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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2017 Google Inc. All rights reserved.
+// http://ceres-solver.org/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+// * Neither the name of Google Inc. nor the names of its contributors may be
+//   used to endorse or promote products derived from this software without
+//   specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: sameeragarwal@google.com (Sameer Agarwal)
+
+#ifndef CERES_INTERNAL_SUBSET_PRECONDITIONER_H_
+#define CERES_INTERNAL_SUBSET_PRECONDITIONER_H_
+
+#include <memory>
+#include "ceres/preconditioner.h"
+
+namespace ceres {
+namespace internal {
+
+class BlockSparseMatrix;
+class SparseCholesky;
+class InnerProductComputer;
+
+// Subset preconditioning, uses a subset of the rows of the Jacobian
+// to construct a preconditioner for the normal equations.
+//
+// To keep the interface simple, we assume that the matrix A has
+// already been re-ordered that the user wishes to some subset of the
+// bottom row blocks of the matrix as the preconditioner. This is
+// controlled by
+// Preconditioner::Options::subset_preconditioner_start_row_block.
+//
+// When using the subset preconditioner, all row blocks starting
+// from this row block are used to construct the preconditioner.
+//
+// More precisely the matrix A is horizontally partitioned as
+//
+// A = [P]
+//     [Q]
+//
+// where P has subset_preconditioner_start_row_block row blocks,
+// and the preconditioner is the inverse of the matrix Q'Q.
+//
+// Obviously, the smaller this number, the more accurate and
+// computationally expensive this preconditioner will be.
+//
+// See the tests for example usage.
+class SubsetPreconditioner : public BlockSparseMatrixPreconditioner {
+ public:
+  SubsetPreconditioner(const Preconditioner::Options& options,
+                       const BlockSparseMatrix& A);
+  virtual ~SubsetPreconditioner();
+
+  // Preconditioner interface
+  void RightMultiply(const double* x, double* y) const final;
+  int num_rows() const final { return num_cols_; }
+  int num_cols() const final { return num_cols_; }
+
+ private:
+  bool UpdateImpl(const BlockSparseMatrix& A, const double* D) final;
+
+  const Preconditioner::Options options_;
+  const int num_cols_;
+  std::unique_ptr<SparseCholesky> sparse_cholesky_;
+  std::unique_ptr<InnerProductComputer> inner_product_computer_;
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_SUBSET_PRECONDITIONER_H_