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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: keir@google.com (Keir Mierle)
+//
+// A jacobian writer that writes to block sparse matrices. The "writer" name is
+// misleading, since the Write() operation on the block jacobian writer does not
+// write anything. Instead, the Prepare() method on the BlockEvaluatePreparers
+// makes a jacobians array which has direct pointers into the block sparse
+// jacobian. When the cost function is evaluated, the jacobian blocks get placed
+// directly in their final location.
+
+#ifndef CERES_INTERNAL_BLOCK_JACOBIAN_WRITER_H_
+#define CERES_INTERNAL_BLOCK_JACOBIAN_WRITER_H_
+
+#include <vector>
+#include "ceres/evaluator.h"
+#include "ceres/internal/port.h"
+
+namespace ceres {
+namespace internal {
+
+class BlockEvaluatePreparer;
+class Program;
+class SparseMatrix;
+
+class BlockJacobianWriter {
+ public:
+  BlockJacobianWriter(const Evaluator::Options& options,
+                      Program* program);
+
+  // JacobianWriter interface.
+
+  // Create evaluate prepareres that point directly into the final jacobian.
+  // This makes the final Write() a nop.
+  BlockEvaluatePreparer* CreateEvaluatePreparers(int num_threads);
+
+  SparseMatrix* CreateJacobian() const;
+
+  void Write(int /* residual_id */,
+             int /* residual_offset */,
+             double** /* jacobians */,
+             SparseMatrix* /* jacobian */) {
+    // This is a noop since the blocks were written directly into their final
+    // position by the outside evaluate call, thanks to the jacobians array
+    // prepared by the BlockEvaluatePreparers.
+  }
+
+ private:
+  Program* program_;
+
+  // Stores the position of each residual / parameter jacobian.
+  //
+  // The block sparse matrix that this writer writes to is stored as a set of
+  // contiguos dense blocks, one after each other; see BlockSparseMatrix. The
+  // "double* values_" member of the block sparse matrix contains all of these
+  // blocks. Given a pointer to the first element of a block and the size of
+  // that block, it's possible to write to it.
+  //
+  // In the case of a block sparse jacobian, the jacobian writer needs a way to
+  // find the offset in the values_ array of each residual/parameter jacobian
+  // block.
+  //
+  // That is the purpose of jacobian_layout_.
+  //
+  // In particular, jacobian_layout_[i][j] is the offset in the values_ array of
+  // the derivative of residual block i with respect to the parameter block at
+  // active argument position j.
+  //
+  // The active qualifier means that non-active parameters do not count. Care
+  // must be taken when indexing into jacobian_layout_ to account for this.
+  // Consider a single residual example:
+  //
+  //   r(x, y, z)
+  //
+  // with r in R^3, x in R^4, y in R^2, and z in R^5.
+  // Take y as a constant (non-active) parameter.
+  // Take r as residual number 0.
+  //
+  // In this case, the active arguments are only (x, z), so the active argument
+  // position for x is 0, and the active argument position for z is 1. This is
+  // similar to thinking of r as taking only 2 parameters:
+  //
+  //   r(x, z)
+  //
+  // There are only 2 jacobian blocks: dr/dx and dr/dz. jacobian_layout_ would
+  // have the following contents:
+  //
+  //   jacobian_layout_[0] = { 0, 12 }
+  //
+  // which indicates that dr/dx is located at values_[0], and dr/dz is at
+  // values_[12]. See BlockEvaluatePreparer::Prepare()'s comments about 'j'.
+  vector<int*> jacobian_layout_;
+
+  // The pointers in jacobian_layout_ point directly into this vector.
+  vector<int> jacobian_layout_storage_;
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_BLOCK_JACOBIAN_WRITER_H_