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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)
+//
+// Interface for matrices that allow block based random access.
+
+#ifndef CERES_INTERNAL_BLOCK_RANDOM_ACCESS_MATRIX_H_
+#define CERES_INTERNAL_BLOCK_RANDOM_ACCESS_MATRIX_H_
+
+#include <mutex>
+
+namespace ceres {
+namespace internal {
+
+// A matrix implementing the BlockRandomAccessMatrix interface is a
+// matrix whose rows and columns are divided into blocks. For example
+// the matrix A:
+//
+// 3 4 5
+// A = 5 [c_11 c_12 c_13]
+// 4 [c_21 c_22 c_23]
+//
+// has row blocks of size 5 and 4, and column blocks of size 3, 4 and
+// 5. It has six cells corresponding to the six row-column block
+// combinations.
+//
+// BlockRandomAccessMatrix objects provide access to cells c_ij using
+// the GetCell method. when a cell is present, GetCell will return a
+// CellInfo object containing a pointer to an array which contains the
+// cell as a submatrix and a mutex that guards this submatrix. If the
+// user is accessing the matrix concurrently, it is his responsibility
+// to use the mutex to exclude other writers from writing to the cell
+// concurrently.
+//
+// There is no requirement that all cells be present, i.e. the matrix
+// itself can be block sparse. When a cell is not present, the GetCell
+// method will return a NULL pointer.
+//
+// There is no requirement about how the cells are stored beyond that
+// form a dense submatrix of a larger dense matrix. Like everywhere
+// else in Ceres, RowMajor storage assumed.
+//
+// Example usage:
+//
+// BlockRandomAccessMatrix* A = new BlockRandomAccessMatrixSubClass(...)
+//
+// int row, col, row_stride, col_stride;
+// CellInfo* cell = A->GetCell(row_block_id, col_block_id,
+// &row, &col,
+// &row_stride, &col_stride);
+//
+// if (cell != NULL) {
+// MatrixRef m(cell->values, row_stride, col_stride);
+// std::lock_guard<std::mutex> l(&cell->m);
+// m.block(row, col, row_block_size, col_block_size) = ...
+// }
+
+// Structure to carry a pointer to the array containing a cell and the
+// mutex guarding it.
+struct CellInfo {
+ CellInfo()
+ : values(NULL) {
+ }
+
+ explicit CellInfo(double* ptr)
+ : values(ptr) {
+ }
+
+ double* values;
+ std::mutex m;
+};
+
+class BlockRandomAccessMatrix {
+ public:
+ virtual ~BlockRandomAccessMatrix();
+
+ // If the cell (row_block_id, col_block_id) is present, then return
+ // a CellInfo with a pointer to the dense matrix containing it,
+ // otherwise return NULL. The dense matrix containing this cell has
+ // size row_stride, col_stride and the cell is located at position
+ // (row, col) within this matrix.
+ //
+ // The size of the cell is row_block_size x col_block_size is
+ // assumed known to the caller. row_block_size less than or equal to
+ // row_stride and col_block_size is upper bounded by col_stride.
+ virtual CellInfo* GetCell(int row_block_id,
+ int col_block_id,
+ int* row,
+ int* col,
+ int* row_stride,
+ int* col_stride) = 0;
+
+ // Zero out the values of the array. The structure of the matrix
+ // (size and sparsity) is preserved.
+ virtual void SetZero() = 0;
+
+ // Number of scalar rows and columns in the matrix, i.e the sum of
+ // all row blocks and column block sizes respectively.
+ virtual int num_rows() const = 0;
+ virtual int num_cols() const = 0;
+};
+
+} // namespace internal
+} // namespace ceres
+
+#endif // CERES_INTERNAL_BLOCK_RANDOM_ACCESS_MATRIX_H_