// 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: 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;

// TODO(sameeragarwal): This class needs documemtation.

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'.

  std::vector<int*> jacobian_layout_;

  // The pointers in jacobian_layout_ point directly into this vector.

  std::vector<int> jacobian_layout_storage_;

};

}  // namespace internal

}  // namespace ceres

#endif  // CERES_INTERNAL_BLOCK_JACOBIAN_WRITER_H_