diff --git a/include/ceres/cost_function.h b/include/ceres/cost_function.h
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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2010, 2011, 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)
+//         keir@google.m (Keir Mierle)
+//
+// This is the interface through which the least squares solver accesses the
+// residual and Jacobian of the least squares problem. Users are expected to
+// subclass CostFunction to define their own terms in the least squares problem.
+//
+// It is recommended that users define templated residual functors for use as
+// arguments for AutoDiffCostFunction (see autodiff_cost_function.h), instead of
+// directly implementing the CostFunction interface. This often results in both
+// shorter code and faster execution than hand-coded derivatives. However,
+// specialized cases may demand direct implementation of the lower-level
+// CostFunction interface; for example, this is true when calling legacy code
+// which is not templated on numeric types.
+
+#ifndef CERES_PUBLIC_COST_FUNCTION_H_
+#define CERES_PUBLIC_COST_FUNCTION_H_
+
+#include <vector>
+#include "ceres/internal/macros.h"
+#include "ceres/internal/port.h"
+#include "ceres/types.h"
+
+namespace ceres {
+
+// This class implements the computation of the cost (a.k.a. residual) terms as
+// a function of the input (control) variables, and is the interface for users
+// to describe their least squares problem to Ceres. In other words, this is the
+// modelling layer between users and the Ceres optimizer. The signature of the
+// function (number and sizes of input parameter blocks and number of outputs)
+// is stored in parameter_block_sizes_ and num_residuals_ respectively. User
+// code inheriting from this class is expected to set these two members with the
+// corresponding accessors. This information will be verified by the Problem
+// when added with AddResidualBlock().
+class CostFunction {
+ public:
+  CostFunction() : num_residuals_(0) {}
+
+  virtual ~CostFunction() {}
+
+  // Inputs:
+  //
+  // parameters is an array of pointers to arrays containing the
+  // various parameter blocks. parameters has the same number of
+  // elements as parameter_block_sizes_.  Parameter blocks are in the
+  // same order as parameter_block_sizes_.i.e.,
+  //
+  //   parameters_[i] = double[parameter_block_sizes_[i]]
+  //
+  // Outputs:
+  //
+  // residuals is an array of size num_residuals_.
+  //
+  // jacobians is an array of size parameter_block_sizes_ containing
+  // pointers to storage for jacobian blocks corresponding to each
+  // parameter block. Jacobian blocks are in the same order as
+  // parameter_block_sizes, i.e. jacobians[i], is an
+  // array that contains num_residuals_* parameter_block_sizes_[i]
+  // elements. Each jacobian block is stored in row-major order, i.e.,
+  //
+  //   jacobians[i][r*parameter_block_size_[i] + c] =
+  //                              d residual[r] / d parameters[i][c]
+  //
+  // If jacobians is NULL, then no derivatives are returned; this is
+  // the case when computing cost only. If jacobians[i] is NULL, then
+  // the jacobian block corresponding to the i'th parameter block must
+  // not to be returned.
+  virtual bool Evaluate(double const* const* parameters,
+                        double* residuals,
+                        double** jacobians) const = 0;
+
+  const vector<int16>& parameter_block_sizes() const {
+    return parameter_block_sizes_;
+  }
+
+  int num_residuals() const {
+    return num_residuals_;
+  }
+
+ protected:
+  vector<int16>* mutable_parameter_block_sizes() {
+    return &parameter_block_sizes_;
+  }
+
+  void set_num_residuals(int num_residuals) {
+    num_residuals_ = num_residuals;
+  }
+
+ private:
+  // Cost function signature metadata: number of inputs & their sizes,
+  // number of outputs (residuals).
+  vector<int16> parameter_block_sizes_;
+  int num_residuals_;
+  DISALLOW_COPY_AND_ASSIGN(CostFunction);
+};
+
+}  // namespace ceres
+
+#endif  // CERES_PUBLIC_COST_FUNCTION_H_