[ML] Eigen block matrix view CwiseNullaryOp

including tests.

MathLib/EigenBlockMatrixView.h

+/**
+ * \file
+ * \copyright
+ * Copyright (c) 2012-2023, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/project/license
+ */
+
+#pragma once
+
+#include <Eigen/Core>
+
+namespace MathLib
+{
+template <int D, typename M>
+class EigenBlockMatrixViewFunctor
+{
+public:
+    static constexpr int rows = M::RowsAtCompileTime == Eigen::Dynamic
+                                    ? Eigen::Dynamic
+                                    : D * M::RowsAtCompileTime;
+    static constexpr int cols = M::ColsAtCompileTime == Eigen::Dynamic
+                                    ? Eigen::Dynamic
+                                    : D * M::ColsAtCompileTime;
+    using Scalar = typename M::Scalar;
+    using Matrix = Eigen::Matrix<Scalar, rows, cols, Eigen::ColMajor>;
+
+    constexpr explicit EigenBlockMatrixViewFunctor(const M& matrix)
+        : matrix_(matrix){};
+
+    constexpr const Scalar operator()(Eigen::Index row, Eigen::Index col) const
+    {
+        if (row / matrix_.rows() != col / matrix_.cols())
+        {
+            return 0;
+        }
+        return matrix_(row % matrix_.rows(), col % matrix_.cols());
+    }
+
+private:
+    const typename M::Nested& matrix_;
+};
+
+template <int D, typename M>
+constexpr Eigen::CwiseNullaryOp<
+    EigenBlockMatrixViewFunctor<D, M>,
+    typename EigenBlockMatrixViewFunctor<D, M>::Matrix>
+eigenBlockMatrixView(const Eigen::MatrixBase<M>& matrix)
+{
+    using Matrix = typename EigenBlockMatrixViewFunctor<D, M>::Matrix;
+    return Matrix::NullaryExpr(
+        D * matrix.rows(), D * matrix.cols(),
+        EigenBlockMatrixViewFunctor<D, M>(matrix.derived()));
+}
+}  // namespace MathLib

Tests/MathLib/EigenBlockMatrixView.cpp

+/**
+ * \file
+ * \copyright
+ * Copyright (c) 2012-2023, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/project/license
+ */
+
+#include "MathLib/EigenBlockMatrixView.h"
+
+#include <gtest/gtest.h>
+
+#include <Eigen/Core>
+
+TEST(MathLib_EigenBlockMatriView, 1by1)
+{
+    Eigen::Matrix<double, 4, 4> expected =
+        Eigen::Vector4d::Constant(42).asDiagonal();
+
+    // fixed size
+    Eigen::Matrix<double, 1, 1> A{42};
+    ASSERT_EQ(expected, MathLib::eigenBlockMatrixView<4>(A));
+
+    // dynamic size
+    Eigen::MatrixXd B(1, 1);
+    B << 42;
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<4>(B));
+}
+
+TEST(MathLib_EigenBlockMatriView, 1byN)
+{
+    Eigen::Matrix<double, 6, 3> expected;
+    // clang-format off
+    expected << 1, 0, 0,
+                2, 0, 0,
+                0, 1, 0,
+                0, 2, 0,
+                0, 0, 1,
+                0, 0, 2;
+    // clang-format on
+
+    Eigen::Vector2d v{1, 2};
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<3>(v));
+
+    // dynamic
+    Eigen::MatrixXd w(2, 1);
+    w << 1, 2;
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<3>(w));
+}
+
+TEST(MathLib_EigenBlockMatriView, Nby1)
+{
+    Eigen::Matrix<double, 3, 6> expected;
+    // clang-format off
+    expected << 1, 2, 0, 0, 0, 0,
+                0, 0, 1, 2, 0, 0,
+                0, 0, 0, 0, 1, 2;
+    // clang-format on
+
+    Eigen::RowVector2d v{1, 2};
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<3>(v));
+
+    // dynamic
+    Eigen::MatrixXd w(1, 2);
+    w << 1, 2;
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<3>(w));
+}
+
+TEST(MathLib_EigenBlockMatriView, NbyM)
+{
+    Eigen::Matrix<double, 6, 6> expected;
+    // clang-format off
+    expected << 1, 2, 0, 0, 0, 0,
+                3, 4, 0, 0, 0, 0,
+                0, 0, 1, 2, 0, 0,
+                0, 0, 3, 4, 0, 0,
+                0, 0, 0, 0, 1, 2,
+                0, 0, 0, 0, 3, 4;
+    // clang-format on
+
+    Eigen::Matrix2d A;
+    A << 1, 2, 3, 4;
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<3>(A));
+
+    // dynamic
+    Eigen::MatrixXd B(2, 2);
+    B << 1, 2, 3, 4;
+    ASSERT_TRUE(expected == MathLib::eigenBlockMatrixView<3>(B));
+}