append_as_triplets.hpp

// Copyright (c) Sleipnir contributors
 
#pragma once
 
#include <initializer_list>
 
#include <Eigen/SparseCore>
#include <gch/small_vector.hpp>
 
template <typename Scalar>
void append_as_triplets(
    gch::small_vector<Eigen::Triplet<Scalar>>& triplets, int row_offset,
    int col_offset, std::initializer_list<Eigen::SparseMatrix<Scalar>> mats) {
  // Compute row offset for each matrix
  gch::small_vector<int> mat_row_offsets;
  int mat_row_offset = 0;
  for (const auto& mat : mats) {
    mat_row_offsets.emplace_back(mat_row_offset);
    mat_row_offset += mat.rows();
  }
 
  // Append elements in column-major order
  for (int col = 0; col < mats.begin()[0].cols(); ++col) {
    for (size_t i = 0; i < mats.size(); ++i) {
      for (typename Eigen::SparseMatrix<Scalar>::InnerIterator it{
               mats.begin()[i], col};
           it; ++it) {
        triplets.emplace_back(row_offset + mat_row_offsets[i] + it.row(),
                              col_offset + it.col(), it.value());
      }
    }
  }
}