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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2015 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)
+//         keir@google.com (Keir Mierle)
+
+#ifndef CERES_INTERNAL_EVALUATOR_H_
+#define CERES_INTERNAL_EVALUATOR_H_
+
+#include <map>
+#include <string>
+#include <vector>
+
+#include "ceres/context_impl.h"
+#include "ceres/execution_summary.h"
+#include "ceres/internal/port.h"
+#include "ceres/types.h"
+
+namespace ceres {
+
+struct CRSMatrix;
+class EvaluationCallback;
+
+namespace internal {
+
+class Program;
+class SparseMatrix;
+
+// The Evaluator interface offers a way to interact with a least squares cost
+// function that is useful for an optimizer that wants to minimize the least
+// squares objective. This insulates the optimizer from issues like Jacobian
+// storage, parameterization, etc.
+class Evaluator {
+ public:
+  virtual ~Evaluator();
+
+  struct Options {
+    int num_threads = 1;
+    int num_eliminate_blocks = -1;
+    LinearSolverType linear_solver_type = DENSE_QR;
+    bool dynamic_sparsity = false;
+    ContextImpl* context = nullptr;
+    EvaluationCallback* evaluation_callback = nullptr;
+  };
+
+  static Evaluator* Create(const Options& options,
+                           Program* program,
+                           std::string* error);
+
+  // Build and return a sparse matrix for storing and working with the Jacobian
+  // of the objective function. The jacobian has dimensions
+  // NumEffectiveParameters() by NumParameters(), and is typically extremely
+  // sparse. Since the sparsity pattern of the Jacobian remains constant over
+  // the lifetime of the optimization problem, this method is used to
+  // instantiate a SparseMatrix object with the appropriate sparsity structure
+  // (which can be an expensive operation) and then reused by the optimization
+  // algorithm and the various linear solvers.
+  //
+  // It is expected that the classes implementing this interface will be aware
+  // of their client's requirements for the kind of sparse matrix storage and
+  // layout that is needed for an efficient implementation. For example
+  // CompressedRowOptimizationProblem creates a compressed row representation of
+  // the jacobian for use with CHOLMOD, where as BlockOptimizationProblem
+  // creates a BlockSparseMatrix representation of the jacobian for use in the
+  // Schur complement based methods.
+  virtual SparseMatrix* CreateJacobian() const = 0;
+
+  // Options struct to control Evaluator::Evaluate;
+  struct EvaluateOptions {
+    // If false, the loss function correction is not applied to the
+    // residual blocks.
+    bool apply_loss_function = true;
+
+    // If false, this evaluation point is the same as the last one.
+    bool new_evaluation_point = true;
+  };
+
+  // Evaluate the cost function for the given state. Returns the cost,
+  // residuals, and jacobian in the corresponding arguments. Both residuals and
+  // jacobian are optional; to avoid computing them, pass NULL.
+  //
+  // If non-NULL, the Jacobian must have a suitable sparsity pattern; only the
+  // values array of the jacobian is modified.
+  //
+  // state is an array of size NumParameters(), cost is a pointer to a single
+  // double, and residuals is an array of doubles of size NumResiduals().
+  virtual bool Evaluate(const EvaluateOptions& evaluate_options,
+                        const double* state,
+                        double* cost,
+                        double* residuals,
+                        double* gradient,
+                        SparseMatrix* jacobian) = 0;
+
+  // Variant of Evaluator::Evaluate where the user wishes to use the
+  // default EvaluateOptions struct. This is mostly here as a
+  // convenience method.
+  bool Evaluate(const double* state,
+                double* cost,
+                double* residuals,
+                double* gradient,
+                SparseMatrix* jacobian) {
+    return Evaluate(EvaluateOptions(),
+                    state,
+                    cost,
+                    residuals,
+                    gradient,
+                    jacobian);
+  }
+
+  // Make a change delta (of size NumEffectiveParameters()) to state (of size
+  // NumParameters()) and store the result in state_plus_delta.
+  //
+  // In the case that there are no parameterizations used, this is equivalent to
+  //
+  //   state_plus_delta[i] = state[i] + delta[i] ;
+  //
+  // however, the mapping is more complicated in the case of parameterizations
+  // like quaternions. This is the same as the "Plus()" operation in
+  // local_parameterization.h, but operating over the entire state vector for a
+  // problem.
+  virtual bool Plus(const double* state,
+                    const double* delta,
+                    double* state_plus_delta) const = 0;
+
+  // The number of parameters in the optimization problem.
+  virtual int NumParameters() const = 0;
+
+  // This is the effective number of parameters that the optimizer may adjust.
+  // This applies when there are parameterizations on some of the parameters.
+  virtual int NumEffectiveParameters()  const = 0;
+
+  // The number of residuals in the optimization problem.
+  virtual int NumResiduals() const = 0;
+
+  // The following two methods return copies instead of references so
+  // that the base class implementation does not have to worry about
+  // life time issues. Further, these calls are not expected to be
+  // frequent or performance sensitive.
+  virtual std::map<std::string, CallStatistics> Statistics() const {
+    return std::map<std::string, CallStatistics>();
+  }
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_EVALUATOR_H_