// Ceres Solver - A fast non-linear least squares minimizer
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// 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>

#include "ceres/internal/export.h"

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 nullptr 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 != nullptr) {
//     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 CERES_NO_EXPORT CellInfo {
  CellInfo() = default;
  explicit CellInfo(double* values) : values(values) {}

  double* values{nullptr};
  std::mutex m;
};

class CERES_NO_EXPORT 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 nullptr. 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_