// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2015 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
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// modification, are permitted provided that the following conditions are met:
//
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//   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.
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//   used to endorse or promote products derived from this software without
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// Author: sameeragarwal@google.com (Sameer Agarwal)

#ifndef CERES_INTERNAL_LEVENBERG_MARQUARDT_STRATEGY_H_
#define CERES_INTERNAL_LEVENBERG_MARQUARDT_STRATEGY_H_

#include "ceres/internal/disable_warnings.h"
#include "ceres/internal/eigen.h"
#include "ceres/internal/export.h"
#include "ceres/trust_region_strategy.h"

namespace ceres::internal {

// Levenberg-Marquardt step computation and trust region sizing
// strategy based on on "Methods for Nonlinear Least Squares" by
// K. Madsen, H.B. Nielsen and O. Tingleff. Available to download from
//
// http://www2.imm.dtu.dk/pubdb/views/edoc_download.php/3215/pdf/imm3215.pdf
class CERES_NO_EXPORT LevenbergMarquardtStrategy final
    : public TrustRegionStrategy {
 public:
  explicit LevenbergMarquardtStrategy(
      const TrustRegionStrategy::Options& options);
  ~LevenbergMarquardtStrategy() override;

  // TrustRegionStrategy interface
  TrustRegionStrategy::Summary ComputeStep(
      const TrustRegionStrategy::PerSolveOptions& per_solve_options,
      SparseMatrix* jacobian,
      const double* residuals,
      double* step) final;
  void StepAccepted(double step_quality) final;
  void StepRejected(double step_quality) final;
  void StepIsInvalid() final {
    // Treat the current step as a rejected step with no increase in
    // solution quality. Since rejected steps lead to decrease in the
    // size of the trust region, the next time ComputeStep is called,
    // this will lead to a better conditioned system.
    StepRejected(0.0);
  }

  double Radius() const final;

 private:
  LinearSolver* linear_solver_;
  double radius_;
  double max_radius_;
  const double min_diagonal_;
  const double max_diagonal_;
  double decrease_factor_;
  bool reuse_diagonal_;
  Vector diagonal_;  // diagonal_ =  diag(J'J)
  // Scaled copy of diagonal_. Stored here as optimization to prevent
  // allocations in every iteration and reuse when a step fails and
  // ComputeStep is called again.
  Vector lm_diagonal_;  // lm_diagonal_ = sqrt(diagonal_ / radius_);
};

}  // namespace ceres::internal

#include "ceres/internal/reenable_warnings.h"

#endif  // CERES_INTERNAL_LEVENBERG_MARQUARDT_STRATEGY_H_