From 5ffe169da2b56ac275977744b7b87e63586167ff Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Sun, 18 Feb 2024 11:11:16 +0100
Subject: [PATCH] Add examples for Eigen.

---
 src/examples/CMakeLists.txt   | 12 +++++++++
 src/examples/eigen_map.cpp    | 48 +++++++++++++++++++++++++++++++++++
 src/examples/eigen_matrix.cpp | 33 ++++++++++++++++++++++++
 src/examples/eigen_vector.cpp | 33 ++++++++++++++++++++++++
 4 files changed, 126 insertions(+)
 create mode 100644 src/examples/eigen_map.cpp
 create mode 100644 src/examples/eigen_matrix.cpp
 create mode 100644 src/examples/eigen_vector.cpp

diff --git a/src/examples/CMakeLists.txt b/src/examples/CMakeLists.txt
index ab5700c6a..47c43e00a 100644
--- a/src/examples/CMakeLists.txt
+++ b/src/examples/CMakeLists.txt
@@ -30,6 +30,12 @@ set(boost_examples
   ${CMAKE_CURRENT_SOURCE_DIR}/boost_ublas_double.cpp
 )
 
+set(eigen_examples
+  ${CMAKE_CURRENT_SOURCE_DIR}/eigen_matrix.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/eigen_vector.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/eigen_map.cpp
+)
+
 set(hl_hdf5_examples
   ${CMAKE_CURRENT_SOURCE_DIR}/hl_hdf5_inmemory_files.cpp
 )
@@ -70,6 +76,12 @@ if(HIGHFIVE_TEST_BOOST)
   endforeach()
 endif()
 
+if(HIGHFIVE_TEST_EIGEN)
+  foreach(example_source ${eigen_examples})
+    compile_example(${example_source} HighFiveEigenDependency)
+  endforeach()
+endif()
+
 if(HDF5_IS_PARALLEL)
   foreach(example_source ${parallel_hdf5_examples})
     compile_example(${example_source})
diff --git a/src/examples/eigen_map.cpp b/src/examples/eigen_map.cpp
new file mode 100644
index 000000000..caf477b3b
--- /dev/null
+++ b/src/examples/eigen_map.cpp
@@ -0,0 +1,48 @@
+#include <highfive/highfive.hpp>
+#include <highfive/eigen.hpp>
+
+// Example showing reading and writing of `Eigen::Map`. Using
+// `Map<Matrix, ...>` as an example, but `Map<Array, ...>` works
+// analogously.
+//
+// Both `Eigen::Matrix` and `Eigen::Vector` have their own examples.
+
+int main() {
+    HighFive::File file("eigen_map.h5", HighFive::File::Truncate);
+
+    // Somehow allocate some memory:
+    double* p1 = (double*) malloc(4*3 * sizeof(double));
+
+    Eigen::Map<Eigen::MatrixXd> A(p1, 4, 3);
+
+    // clang-format off
+    A <<  1,  2,  3,
+          4,  5,  6,
+          7,  8,  9,
+         10, 11, 12;
+    // clang-format on
+    std::cout << "A = \n" << A << "\n\n";
+
+    // Write it to the file:
+    file.createDataSet("mat", A);
+
+    // ... and read it back as fixed-size and row-major:
+    using Matrix43d = Eigen::Matrix<double, 4, 3, Eigen::RowMajor>;
+
+    // Again, memory was obtain somehow, and we create an `Eigen::Map`
+    // from it:
+    double* p2 = (double*) malloc(4*3 * sizeof(double));
+    Eigen::Map<Matrix43d> B(p2, 4, 3);
+
+    // Since, we've pre-allocated the memory, we use the overload of `read`
+    // accepts `B` and an argument. Note, this will throw if `B` needs to be
+    // resized, because a map shouldn't resize the underlying memory:
+    file.getDataSet("mat").read(B);
+
+    std::cout << "B = \n" << B << "\n";
+
+    free(p1);
+    free(p2);
+
+    return 0;
+}
diff --git a/src/examples/eigen_matrix.cpp b/src/examples/eigen_matrix.cpp
new file mode 100644
index 000000000..7a40445d7
--- /dev/null
+++ b/src/examples/eigen_matrix.cpp
@@ -0,0 +1,33 @@
+#include <highfive/highfive.hpp>
+#include <highfive/eigen.hpp>
+
+// Example showing reading and writing of `Eigen::Matrix`. Using
+// `Eigen::Matrix` as an example, but `Eigen::Array` works analogously.
+//
+// Both `Eigen::Vector` and `Eigen::Map` have their own examples.
+
+int main() {
+    HighFive::File file("eigen_matrix.h5", HighFive::File::Truncate);
+
+    // Create a matrix.
+    Eigen::MatrixXd A(4, 3);
+    // clang-format off
+    A <<  1,  2,  3,
+          4,  5,  6,
+          7,  8,  9,
+         10, 11, 12;
+    // clang-format on
+    //
+    std::cout << "A = \n" << A << "\n\n";
+
+    // Write it to the file:
+    file.createDataSet("mat", A);
+
+    // ... and read it back as fixed-size and row-major:
+    using Matrix43d = Eigen::Matrix<double, 4, 3, Eigen::RowMajor>;
+    auto B = file.getDataSet("mat").read<Matrix43d>();
+
+    std::cout << "B = \n" << B << "\n";
+
+    return 0;
+}
diff --git a/src/examples/eigen_vector.cpp b/src/examples/eigen_vector.cpp
new file mode 100644
index 000000000..c5f723de2
--- /dev/null
+++ b/src/examples/eigen_vector.cpp
@@ -0,0 +1,33 @@
+#include <highfive/highfive.hpp>
+#include <highfive/eigen.hpp>
+
+// Example showing reading and writing of `Eigen::Matrix`. Using
+// `Eigen::Matrix` as an example, but `Eigen::Array` works analogously.
+//
+// Both `Eigen::Vector` and `Eigen::Map` have their own examples.
+
+int main() {
+    HighFive::File file("eigen_vector.h5", HighFive::File::Truncate);
+
+    // Create a matrix.
+    Eigen::VectorXd v(3);
+    v <<  1,  2,  3;
+    std::cout << "v = \n" << v << "\n\n";
+
+    // Write it to the file:
+    file.createDataSet("col_vec", v);
+
+    // The twist is that Eigen typedefs:
+    //   using VectorXd = Matrix<double, Dynamic, 1>;
+    //
+    // Therefore, for HighFive it's indistinguishable from a Nx1 matrix. Since,
+    // Eigen distinguishes row and column vectors, the HighFive chooses to
+    // respect the distinction and deduces the shape of vector as Nx1.
+
+    // ... and read it back as fixed-size:
+    auto w = file.getDataSet("col_vec").read<Eigen::Vector3d>();
+
+    std::cout << "w = \n" << w << "\n";
+
+    return 0;
+}