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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2023 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_DENSE_QR_H_
+#define CERES_INTERNAL_DENSE_QR_H_
+
+// This include must come before any #ifndef check on Ceres compile options.
+// clang-format off
+#include "ceres/internal/config.h"
+// clang-format on
+
+#include <memory>
+#include <vector>
+
+#include "Eigen/Dense"
+#include "ceres/context_impl.h"
+#include "ceres/internal/disable_warnings.h"
+#include "ceres/internal/eigen.h"
+#include "ceres/internal/export.h"
+#include "ceres/linear_solver.h"
+#include "glog/logging.h"
+
+#ifndef CERES_NO_CUDA
+#include "ceres/context_impl.h"
+#include "ceres/cuda_buffer.h"
+#include "cublas_v2.h"
+#include "cuda_runtime.h"
+#include "cusolverDn.h"
+#endif  // CERES_NO_CUDA
+
+namespace ceres::internal {
+
+// An interface that abstracts away the internal details of various dense linear
+// algebra libraries and offers a simple API for solving dense linear systems
+// using a QR factorization.
+class CERES_NO_EXPORT DenseQR {
+ public:
+  static std::unique_ptr<DenseQR> Create(const LinearSolver::Options& options);
+
+  virtual ~DenseQR();
+
+  // Computes the QR factorization of the given matrix.
+  //
+  // The input matrix lhs is assumed to be a column-major num_rows x num_cols
+  // matrix.
+  //
+  // The input matrix lhs may be modified by the implementation to store the
+  // factorization, irrespective of whether the factorization succeeds or not.
+  // As a result it is the user's responsibility to ensure that lhs is valid
+  // when Solve is called.
+  virtual LinearSolverTerminationType Factorize(int num_rows,
+                                                int num_cols,
+                                                double* lhs,
+                                                std::string* message) = 0;
+
+  // Computes the solution to the equation
+  //
+  // lhs * solution = rhs
+  //
+  // Calling Solve without calling Factorize is undefined behaviour. It is the
+  // user's responsibility to ensure that the input matrix lhs passed to
+  // Factorize has not been freed/modified when Solve is called.
+  virtual LinearSolverTerminationType Solve(const double* rhs,
+                                            double* solution,
+                                            std::string* message) = 0;
+
+  // Convenience method which combines a call to Factorize and Solve. Solve is
+  // only called if Factorize returns LinearSolverTerminationType::SUCCESS.
+  //
+  // The input matrix lhs may be modified by the implementation to store the
+  // factorization, irrespective of whether the method succeeds or not. It is
+  // the user's responsibility to ensure that lhs is valid if and when Solve is
+  // called again after this call.
+  LinearSolverTerminationType FactorAndSolve(int num_rows,
+                                             int num_cols,
+                                             double* lhs,
+                                             const double* rhs,
+                                             double* solution,
+                                             std::string* message);
+};
+
+class CERES_NO_EXPORT EigenDenseQR final : public DenseQR {
+ public:
+  LinearSolverTerminationType Factorize(int num_rows,
+                                        int num_cols,
+                                        double* lhs,
+                                        std::string* message) override;
+  LinearSolverTerminationType Solve(const double* rhs,
+                                    double* solution,
+                                    std::string* message) override;
+
+ private:
+  using QRType = Eigen::HouseholderQR<Eigen::Ref<ColMajorMatrix>>;
+  std::unique_ptr<QRType> qr_;
+};
+
+#ifndef CERES_NO_LAPACK
+class CERES_NO_EXPORT LAPACKDenseQR final : public DenseQR {
+ public:
+  LinearSolverTerminationType Factorize(int num_rows,
+                                        int num_cols,
+                                        double* lhs,
+                                        std::string* message) override;
+  LinearSolverTerminationType Solve(const double* rhs,
+                                    double* solution,
+                                    std::string* message) override;
+
+ private:
+  double* lhs_ = nullptr;
+  int num_rows_;
+  int num_cols_;
+  LinearSolverTerminationType termination_type_ =
+      LinearSolverTerminationType::FATAL_ERROR;
+  Vector work_;
+  Vector tau_;
+  Vector q_transpose_rhs_;
+};
+#endif  // CERES_NO_LAPACK
+
+#ifndef CERES_NO_CUDA
+// Implementation of DenseQR using the 32-bit cuSolverDn interface. A
+// requirement for using this solver is that the lhs must not be rank deficient.
+// This is because cuSolverDn does not implement the singularity-checking
+// wrapper trtrs, hence this solver directly uses trsv from CUBLAS for the
+// backsubstitution.
+class CERES_NO_EXPORT CUDADenseQR final : public DenseQR {
+ public:
+  static std::unique_ptr<CUDADenseQR> Create(
+      const LinearSolver::Options& options);
+  CUDADenseQR(const CUDADenseQR&) = delete;
+  CUDADenseQR& operator=(const CUDADenseQR&) = delete;
+  LinearSolverTerminationType Factorize(int num_rows,
+                                        int num_cols,
+                                        double* lhs,
+                                        std::string* message) override;
+  LinearSolverTerminationType Solve(const double* rhs,
+                                    double* solution,
+                                    std::string* message) override;
+
+ private:
+  explicit CUDADenseQR(ContextImpl* context);
+
+  ContextImpl* context_ = nullptr;
+  // Number of rowns in the A matrix, to be cached between calls to *Factorize
+  // and *Solve.
+  size_t num_rows_ = 0;
+  // Number of columns in the A matrix, to be cached between calls to *Factorize
+  // and *Solve.
+  size_t num_cols_ = 0;
+  // GPU memory allocated for the A matrix (lhs matrix).
+  CudaBuffer<double> lhs_;
+  // GPU memory allocated for the B matrix (rhs vector).
+  CudaBuffer<double> rhs_;
+  // GPU memory allocated for the TAU matrix (scaling of householder vectors).
+  CudaBuffer<double> tau_;
+  // Scratch space for cuSOLVER on the GPU.
+  CudaBuffer<double> device_workspace_;
+  // Required for error handling with cuSOLVER.
+  CudaBuffer<int> error_;
+  // Cache the result of Factorize to ensure that when Solve is called, the
+  // factiorization of lhs is valid.
+  LinearSolverTerminationType factorize_result_ =
+      LinearSolverTerminationType::FATAL_ERROR;
+};
+
+#endif  // CERES_NO_CUDA
+
+}  // namespace ceres::internal
+
+#include "ceres/internal/reenable_warnings.h"
+
+#endif  // CERES_INTERNAL_DENSE_QR_H_