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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2017 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_FUNCTION_SAMPLE_H_
+#define CERES_INTERNAL_FUNCTION_SAMPLE_H_
+
+#include <string>
+#include "ceres/internal/eigen.h"
+
+namespace ceres {
+namespace internal {
+
+// FunctionSample is used by the line search routines to store and
+// communicate the value and (optionally) the gradient of the function
+// being minimized.
+//
+// Since line search as the name implies happens along a certain
+// line/direction. FunctionSample contains the information in two
+// ways. Information in the ambient space and information along the
+// direction of search.
+struct FunctionSample {
+  FunctionSample();
+  FunctionSample(double x, double value);
+  FunctionSample(double x, double value, double gradient);
+
+  std::string ToDebugString() const;
+
+  // x is the location of the sample along the search direction.
+  double x;
+
+  // Let p be a point and d be the search direction then
+  //
+  // vector_x = p + x * d;
+  Vector vector_x;
+  // True if vector_x has been assigned a valid value.
+  bool vector_x_is_valid;
+
+  // value = f(vector_x)
+  double value;
+  // True of the evaluation was successful and value is a finite
+  // number.
+  bool value_is_valid;
+
+  // vector_gradient = Df(vector_position);
+  //
+  // D is the derivative operator.
+  Vector vector_gradient;
+  // True if the vector gradient was evaluated and the evaluation was
+  // successful (the value is a finite number).
+  bool vector_gradient_is_valid;
+
+  // gradient = d.transpose() * vector_gradient
+  //
+  // where d is the search direction.
+  double gradient;
+  // True if the evaluation of the gradient was sucessful and the
+  // value is a finite number.
+  bool gradient_is_valid;
+};
+
+
+
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_FUNCTION_SAMPLE_H_