diff --git a/internal/ceres/function_sample.cc b/internal/ceres/function_sample.cc
new file mode 100644
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+++ b/internal/ceres/function_sample.cc
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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)
+
+#include "ceres/function_sample.h"
+#include "ceres/stringprintf.h"
+
+namespace ceres {
+namespace internal {
+
+FunctionSample::FunctionSample()
+    : x(0.0),
+      vector_x_is_valid(false),
+      value(0.0),
+      value_is_valid(false),
+      vector_gradient_is_valid(false),
+      gradient(0.0),
+      gradient_is_valid(false) {}
+
+FunctionSample::FunctionSample(const double x, const double value)
+    : x(x),
+      vector_x_is_valid(false),
+      value(value),
+      value_is_valid(true),
+      vector_gradient_is_valid(false),
+      gradient(0.0),
+      gradient_is_valid(false) {}
+
+FunctionSample::FunctionSample(const double x,
+                               const double value,
+                               const double gradient)
+    : x(x),
+      vector_x_is_valid(false),
+      value(value),
+      value_is_valid(true),
+      vector_gradient_is_valid(false),
+      gradient(gradient),
+      gradient_is_valid(true) {}
+
+std::string FunctionSample::ToDebugString() const {
+  return StringPrintf("[x: %.8e, value: %.8e, gradient: %.8e, "
+                      "value_is_valid: %d, gradient_is_valid: %d]",
+                      x, value, gradient, value_is_valid, gradient_is_valid);
+}
+
+}  // namespace internal
+}  // namespace ceres