Add C wrapper to basix for tabulation in C-code

cpp/CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.16)
 
 # Set the version
-project(Basix VERSION "0.6.0.0" LANGUAGES CXX)
+project(Basix VERSION "0.6.0.0" LANGUAGES CXX C)
 include(GNUInstallDirs)
 
 if(SKBUILD)
@@ -62,7 +62,8 @@ set(HEADERS_basix
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/e-hermite.h
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/mdspan.hpp
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/docs.h
-  ${CMAKE_CURRENT_BINARY_DIR}/basix/version.h)
+  ${CMAKE_CURRENT_BINARY_DIR}/basix/version.h
+  ${CMAKE_CURRENT_SOURCE_DIR}/basix/cwrapper.h)
 
 target_sources(basix PRIVATE
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/cell.cpp
@@ -87,10 +88,12 @@ target_sources(basix PRIVATE
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/e-crouzeix-raviart.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/e-bubble.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/basix/e-hermite.cpp
-  ${CMAKE_CURRENT_SOURCE_DIR}/basix/e-serendipity.cpp)
+  ${CMAKE_CURRENT_SOURCE_DIR}/basix/e-serendipity.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/basix/cwrapper.cpp)
 
 # Configure the library
 set_target_properties(basix PROPERTIES PUBLIC_HEADER basix/finite-element.h)
+set_target_properties(basix PROPERTIES PUBLIC_HEADER basix/cwrapper.h)
 set_target_properties(basix PROPERTIES PRIVATE_HEADER "${HEADERS_basix}")
 target_include_directories(basix PUBLIC
   $<INSTALL_INTERFACE:include>

cpp/basix/cwrapper.cpp

@@ -0,0 +1,61 @@
+// Copyright (c) 2022 Susanne Claus
+// FEniCS Project
+// SPDX-License-Identifier:    MIT
+
+// This file defines the C-API functions listed in cwrapper.h 
+// This is a C++ file that defines C calls 
+#include <basix/finite-element.h>
+#include <basix/cell.h>
+#include <span>
+#include "cwrapper.h"
+
+extern "C" {
+
+basix_element* basix_element_create(const int basix_family, const int basix_cell_type, const int degree, 
+                                    const int lagrange_variant, const int dpc_variant, const bool discontinuous)
+{
+    //Specify which element is needed
+    basix::element::family family = static_cast<basix::element::family>(basix_family); 
+    basix::cell::type cell_type = static_cast<basix::cell::type>(basix_cell_type); 
+    int k = degree;
+    basix::element::lagrange_variant lvariant = static_cast<basix::element::lagrange_variant>(lagrange_variant);
+    basix::element::dpc_variant dvariant = static_cast<basix::element::dpc_variant>(dpc_variant);
+
+    //Create C++ basix element object
+    basix::FiniteElement finite_element = basix::create_element(family, cell_type, k, lvariant,dvariant,discontinuous);
+
+    basix_element* element = reinterpret_cast<basix_element*>(new basix::FiniteElement(finite_element));
+
+    return element; 
+}
+
+void basix_element_destroy(basix_element *element)
+{
+    delete reinterpret_cast<basix::FiniteElement*>(element);
+}
+
+void basix_element_tabulate_shape(basix_element *element, const unsigned int num_points, 
+                                  const int nd, int* cshape)
+{
+    //Determine shape of tabulated values to allocate sufficient memory 
+    std::array<std::size_t, 4> shape = reinterpret_cast<basix::FiniteElement*>(element)->tabulate_shape(nd, num_points);
+
+    //Copy shape values 
+    std::copy(shape.begin(),shape.end(),cshape);
+}
+
+void basix_element_tabulate(basix_element *element, const unsigned int gdim, const double* points,
+                            const unsigned int num_points, const int nd, 
+                            double* table_data, long unsigned int table_data_size)
+{
+    basix::FiniteElement* finite_element = reinterpret_cast<basix::FiniteElement*>(element);
+
+    //Create span views on points and table values
+    std::span<const double> points_view{points,num_points*gdim};
+    std::span<double> basis{table_data, table_data_size};
+    std::array<std::size_t, 2> xshape{num_points,gdim};
+
+    finite_element->tabulate(nd, points_view, xshape, basis);
+}
+
+}

cpp/basix/cwrapper.h

@@ -0,0 +1,43 @@
+// Copyright (c) 2022 Susanne Claus
+// FEniCS Project
+// SPDX-License-Identifier:    MIT
+
+// Declares a simple C API wrapper around C++.
+
+#ifndef CWRAPPER_H_
+#define CWRAPPER_H_
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+// extern C indicates to C++ compiler that the following code 
+// is meant to be called by a C compiler 
+// i.e. tells the compiler that C naming conventions etc should be used
+extern "C" {
+#endif
+
+//@todo Could this be replaced by ufcx_finite_element? 
+/// Basix element structure to map c basix element to C++ basix finite_element
+/// The following int values indicate the position of the values in the corresponding 
+/// enum classes in basix
+typedef struct basix_element basix_element;
+
+basix_element* basix_element_create(const int basix_family, const int basix_cell_type, const int degree, 
+                                    const int lagrange_variant, const int dpc_variant, const bool discontinuous);
+
+void basix_element_destroy(basix_element *element);
+
+void basix_element_tabulate_shape(basix_element *element, const unsigned int num_points, 
+                                  const int nd, int* cshape);
+
+void basix_element_tabulate(basix_element *element, const unsigned int gdim, const double* points,
+                            const unsigned int num_points, const int nd, 
+                            double* table_data, long unsigned int table_data_size);
+
+
+#ifdef __cplusplus
+}
+#endif
+
+
+#endif /*CWRAPPER_H_*/