diff --git a/internal/ceres/trust_region_strategy.h b/internal/ceres/trust_region_strategy.h
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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2012 Google Inc. All rights reserved.
+// http://code.google.com/p/ceres-solver/
+//
+// 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_TRUST_REGION_STRATEGY_H_
+#define CERES_INTERNAL_TRUST_REGION_STRATEGY_H_
+
+#include "ceres/linear_solver.h"
+
+namespace ceres {
+namespace internal {
+
+// Interface for classes implementing various trust region strategies
+// for nonlinear least squares problems.
+//
+// The object is expected to maintain and update a trust region
+// radius, which it then uses to solve for the trust region step using
+// the jacobian matrix and residual vector.
+//
+// Here the term trust region radius is used loosely, as the strategy
+// is free to treat it as guidance and violate it as need be. e.g.,
+// the LevenbergMarquardtStrategy uses the inverse of the trust region
+// radius to scale the damping term, which controls the step size, but
+// does not set a hard limit on its size.
+class TrustRegionStrategy {
+public:
+  struct Options {
+    Options()
+        : trust_region_strategy_type(LEVENBERG_MARQUARDT),
+          initial_radius(1e4),
+          max_radius(1e32),
+          lm_min_diagonal(1e-6),
+          lm_max_diagonal(1e32) {
+    }
+
+    TrustRegionStrategyType trust_region_strategy_type;
+    // Linear solver used for actually solving the trust region step.
+    LinearSolver* linear_solver;
+    double initial_radius;
+    double max_radius;
+
+    // Minimum and maximum values of the diagonal damping matrix used
+    // by LevenbergMarquardtStrategy.
+    double lm_min_diagonal;
+    double lm_max_diagonal;
+  };
+
+  // Per solve options.
+  struct PerSolveOptions {
+    // Forcing sequence for inexact solves.
+    double eta;
+  };
+
+  ~TrustRegionStrategy();
+
+  // Use the current radius to solve for the trust region step.
+  virtual LinearSolver::Summary ComputeStep(
+      const PerSolveOptions& per_solve_options,
+      SparseMatrix* jacobian,
+      const double* residuals,
+      double* step) = 0;
+
+  // Inform the strategy that the current step has been accepted, and
+  // that the ratio of the decrease in the non-linear objective to the
+  // decrease in the trust region model is step_quality.
+  virtual void StepAccepted(double step_quality) = 0;
+
+  // Inform the strategy that the current step has been rejected, and
+  // that the ratio of the decrease in the non-linear objective to the
+  // decrease in the trust region model is step_quality.
+  virtual void StepRejected(double step_quality) = 0;
+
+  // Current trust region radius.
+  virtual double Radius() const = 0;
+
+  // Factory.
+  static TrustRegionStrategy* Create(const Options& options);
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_TRUST_REGION_STRATEGY_H_