text: // Ceres Solver - A fast non-linear least squares minimizer
  // Copyright 2015 Google Inc. All rights reserved.
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  // Author: sameeragarwal@google.com (Sameer Agarwal)
  
  #include "ceres/schur_jacobi_preconditioner.h"
  
  #include <utility>
  #include <vector>
  
  #include "ceres/block_random_access_diagonal_matrix.h"
  #include "ceres/block_sparse_matrix.h"
  #include "ceres/linear_solver.h"
  #include "ceres/schur_eliminator.h"
  #include "glog/logging.h"
  
  namespace ceres::internal {
  
  SchurJacobiPreconditioner::SchurJacobiPreconditioner(
      const CompressedRowBlockStructure& bs, Preconditioner::Options options)
      : options_(std::move(options)) {
    CHECK_GT(options_.elimination_groups.size(), 1);
    CHECK_GT(options_.elimination_groups[0], 0);
    const int num_blocks = bs.cols.size() - options_.elimination_groups[0];
    CHECK_GT(num_blocks, 0) << "Jacobian should have at least 1 f_block for "
                            << "SCHUR_JACOBI preconditioner.";
    CHECK(options_.context != nullptr);
  
    std::vector<int> blocks(num_blocks);
    for (int i = 0; i < num_blocks; ++i) {
      blocks[i] = bs.cols[i + options_.elimination_groups[0]].size;
    }
  
    m_ = std::make_unique<BlockRandomAccessDiagonalMatrix>(blocks);
    InitEliminator(bs);
  }
  
  SchurJacobiPreconditioner::~SchurJacobiPreconditioner() = default;
  
  // Initialize the SchurEliminator.
  void SchurJacobiPreconditioner::InitEliminator(
      const CompressedRowBlockStructure& bs) {
    LinearSolver::Options eliminator_options;
    eliminator_options.elimination_groups = options_.elimination_groups;
    eliminator_options.num_threads = options_.num_threads;
    eliminator_options.e_block_size = options_.e_block_size;
    eliminator_options.f_block_size = options_.f_block_size;
    eliminator_options.row_block_size = options_.row_block_size;
    eliminator_options.context = options_.context;
    eliminator_ = SchurEliminatorBase::Create(eliminator_options);
    const bool kFullRankETE = true;
    eliminator_->Init(
        eliminator_options.elimination_groups[0], kFullRankETE, &bs);
  }
  
  // Update the values of the preconditioner matrix and factorize it.
  bool SchurJacobiPreconditioner::UpdateImpl(const BlockSparseMatrix& A,
                                             const double* D) {
    const int num_rows = m_->num_rows();
    CHECK_GT(num_rows, 0);
  
    // Compute a subset of the entries of the Schur complement.
    eliminator_->Eliminate(
        BlockSparseMatrixData(A), nullptr, D, m_.get(), nullptr);
    m_->Invert();
    return true;
  }
  
  void SchurJacobiPreconditioner::RightMultiply(const double* x,
                                                double* y) const {
    m_->RightMultiply(x, y);
  }
  
  int SchurJacobiPreconditioner::num_rows() const { return m_->num_rows(); }
  
  }  // namespace ceres::internal
  