const { return m_bidiagonal; }

  const HouseholderUSequenceType householderU() const {
    eigen_assert(m_isInitialized && "UpperBidiagonalization is not initialized.");
    return HouseholderUSequenceType(m_householder, m_householder.diagonal().conjugate());
  }

  const HouseholderVSequenceType householderV()  // const here gives nasty errors and i'm lazy
  {
    eigen_assert(m_isInitialized && "UpperBidiagonalization is not initialized.");
    return HouseholderVSequenceType(m_householder.conjugate(), m_householder.const_derived().template diagonal<1>())
        .setLength(m_householder.cols() - 1)
        .setShift(1);
  }

 protected:
  MatrixType m_householder;
  BidiagonalType m_bidiagonal;
  bool m_isInitialized;
};

// Standard upper bidiagonalization without fancy optimizations
// This version should be faster for small matrix size
template <typename MatrixType>
void upperbidiagonalization_inplace_unblocked(MatrixType& mat, typename MatrixType::RealScalar* diagonal,
                                              typename MatrixType::RealScalar* upper_diagonal,
                                              typename MatrixType::Scalar* tempData = 0) {
  typedef typename MatrixType::Scalar Scalar;

  Index rows = mat.rows();
  Index cols = mat.cols();

  typedef Matrix<Scalar, Dynamic, 1, ColMajor, MatrixType::MaxRowsAtCompileTime, 1> TempType;
  TempType tempVector;
  if (tempData == 0) {
    tempVector.resize(rows);
    tempData = tempVector.data();
  }

  for (Index k = 0; /* breaks at k==cols-1 below */; ++k) {
    Index remainingRows = rows - k;
    Index remainingCols = cols - k - 1;

    // construct left householder transform in-place in A
    mat.col(k).tail(remainingRows).makeHouseholderInPlace(mat.coeffRef(k, k), diagonal[k]);
    // apply householder transform to remaining part of A on the left
    mat.bottomRightCorner(remainingRows, remainingCols)
        .applyHouseholderOnTheLeft(mat.col(k).tail(remainingRows - 1), mat.coeff(k, k), tempData);

    if (k == cols - 1) break;

    // construct right householder transform in-place in mat
    mat.row(k).tail(remainingCols).makeHouseholderInPlace(mat.coeffRef(k, k + 1), upper_diagonal[k]);
    // apply householder transform to remaining part of mat on the left
    mat.bottomRightCorner(remainingRows - 1, remainingCols)
        .applyHouseholderOnTheRight(mat.row(k).tail(remainingCols - 1).adjoint(), mat.coeff(k, k + 1), tempData);
  }
}

template <typename MatrixType>
void upperbidiagonalization_blocked_helper(
    MatrixType& A, typename MatrixType::RealScalar* diagonal, typename MatrixType::RealScalar* upper_diagonal, Index bs,
    Ref<Matrix<typename MatrixType::Scalar, Dynamic, Dynamic, traits<MatrixType>::Flags & RowMajorBit> > X,
    Ref<Matrix<typename MatrixType::Scalar, Dynamic, Dynamic, traits<MatrixType>::Flags & RowMajorBit> > Y) {
  typedef typename MatrixType::Scalar Scalar;
  typedef typename MatrixType::RealScalar RealScalar;
  typedef typename NumTraits<RealScalar>::Literal Literal;
  static constexpr int StorageOrder = (traits<MatrixType>::Flags & RowMajorBit) ? RowMajor : ColMajor;
  typedef InnerStride<StorageOrder == ColMajor ? 1 : Dynamic> ColInnerStride;
  typedef InnerStride<StorageOrder == ColMajor ? Dynamic : 1> RowInnerStride;
  typedef Ref<Matrix<Scalar, Dynamic, 1>, 0, ColInnerStride> SubColumnType;
  typedef Ref<Matrix<Scalar, 1, Dynamic>, 0, RowInnerStride> SubRowType;
  typedef Ref<Matrix<Scalar, Dynamic, Dynamic, StorageOrder> > SubMatType;

  Index brows = A.rows();
  Index bcols = A.cols();

  Scalar tau_u, tau_u_prev(0), tau_v;

  for (Index k = 0; k < bs; ++k) {
    Index remainingRows = brows - k;
    Index remainingCols = bcols - k - 1;

    SubMatType X_k1(X.block(k, 0, remainingRows, k));
    SubMatType V_k1(A.block(k, 0, remainingRows, k));

    // 1 - update the k-th column of A
    SubColumnType v_k = A.col(k).tail(remainingRows);
    v_k -= V_k1 * Y.row(k).head(k).adjoint();
    if (k) v_k.noalias() -= X_k1 * A.col(k).head(k);

    // 2 - construct left Householder transform in-place
    v_k.makeHouseholderInPlace(tau_v, diagonal[k]);

    if (k + 1 < bcols) {
      SubMatType Y_k(Y.block(k + 1, 0, remainingCols, k + 1));
      SubMatType U_k1(A.block(0, k + 1, k, remainingCols));

      // this eases the application of Householder transforAions
      // A(k,k) will store tau_v later
      A(k, k) = Scalar(1);

      // 3 - Compute y_k^T = tau_v * ( A^T*v_k - Y_k-1*V_k-1^T*v_k - U_k-1*X_k-1^T*v_k )
      {
        SubColumnType y_k(Y.col(k).tail(remainingCols));

        // let's use the beginning of column k of Y as a temporary vector
        SubColumnType tmp(Y.col(k).head(k));
        y_k.noalias() = A.block(k, k + 1, remainingRows, remainingCols).adjoint() * v_k;  // bottleneck
        tmp.noalias() = V_k1.adjoint() * v_k;
        y_k.noalias() -= Y_k.leftCols(k) * tmp;
        tmp.noalias() = X_k1.adjoint() * v_k;
        y_k.noalias() -= U_k1.adjoint() * tmp;
        y_k *= numext::conj(tau_v);
      }

      // 4 - update k-th row of A (it will become u_k)
      SubRowType u_k(A.row(k).tail(remainingCols));
      u_k = u_k.conjugate();
      {
        u_k.noalias() -= Y_k * A.row(k).head(k + 1).adjoint();
        if (k) u_k -= U_k1.adjoint() * X.row(k).head(k).adjoint();
      }

      // 5 - construct right Householder transform in-place
      u_k.makeHouseholderInPlace(tau_u, upper_diagonal[k]);

      // this eases the application of Householder transformations
      // A(k,k+1) will store tau_u later
      A(k, k + 1) = Scalar(1);

      // 6 - Compute x_k = tau_u * ( A*u_k - X_k-1*U_k-1^T*u_k - V_k*Y_k^T*u_k )
      {
        SubColumnType x_k(X.col(k).tail(remainingRows - 1));

        // let's use the beginning of column k of X as a temporary vectors
        // note that tmp0 and tmp1 overlaps
        SubColumnType tmp0(X.col(k).head(k)), tmp1(X.col(k).head(k + 1));

        x_k.noalias() = A.block(k + 1, k + 1, remainingRows - 1, remainingCols) * u_k.transpose();  // bottleneck
        tmp0.noalias() = U_k1 * u_k.transpose();
        x_k.noalias() -= X_k1.bottomRows(remainingRows - 1) * tmp0;
        tmp1.noalias() = Y_k.adjoint() * u_k.transpose();
        x_k.noalias() -= A.block(k + 1, 0, remainingRows - 1, k + 1) * tmp1;
        x_k *= numext::conj(tau_u);
        tau_u = numext::conj(tau_u);
        u_k = u_k.conjugate();
      }

      if (k > 0) A.coeffRef(k - 1, k) = tau_u_prev;
      tau_u_prev = tau_u;
    } else
      A.coeffRef(k - 1, k) = tau_u_prev;

    A.coeffRef(k, k) = tau_v;
  }

  if (bs < bcols) A.coeffRef(bs - 1, bs) = tau_u_prev;

  // update A22
  if (bcols > bs && brows > bs) {
    SubMatType A11(A.bottomRightCorner(brows - bs, bcols - bs));
    SubMatType A10(A.block(bs, 0, brows - bs, bs));
    SubMatType A01(A.block(0, bs, bs, bcols - bs));
    Scalar tmp = A01(bs - 1, 0);
    A01(bs - 1, 0) = Literal(1);
    A11.noalias() -= A10 * Y.topLeftCorner(bcols, bs).bottomRows(bcols - bs).adjoint();
    A11.noalias() -= X.topLeftCorner(brows, bs).bottomRows(brows - bs) * A01;
    A01(bs - 1, 0) = tmp;
  }
}

template <typename MatrixType, typename BidiagType>
void upperbidiagonalization_inplace_blocked(MatrixType& A, BidiagType& bidiagonal, Index maxBlockSize = 32,
                                            typename MatrixType::Scalar* /*tempData*/ = 0) {
  typedef typename MatrixType::Scalar Scalar;
  typedef Block<MatrixType, Dynamic, Dynamic> BlockType;

  Index rows = A.rows();
  Index cols = A.cols();
  Index size = (std::min)(rows, cols);

  // X and Y are work space
  static constexpr int StorageOrder = (traits<MatrixType>::Flags & RowMajorBit) ? RowMajor : ColMajor;
  Matrix<Scalar, MatrixType::RowsAtCompileTime, Dynamic, StorageOrder, MatrixType::MaxRowsAtCompileTime> X(
      rows, maxBlockSize);
  Matrix<Scalar, MatrixType::ColsAtCompileTime, Dynamic, StorageOrder, MatrixType::MaxColsAtCompileTime> Y(
      cols, maxBlockSize);
  Index blockSize = (std::min)(maxBlockSize, size);

  Index k = 0;
  for (k = 0; k < size; k += blockSize) {
    Index bs = (std::min)(size - k, blockSize);  // actual size of the block
    Index brows = rows - k;                      // rows of the block
    Index bcols = cols - k;                      // columns of the block

    // partition the matrix A:
    //
    //      | A00 A01 A02 |
    //      |             |
    // A  = | A10 A11 A12 |
    //      |             |
    //      | A20 A21 A22 |
    //
    // where A11 is a bs x bs diagonal block,
    // and let:
    //      | A11 A12 |
    //  B = |         |
    //      | A21 A22 |

    BlockType B = A.block(k, k, brows, bcols);

    // This stage performs the bidiagonalization of A11, A21, A12, and updating of A22.
    // Finally, the algorithm continue on the updated A22.
    //
    // However, if B is too small, or A22 empty, then let's use an unblocked strategy

    auto upper_diagonal = bidiagonal.template diagonal<1>();
    typename MatrixType::RealScalar* upper_diagonal_ptr =
        upper_diagonal.size() > 0 ? &upper_diagonal.coeffRef(k) : nullptr;

    if (k + bs == cols || bcols < 48)  // somewhat arbitrary threshold
    {
      upperbidiagonalization_inplace_unblocked(B, &(bidiagonal.template diagonal<0>().coeffRef(k)), upper_diagonal_ptr,
                                               X.data());
      break;  // We're done
    } else {
      upperbidiagonalization_blocked_helper<BlockType>(B, &(bidiagonal.template diagonal<0>().coeffRef(k)),
                                                       upper_diagonal_ptr, bs, X.topLeftCorner(brows, bs),
                                                       Y.topLeftCorner(bcols, bs));
    }
  }
}

template <typename MatrixType_>
UpperBidiagonalization<MatrixType_>& UpperBidiagonalization<MatrixType_>::computeUnblocked(const MatrixType_& matrix) {
  Index rows = matrix.rows();
  Index cols = matrix.cols();
  EIGEN_ONLY_USED_FOR_DEBUG(cols);

  eigen_assert(rows >= cols && "UpperBidiagonalization is only for Arices satisfying rows>=cols.");

  m_householder = matrix;

  ColVectorType temp(rows);

  upperbidiagonalization_inplace_unblocked(m_householder, &(m_bidiagonal.template diagonal<0>().coeffRef(0)),
                                           &(m_bidiagonal.template diagonal<1>().coeffRef(0)), temp.data());

  m_isInitialized = true;
  return *this;
}

template <typename MatrixType_>
UpperBidiagonalization<MatrixType_>& UpperBidiagonalization<MatrixType_>::compute(const MatrixType_& matrix) {
  Index rows = matrix.rows();
  Index cols = matrix.cols();
  EIGEN_ONLY_USED_FOR_DEBUG(rows);
  EIGEN_ONLY_USED_FOR_DEBUG(cols);

  eigen_assert(rows >= cols && "UpperBidiagonalization is only for Arices satisfying rows>=cols.");

  m_householder = matrix;
  upperbidiagonalization_inplace_blocked(m_householder, m_bidiagonal);

  m_isInitialized = true;
  return *this;
}

#if 0

template<typename Derived>
const UpperBidiagonalization<typename MatrixBase<Derived>::PlainObject>
MatrixBase<Derived>::bidiagonalization() const
{
  return UpperBidiagonalization<PlainObject>(eval());
}
#endif

}  // end namespace internal

}  // end namespace Eigen

#endif  // EIGEN_BIDIAGONALIZATION_H