examples/natmod: Add very simple features0 example to compute factorial.

examples/natmod/features0/Makefile

@@ -0,0 +1,14 @@
+# Location of top-level MicroPython directory
+MPY_DIR = ../../..
+
+# Name of module
+MOD = features0
+
+# Source files (.c or .py)
+SRC = features0.c
+
+# Architecture to build for (x86, x64, armv7m, xtensa, xtensawin)
+ARCH = x64
+
+# Include to get the rules for compiling and linking the module
+include $(MPY_DIR)/py/dynruntime.mk

examples/natmod/features0/features0.c

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+/* This example demonstrates the following features in a native module:
+    - defining a simple function exposed to Python
+    - defining a local, helper C function
+    - getting and creating integer objects
+*/
+
+// Include the header file to get access to the MicroPython API
+#include "py/dynruntime.h"
+
+// Helper function to compute factorial
+STATIC mp_int_t factorial_helper(mp_int_t x) {
+    if (x == 0) {
+        return 1;
+    }
+    return x * factorial_helper(x - 1);
+}
+
+// This is the function which will be called from Python, as factorial(x)
+STATIC mp_obj_t factorial(mp_obj_t x_obj) {
+    // Extract the integer from the MicroPython input object
+    mp_int_t x = mp_obj_get_int(x_obj);
+    // Calculate the factorial
+    mp_int_t result = factorial_helper(x);
+    // Convert the result to a MicroPython integer object and return it
+    return mp_obj_new_int(result);
+}
+// Define a Python reference to the function above
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(factorial_obj, factorial);
+
+// This is the entry point and is called when the module is imported
+mp_obj_t mpy_init(mp_obj_fun_bc_t *self, size_t n_args, size_t n_kw, mp_obj_t *args) {
+    // This must be first, it sets up the globals dict and other things
+    MP_DYNRUNTIME_INIT_ENTRY
+
+    // Make the function available in the module's namespace
+    mp_store_global(MP_QSTR_factorial, MP_OBJ_FROM_PTR(&factorial_obj));
+
+    // This must be last, it restores the globals dict
+    MP_DYNRUNTIME_INIT_EXIT
+}