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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)
+//
+// Class definition for the object that is responsible for applying a
+// second order correction to the Gauss-Newton based on the ideas in
+// BANS by Triggs et al.
+
+#ifndef CERES_INTERNAL_CORRECTOR_H_
+#define CERES_INTERNAL_CORRECTOR_H_
+
+namespace ceres {
+namespace internal {
+
+// Corrector is responsible for applying the second order correction
+// to the residual and jacobian of a least squares problem based on a
+// radial robust loss.
+//
+// The key idea here is to look at the expressions for the robustified
+// gauss newton approximation and then take its squareroot to get the
+// corresponding corrections to the residual and jacobian.  For the
+// full expressions see Eq. 10 and 11 in BANS by Triggs et al.
+class Corrector {
+ public:
+  // The constructor takes the squared norm, the value, the first and
+  // second derivatives of the LossFunction. It precalculates some of
+  // the constants that are needed to apply the correction. The
+  // correction constant alpha is constrained to be smaller than 1, if
+  // it becomes larger than 1, then it will reverse the sign of the
+  // residual and the correction. If alpha is equal to 1 will result
+  // in a divide by zero error. Thus we constrain alpha to be upper
+  // bounded by 1 - epsilon_.
+  //
+  // rho[1] needs to be positive. The constructor will crash if this
+  // condition is not met.
+  //
+  // In practical use CorrectJacobian should always be called before
+  // CorrectResidual, because the jacobian correction depends on the
+  // value of the uncorrected residual values.
+  explicit Corrector(double sq_norm, const double rho[3]);
+
+  // residuals *= sqrt(rho[1]) / (1 - alpha)
+  void CorrectResiduals(int nrow, double* residuals);
+
+  // jacobian = sqrt(rho[1]) * jacobian -
+  // sqrt(rho[1]) * alpha / sq_norm * residuals residuals' * jacobian.
+  //
+  // The method assumes that the jacobian has row-major storage. It is
+  // the caller's responsibility to ensure that the pointer to
+  // jacobian is not null.
+  void CorrectJacobian(int nrow, int ncol,
+                       double* residuals, double* jacobian);
+
+ private:
+  double sqrt_rho1_;
+  double residual_scaling_;
+  double alpha_sq_norm_;
+};
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_CORRECTOR_H_