[WIP] C++ migration

.github/workflows/ubuntu.yml

@@ -25,33 +25,26 @@ jobs:
           {name: 'gcc', version: 11}
         ]
         build-type: ["Release"]
+        cxx: ["ON", "OFF"]
         mpi: ["ON", "OFF"]
         mtln: ["ON", "OFF"]
         hdf: ["ON"]
         double-precision: ["OFF"]
 
         include:
-          # Disable by lack of space on github action
-          # - os: ubuntu-latest
-          #   compiler: {name: 'nvidia-hpc', version: '24.5'}
-          #   build-type: "Release"
-          #   mpi: "OFF"
-          #   mtln: "OFF"
-          #   hdf: "OFF"
-          #   double-precision: "OFF"
-
           - os: ubuntu-latest # This is the only test with double precision.
             compiler: {name: 'intel', version: '2025.1'}
             build-type: "Release"
             mpi: "ON"
             mtln: "OFF"
             hdf: "ON"
             double-precision: "ON"
+            cxx: "OFF"
 
       fail-fast: false
 
     runs-on: ${{ matrix.os }}
-    name: ${{matrix.os}} / ${{matrix.compiler.name}} / ${{matrix.build-type}}-mpi(${{matrix.mpi}})-mtln(${{matrix.mtln}})-hdf(${{matrix.hdf}})-double(${{matrix.double-precision}})
+    name: ${{matrix.os}} / ${{matrix.compiler.name}} / ${{matrix.build-type}}-cxx(${{matrix.cxx}})-mpi(${{matrix.mpi}})-mtln(${{matrix.mtln}})-hdf(${{matrix.hdf}})-double(${{matrix.double-precision}})
 
     steps:       
     - name: Checkout
@@ -74,7 +67,13 @@ jobs:
       run: |
         sudo apt update
         sudo apt install libhdf5-dev libopenmpi-dev
-
+
+    - name: Install cxx required packages
+      if: matrix.cxx=='ON'
+      run: |
+        sudo apt update
+        sudo apt install nlohmann-json3-dev
+
     - name: Setup fortran compiler
       uses: fortran-lang/setup-fortran@v1
       id: setup-fortran
@@ -86,17 +85,24 @@ jobs:
       run: |       
         cmake -S . -B build \
           -DCMAKE_BUILD_TYPE=${{matrix.build-type}} \
+          -DSEMBA_FDTD_BUILD_CXX=${{matrix.cxx}} \
           -DSEMBA_FDTD_ENABLE_MPI=${{matrix.mpi}} \
           -DSEMBA_FDTD_ENABLE_HDF=${{matrix.hdf}} \
           -DSEMBA_FDTD_ENABLE_MTLN=${{matrix.mtln}} \
           -DSEMBA_FDTD_ENABLE_DOUBLE_PRECISION=${{matrix.double-precision}}
         cmake --build build -j
 
     - name: Run unit tests 
-      run: build/bin/fdtd_tests
+      run: |
+        if [ "${{ matrix.cxx }}" = "ON" ]; then
+          build/bin/fdtd_cpp_tests
+        else
+          build/bin/fdtd_tests
+        fi
 
     - name: Run python tests
       env:
+        SEMBA_EXE: ${{ github.workspace }}/build/bin/${{ matrix.cxx == 'ON' && 'semba-fdtd-cpp' || 'semba-fdtd' }}
         SEMBA_FDTD_ENABLE_MPI: ${{ matrix.mpi }}
         SEMBA_FDTD_ENABLE_MTLN: ${{ matrix.mtln }}
         SEMBA_FDTD_ENABLE_HDF: ${{ matrix.hdf }}

.gitignore

@@ -40,6 +40,7 @@ venv/
 build/
 build-*/
 build_test/
+cpp_build/
 
 CMakeUserPresets.json
 CMakeSettings.json
@@ -62,16 +63,17 @@ probes*txt
 
 __pycache__/
 */__pycache__/
-src_pyWrapper/__pycache__/*
+pyWrapper/__pycache__/*
 tmp/
 tmp_cases/
 testData/outputs/paul/paul_8.6_square.txt
 testing_hdf5_writing_and_reading.h5
 fort.17
 build
 build*/
+cpp_build*/
 
-src_main_pub/version.F90
+src/main/version.F90
 git_info.txt
 
 .venv/build2/

CLAUDE.md

@@ -43,23 +43,35 @@ python3 -m venv .venv
 source .venv/bin/activate
 python3 -m pip install -r requirements.txt
 
-pytest test/ --durations=20
-
-# Run by marker
-pytest test/ -m mtln
-pytest test/ -m hdf
-pytest test/ -m mpi
+# Fortran full suite (set feature flags to match the binary)
+export SEMBA_EXE=$PWD/build/bin/semba-fdtd
+export SEMBA_FDTD_ENABLE_MTLN=ON
+export SEMBA_FDTD_ENABLE_HDF=ON
+export SEMBA_FDTD_ENABLE_MPI=ON
+PYTHONPATH=. pytest test/ --durations=20
+
+# C++ migration gates (build semba-fdtd-cpp first, then pytest by marker)
+cmake -S . -B cpp_build_nomtln -DSEMBA_FDTD_BUILD_CXX=ON \
+  -DSEMBA_FDTD_ENABLE_MTLN=OFF -DSEMBA_FDTD_ENABLE_MPI=OFF -DSEMBA_FDTD_ENABLE_HDF=OFF \
+  -DSEMBA_FDTD_MAIN_LIB=ON -DSEMBA_FDTD_EXECUTABLE=ON -DSEMBA_FDTD_ENABLE_TEST=ON
+cmake --build cpp_build_nomtln -j --target semba-fdtd-cpp
+export SEMBA_EXE=$PWD/cpp_build_nomtln/bin/semba-fdtd-cpp
+export SEMBA_FDTD_ENABLE_MTLN=OFF SEMBA_FDTD_ENABLE_MPI=OFF SEMBA_FDTD_ENABLE_HDF=OFF
+PYTHONPATH=. pytest test/pyWrapper/ -m cpp_migration
+
+# HDF / MTLN: same pattern with cpp_build_hdf or cpp_build_mtln and -m hdf or
+# test/pyWrapper/test_mtln_standalone.py -m mtln_standalone
 ```
 
-Test markers are defined in `pytest.ini`: `mtln`, `codemodel`, `hdf`, `mpi`.
+Test markers are defined in `pytest.ini` (`cpp_migration`, `hdf`, `mtln_standalone`, `mtln`, `mpi`, …).
 
-Unit test source is under `test/` in subdirectories: `mtln/`, `smbjson/`, `conformal/`, `observation/`, `rotate/`, `vtk/`, `pyWrapper/`.
+Unit test source: Fortran/C++ GoogleTest under `test/fortran/` (`mtln/`, `smbjson/`, etc.); C++ solver tests under `test/cpp/`; Python integration tests under `test/pyWrapper/`.
 
 ## Architecture
 
 ### Language & Build
 - Primary language: Fortran (free-form, ~49K+ lines)
-- C/C++ used only for unit tests (GoogleTest)
+- C++ solver in `src_cpp/` (mirrors `src/` layout); C++ also used for unit tests (GoogleTest)
 - Python used for integration tests and the `pyWrapper/` interface
 - Build system: CMake 3.15+
 
@@ -82,23 +94,23 @@ semba-types          (FDTD/NFDE/MTLN/conformal type definitions)
 
 ### Execution Flow
 
-1. `src_main_pub/launcher.F90` — entry point, creates `semba_fdtd_t`
-2. `src_main_pub/semba_fdtd.F90` — main module:
+1. `src/main/launcher.F90` — entry point, creates `semba_fdtd_t`
+2. `src/main/semba_fdtd.F90` — main module:
    - `init()`: load input (`.fdtd.json` via smbjson, or legacy `.fdtd` NFDE format)
    - `launch()`: run the time-stepping loop
    - `end()`: finalize and write outputs
-3. Time-step loop in `src_main_pub/timestepping.F90`:
+3. Time-step loop in `src/main/timestepping.F90`:
    - Update E-fields → apply materials, boundary conditions, wire coupling
    - Update H-fields → apply MTLN/SPICE if enabled
    - Sample observation probes, write snapshots
 
 ### Key Source Directories
 
-- `src_main_pub/` — core solver, time-stepping, preprocessing, geometry, main types
-- `src_conformal/` — conformal mapping (staircase reduction)
-- `src_mtln/` — MTLN circuit/transmission-line solver and ngspice coupling
-- `src_json_parser/` — `.fdtd.json` input format parser
-- `src_wires_pub/` — wire/thin-wire models
+- `src/main/` — core solver, time-stepping, preprocessing, geometry, main types
+- `src/conformal/` — conformal mapping (staircase reduction)
+- `src/mtln/` — MTLN circuit/transmission-line solver and ngspice coupling
+- `src/json_parser/` — `.fdtd.json` input format parser
+- `src/wires/` — wire/thin-wire models
 - `external/` — submodules: `json-fortran`, `fhash`, `googletest`, `ngspice`, `lapack`
 
 ### Input/Output
@@ -109,7 +121,7 @@ semba-types          (FDTD/NFDE/MTLN/conformal type definitions)
 
 ### Optional Features and Conditional Compilation
 
-Many modules are only compiled when their CMake flag is enabled. The smbjson parser, MTLN solver, and HDF5 output are all conditionally compiled. MPI support wraps communication in `src_main_pub/mpicomm.F90` and is activated via the `SEMBA_FDTD_ENABLE_MPI` flag.
+Many modules are only compiled when their CMake flag is enabled. The smbjson parser, MTLN solver, and HDF5 output are all conditionally compiled. MPI support wraps communication in `src/main/mpicomm.F90` and is activated via the `SEMBA_FDTD_ENABLE_MPI` flag.
 
 ## Platform Notes
 

CMakeLists.txt

@@ -1,5 +1,14 @@
 cmake_minimum_required (VERSION 3.15)
 
+# Allow choosing between Fortran and C++ builds
+option(SEMBA_FDTD_BUILD_CXX "Build C++ version instead of Fortran" OFF)
+
+if(SEMBA_FDTD_BUILD_CXX)
+    message(STATUS "Building C++ version of semba-fdtd")
+    include("${CMAKE_CURRENT_SOURCE_DIR}/cpp_build.cmake")
+    return()
+endif()
+
 project(semba-fdtd Fortran)
 enable_language (Fortran)
 
@@ -21,7 +30,7 @@ option(SEMBA_FDTD_ENABLE_DOUBLE_PRECISION "Use double precision (CompileWithReal
 option(SEMBA_FDTD_ENABLE_TEST "Compile tests" ON)
 
 option(SEMBA_FDTD_ENABLE_INTEL_XHOST_OPTIMIZATION "When compiling in Release, enables the -xHost optimization flag (not supported in github actions)" OFF) 
-option(SEMBA_FDTD_ENABLE_INTEL_IPO "When compiling in Release, enables the interprocedural optimization" OFF)
+option(SEMBA_FDTD_ENABLE_INTEL_IPO "When compiling in Release, enables interprocedural optimization" OFF)
 
 option(SEMBA_FDTD_EXECUTABLE "Compiles executable" ON)
 option(SEMBA_FDTD_MAIN_LIB "Compiles main library" ON)
@@ -117,13 +126,6 @@ elseif(CMAKE_SYSTEM_NAME MATCHES "Windows")
 	if(CMAKE_Fortran_COMPILER_ID MATCHES "IntelLLVM")
 		message(STATUS "Using IntelLLVM (ifx) flags")
 
-		# This prevents a compilation error happening due to visual studio headers being too strict.
-		# It can possibly can be removed in the future.
-		#add_compile_definitions("$<$<COMPILE_LANGUAGE:C,CXX>:-D_ALLOW_COMPILER_AND_STL_VERSION_MISMATCH>")
-		#add_compile_options($<$<COMPILE_LANGUAGE:Fortran>:/nofpp>)
-		#set(CMAKE_Fortran_FLAGS "/Qopenmp")
-		#set(CMAKE_CXX_FLAGS "/Qopenmp")
-
 		# Debug flags - enable full debugging support for breakpoints
 		set(CMAKE_Fortran_FLAGS_DEBUG "/debug:full /Od /traceback /check:all,nouninit /Zi /debug-parameters:all /warn:interfaces")
 		set(CMAKE_CXX_FLAGS_DEBUG "/Zi /Od /RTC1")
@@ -146,23 +148,23 @@ endif()
 add_subdirectory(external)
 
 add_library(semba-types 
-	"src_main_pub/nfde_types.F90"
-	"src_main_pub/fdetypes.F90"
-	"src_mtln/mtln_types.F90"
-	"src_conformal/conformal_types.F90"
-	"src_wires_pub/wires_types.F90"
-	"src_main_pub/lumped_types.F90"
+	"src/main/nfde_types.F90"
+	"src/main/fdetypes.F90"
+	"src/mtln/mtln_types.F90"
+	"src/conformal/conformal_types.F90"
+	"src/wires/wires_types.F90"
+	"src/main/lumped_types.F90"
 )
 target_link_libraries(semba-types ${MPI_Fortran_LIBRARIES})
 
 add_library(semba-reports
-	"src_main_pub/errorreport.F90"
-	"src_main_pub/snapxdmf.F90"
+	"src/main/errorreport.F90"
+	"src/main/snapxdmf.F90"
 )
 target_link_libraries(semba-reports semba-types ${HDF5_LIBRARIES} ${HDF5_HL_LIBRARIES})
 
 if(SEMBA_FDTD_ENABLE_SMBJSON)
-	add_subdirectory(src_json_parser)
+	add_subdirectory(src/json_parser)
 	if(PROJECT_IS_TOP_LEVEL)
 		set(SMBJSON_LIBRARIES smbjson)
 	else()
@@ -172,50 +174,50 @@ endif()
 if (SEMBA_FDTD_ENABLE_MTLN)
 	set(NGSPICE_LIB ngspice)
 	add_definitions(-DCompileWithMTLN)
-	add_subdirectory(src_mtln)
+	add_subdirectory(src/mtln)
 	if (PROJECT_IS_TOP_LEVEL)
 		set(MTLN_LIBRARIES mtlnsolver ngspice_interface)
 	else()
 		set(MTLN_LIBRARIES mtlnsolver ngspice_interface PARENT_SCOPE)
 	endif()
 endif()
 
-add_subdirectory(src_conformal)
+add_subdirectory(src/conformal)
 set(CONFORMAL_LIBRARIES conformal)
 
 if (SEMBA_FDTD_ENABLE_TEST)
 	add_subdirectory(external/googletest/)
-	add_subdirectory(test)
+	add_subdirectory(test/fortran)
 endif()
 
 if(SEMBA_FDTD_COMPONENTS_LIB)
 	add_library(semba-components
-		"src_main_pub/anisotropic.F90"
-		"src_main_pub/borderscpml.F90"
-		"src_main_pub/bordersmur.F90"
-		"src_main_pub/bordersother.F90"
-		"src_main_pub/electricdispersive.F90"
-		"src_main_pub/magneticdispersive.F90"
-		"src_main_pub/nodalsources.F90"
-		"src_main_pub/planewaves.F90"
-		"src_main_pub/pml_bodies.F90"
-		"src_main_pub/maloney_nostoch.F90"
-		"src_main_pub/lumped.F90"
-		"src_main_pub/dmma_thin_slot.F90"
-		"src_main_pub/farfield.F90"
-		"src_wires_pub/wires.F90"
-		"src_wires_pub/wires_mtln.F90"
+		"src/main/anisotropic.F90"
+		"src/main/borderscpml.F90"
+		"src/main/bordersmur.F90"
+		"src/main/bordersother.F90"
+		"src/main/electricdispersive.F90"
+		"src/main/magneticdispersive.F90"
+		"src/main/nodalsources.F90"
+		"src/main/planewaves.F90"
+		"src/main/pml_bodies.F90"
+		"src/main/maloney_nostoch.F90"
+		"src/main/lumped.F90"
+		"src/main/dmma_thin_slot.F90"
+		"src/main/farfield.F90"
+		"src/wires/wires.F90"
+		"src/wires/wires_mtln.F90"
 	)
 	target_link_libraries(semba-components semba-types semba-reports ${MTLN_LIBRARIES})
 endif()
 
 if(SEMBA_FDTD_OUTPUTS_LIB)
 	add_library(semba-outputs
-		"src_main_pub/mpicomm.F90"
-		"src_main_pub/observation.F90"
-		"src_main_pub/vtk.F90"
-		"src_main_pub/xdmf.F90"
-		"src_main_pub/xdmf_h5.F90"
+		"src/main/mpicomm.F90"
+		"src/main/observation.F90"
+		"src/main/vtk.F90"
+		"src/main/xdmf.F90"
+		"src/main/xdmf_h5.F90"
 	)
 	target_link_libraries(semba-outputs 
 		semba-components 
@@ -224,30 +226,28 @@ if(SEMBA_FDTD_OUTPUTS_LIB)
 endif()
 
 if(SEMBA_FDTD_MAIN_LIB)
-	# Prepares variables containing information on commit-hash and build flags.
-	# This information is only available at the configuration stage.
 	set(PROGRAM_NAME "semba-fdtd")
 	include("${CMAKE_CURRENT_SOURCE_DIR}/get_commit_info.cmake")
 	configure_file(
-	${CMAKE_SOURCE_DIR}/src_main_pub/version.F90.in
-	${CMAKE_SOURCE_DIR}/src_main_pub/version.F90
+	${CMAKE_SOURCE_DIR}/src/main/version.F90.in
+	${CMAKE_SOURCE_DIR}/src/main/version.F90
 	@ONLY
 	)
 
 	add_library(semba-main
-		"src_main_pub/semba_fdtd.F90"
-		"src_main_pub/calc_constants.F90"
-		"src_main_pub/nfde_rotate.F90"
-		"src_main_pub/EpsMuTimeScale.F90"
-		"src_main_pub/getargs.F90"
-		"src_main_pub/healer.F90"
-		"src_main_pub/preprocess_geom.F90"
-		"src_main_pub/storegeom.F90"
-		"src_main_pub/version.F90"
-		"src_main_pub/postprocess.F90"
-		"src_main_pub/interpreta_switches.F90"
-		"src_main_pub/resuming.F90"	
-		"src_main_pub/timestepping.F90"
+		"src/main/semba_fdtd.F90"
+		"src/main/calc_constants.F90"
+		"src/main/nfde_rotate.F90"
+		"src/main/EpsMuTimeScale.F90"
+		"src/main/getargs.F90"
+		"src/main/healer.F90"
+		"src/main/preprocess_geom.F90"
+		"src/main/storegeom.F90"
+		"src/main/version.F90"
+		"src/main/postprocess.F90"
+		"src/main/interpreta_switches.F90"
+		"src/main/resuming.F90"	
+		"src/main/timestepping.F90"
 	)
 	target_link_libraries(semba-main
 		semba-outputs 
@@ -257,7 +257,7 @@ endif()
 
 if (SEMBA_FDTD_EXECUTABLE)
 	add_executable(semba-fdtd 
-		"src_main_pub/launcher.F90"
+		"src/main/launcher.F90"
 	)
 	target_link_libraries(semba-fdtd semba-main semba-reports)
 	target_link_libraries(semba-fdtd ${MPI_Fortran_LIBRARIES})