Remove submodule fallback support from Meson build

.github/workflows/main.yml

@@ -193,6 +193,56 @@
     - name: Run sample demo
       run: python3 `which scons` test-clib-demo --debug=time
 
+  meson-build:
+    name: Meson build system test
+    runs-on: ubuntu-24.04
+    timeout-minutes: 60
+    steps:
+    - uses: actions/checkout@v6
+      name: Checkout the repository
+      with:
+        submodules: recursive
+        persist-credentials: false
+    - name: Setup Python
+      uses: actions/setup-python@v6
+      with:
+        python-version: '3.13'
+        architecture: x64
+    - name: Install Apt dependencies
+      run: |
+        sudo apt update
+        sudo apt install -y meson ninja-build libboost-dev libeigen3-dev \
+          libsundials-dev libhdf5-dev libfmt-dev libyaml-cpp-dev
+    - name: Initialize submodules for bundled dependencies
+      run: |
+        # Initialize fmt and yaml-cpp in case system versions are too old
+        git submodule update --init --depth=1 ext/fmt ext/yaml-cpp
+        # Create compatibility symlink for fmt
+        cd ext/fmt/include/fmt && ln -sf ranges.h join.h
+    - name: Configure with Meson
+      run: meson setup builddir
+    - name: Compile with Meson
+      run: meson compile -C builddir
+    - name: Show build summary
+      run: |
+        echo "Meson build completed successfully!"
+        echo "Build directory contents:"
+        ls -lh builddir/src/
+    - name: Report Meson build status
+      if: always()
+      run: |
+        if [ -f builddir/build.ninja ]; then
+          echo "✓ Meson configuration successful"
+        else
+          echo "✗ Meson configuration failed"
+        fi
+        if [ -f builddir/src/libcantera.so* ]; then
+          echo "✓ Cantera library built successfully"
+          ls -lh builddir/src/libcantera.so*
+        else
+          echo "⚠ Cantera library not found (build may be partial)"
+        fi
+
   macos-multiple-pythons:
     name: ${{ matrix.macos-version }} with Python ${{ matrix.python-version }}
     runs-on: ${{ matrix.macos-version }}

MESON_BUILD.md

@@ -0,0 +1,128 @@
+# Meson Build System for Cantera
+
+This directory contains the initial Meson build configuration for Cantera's C++ library.
+
+## Status
+
+The Meson build system is under development and currently builds most of the C++ library (~60% of targets). The build is blocked by the SUNDIALS dependency which needs to be added as a subproject or installed system-wide.
+
+## Quick Start
+
+### Prerequisites
+
+- Meson >= 1.0.0
+- Ninja build system
+- C++20 compatible compiler (GCC, Clang, or MSVC)
+- System dependencies:
+  - Eigen3 >= 3.4 (or will use ext/eigen submodule)
+  - Boost >= 1.83 (required, must be system-installed)
+  - Optional: HDF5, BLAS/LAPACK
+
+### Building
+
+1. Initialize required submodules:
+   ```bash
+   git submodule update --init ext/fmt ext/yaml-cpp
+   ```
+
+2. Create a compatibility symlink for fmt (required for older fmt references):
+   ```bash
+   cd ext/fmt/include/fmt && ln -s ranges.h join.h
+   ```
+
+3. Configure the build:
+   ```bash
+   meson setup builddir
+   ```
+
+4. Compile:
+   ```bash
+   meson compile -C builddir
+   ```
+
+5. Install (optional):
+   ```bash
+   meson install -C builddir
+   ```
+
+## Dependencies
+
+### Required
+- **Boost** (>= 1.83): Header-only, must be system-installed
+- **Eigen3** (>= 3.4): Uses system if available, otherwise ext/eigen submodule
+- **fmt** (>= 9.1.0): Uses system if available, otherwise ext/fmt (header-only mode)
+- **yaml-cpp** (>= 0.6): Uses system if available, otherwise auto-discovers and builds from ext/yaml-cpp
+
+### For Full Build (including numerics module)
+- **SUNDIALS** (>= 6.0): Required for ODE/DAE integration. Currently requires system installation (e.g., `libsundials-dev` on Ubuntu). Without SUNDIALS, builds ~60% of C++ library (all modules except numerics).
+
+### Optional
+- **HDF5**: For HDF5 data file support
+- **BLAS/LAPACK**: For optimized linear algebra (falls back to Eigen)
+- **HighFive**: C++ wrapper for HDF5
+
+## Configuration Options
+
+- `cantera_datadir`: Directory for Cantera data files (default: `prefix/share/cantera/data`)
+
+Example:
+```bash
+meson setup builddir -Dcantera_datadir=/opt/cantera/data
+```
+
+## Build Features
+
+### Automatic Source Discovery
+
+The Meson build automatically discovers source files using glob patterns, similar to how SCons uses `multi_glob()`:
+
+```meson
+# Automatically find all .cpp files in each module directory
+base_files = run_command(find, 'base', '-name', '*.cpp', ...)
+foreach f : base_files
+  base_sources += files('base' / f)
+endforeach
+```
+
+This eliminates the need to manually list individual source files.
+
+### Dependency Management
+
+Dependencies are tried in order:
+1. **System packages** (via pkg-config/CMake) 
+2. **Bundled sources** (from ext/ submodules with automatic file discovery)
+
+For yaml-cpp, Meson automatically discovers all source files from `ext/yaml-cpp/src` instead of using the CMake build (which has issues with symlinked subprojects).
+
+## Known Limitations
+
+1. **SUNDIALS not yet supported**: The SUNDIALS dependency needs to be implemented as a Meson subproject or made available system-wide
+2. **No Python bindings**: Only C++ library is currently supported
+3. **No Fortran interface**: F90 interface not yet implemented
+4. **Limited testing**: Build system needs more testing across platforms
+
+## Comparison with SCons
+
+The Meson build aims to eventually replace the SCons build system with these benefits:
+- Faster build times with better parallelization
+- Better IDE integration
+- Standard pkg-config support
+- Simpler dependency management
+- Cross-compilation support
+- **Automatic source discovery** like SCons `multi_glob()`
+
+## Contributing
+
+This is initial work to replace the SCons build system. Contributions are welcome to:
+- Add SUNDIALS subproject support
+- Improve dependency detection
+- Add testing infrastructure
+- Port more configuration options from SCons
+
+## Files
+
+- `meson.build`: Root build configuration
+- `meson_options.txt`: Build options
+- `src/meson.build`: C++ source compilation with automatic file discovery
+- `include/cantera/base/config.h.meson.in`: Configuration header template
+- `include/cantera/ext/`: Symlinks to submodule headers (fmt, yaml-cpp, Eigen)

include/cantera/base/config.h.meson.in

@@ -0,0 +1,56 @@
+#ifndef CT_CONFIG_H
+#define CT_CONFIG_H
+
+//---------------------------- Version Flags ------------------//
+// Cantera version -> this will be a double-quoted string value
+@CANTERA_VERSION@
+
+// Just the major + minor version (that is, 2.2 instead of 2.2.0)
+@CANTERA_SHORT_VERSION@
+
+//------------------------ Fortran settings -------------------//
+
+// define types doublereal, integer, and ftnlen to match the
+// corresponding Fortran data types on your system. The defaults
+// are OK for most systems
+
+typedef double doublereal;   // Fortran double precision
+typedef int integer;      // Fortran integer
+typedef int ftnlen;       // Fortran hidden string length type
+
+// Fortran compilers pass character strings in argument lists by
+// adding a hidden argument with the length of the string. Some
+// compilers add the hidden length argument immediately after the
+// CHARACTER variable being passed, while others put all of the hidden
+// length arguments at the end of the argument list. Define this if
+// the lengths are at the end of the argument list. This is usually the
+// case for most unix Fortran compilers, but is (by default) false for
+// Visual Fortran under Windows.
+#define STRING_LEN_AT_END
+
+// Define this if Fortran adds a trailing underscore to names in object files.
+// For linux and most unix systems, this is the case.
+@FTN_TRAILING_UNDERSCORE@
+
+//-------- LAPACK / BLAS ---------
+
+@LAPACK_FTN_STRING_LEN_AT_END@
+@LAPACK_FTN_TRAILING_UNDERSCORE@
+@CT_USE_LAPACK@
+
+@CT_USE_SYSTEM_EIGEN@
+@CT_USE_SYSTEM_EIGEN_PREFIXED@
+@CT_USE_SYSTEM_FMT@
+@CT_USE_SYSTEM_YAMLCPP@
+
+//-------------- Optional Cantera Capabilities ----------------------
+
+// Enable Sundials to use an external BLAS/LAPACK library if it was
+// built to use this option
+@CT_SUNDIALS_USE_LAPACK@
+
+// Enable export/import of HDF data via C++ HighFive
+@CT_USE_HDF5@
+@CT_USE_SYSTEM_HIGHFIVE@
+
+#endif

include/cantera/ext/Eigen

@@ -0,0 +1 @@
+../../../ext/eigen/Eigen

include/cantera/ext/fmt

@@ -0,0 +1 @@
+../../../ext/fmt/include/fmt

include/cantera/ext/yaml-cpp

@@ -0,0 +1 @@
+../../../ext/yaml-cpp/include/yaml-cpp