make_projection_matrix.hpp

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// This file is part of the Acts project.
//
// Copyright (C) 2016-2020 CERN for the benefit of the Acts project
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#pragma once

#include "Acts/Utilities/Definitions.hpp"

namespace Acts {
namespace detail {

template <unsigned int columns, unsigned int... rows>
struct make_projection_matrix;

// build projection matrix by iteratively stacking row vectors
template <unsigned int columns, unsigned int i, unsigned int... N>
struct make_projection_matrix<columns, i, N...> {
  static ActsMatrixD<sizeof...(N) + 1, columns> init() {
    ActsRowVectorD<columns> v;
    v.setZero();
    v(i) = 1;

    ActsMatrixD<sizeof...(N) + 1, columns> m;
    m.row(0) << v;
    m.block(1, 0, sizeof...(N), columns)
        << make_projection_matrix<columns, N...>::init();

    return m;
  }
};

// projection matrix for a single local parameter is a simple row vector
template <unsigned int columns, unsigned int i>
struct make_projection_matrix<columns, i> {
  static ActsRowVectorD<columns> init() {
    ActsRowVectorD<columns> v;
    v.setZero();
    v(i) = 1;
    return v;
  }
};

}  // namespace detail
}  // namespace Acts