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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2019 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: mierle@gmail.com (Keir Mierle)
+
+#ifndef CERES_PUBLIC_EVALUATION_CALLBACK_H_
+#define CERES_PUBLIC_EVALUATION_CALLBACK_H_
+
+#include "ceres/internal/port.h"
+
+namespace ceres {
+
+// Using this callback interface, Ceres can notify you when it is
+// about to evaluate the residuals or jacobians. With the callback,
+// you can share computation between residual blocks by doing the
+// shared computation in PrepareForEvaluation() before Ceres calls
+// CostFunction::Evaluate(). It also enables caching results between a
+// pure residual evaluation and a residual & jacobian evaluation, via
+// the new_evaluation_point argument.
+//
+// One use case for this callback is if the cost function compute is
+// moved to the GPU. In that case, the prepare call does the actual
+// cost function evaluation, and subsequent calls from Ceres to the
+// actual cost functions merely copy the results from the GPU onto the
+// corresponding blocks for Ceres to plug into the solver.
+//
+// NOTE: Ceres provides no mechanism to share data other than the
+// notification from the callback. Users must provide access to
+// pre-computed shared data to their cost functions behind the scenes;
+// this all happens without Ceres knowing.
+//
+// One approach is to put a pointer to the shared data in each cost
+// function (recommended) or to use a global shared variable
+// (discouraged; bug-prone).  As far as Ceres is concerned, it is
+// evaluating cost functions like any other; it just so happens that
+// behind the scenes the cost functions reuse pre-computed data to
+// execute faster.
+class CERES_EXPORT EvaluationCallback {
+ public:
+  virtual ~EvaluationCallback() {}
+
+  // Called before Ceres requests residuals or jacobians for a given setting of
+  // the parameters. User parameters (the double* values provided to the cost
+  // functions) are fixed until the next call to PrepareForEvaluation(). If
+  // new_evaluation_point == true, then this is a new point that is different
+  // from the last evaluated point. Otherwise, it is the same point that was
+  // evaluated previously (either jacobian or residual) and the user can use
+  // cached results from previous evaluations.
+  virtual void PrepareForEvaluation(bool evaluate_jacobians,
+                                    bool new_evaluation_point) = 0;
+};
+
+}  // namespace ceres
+
+#endif  // CERES_PUBLIC_EVALUATION_CALLBACK_H_