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src/tutorial/adding-wasm-logic.md

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+# Adding wasm logic
+
+Adding logic on the wasm / Rust side is very much just like writing a (C compatible) shared library.
+
+Let's begin simple.
+This function creates a [Lua](https://docs.rs/mlua/latest/mlua/struct.Lua.html) instance and returns the raw pointer to it.
+```rust,ignore
+#[no_mangle]
+pub extern "C" fn lua_new() -> *mut mlua::Lua {
+    let lua = mlua::Lua::new();
+    Box::into_raw(Box::new(lua))
+}
+```
+
+Alright, good.
+Now we have a Lua instance, but no way to use it, so let us create one.
+<br>
+The function takes the pointer to the Lua struct we create in the `new_lua` function as well as an arbitrary string, which should be lua code, as parameters.
+It then executes this string via the Lua instance and may write to `stderr` if an error occurs.
+```rust,ignore
+#[no_mangle]
+pub unsafe extern "C" fn lua_execute(lua: *mut mlua::Lua, to_execute: *const std::ffi::c_char) {
+    // casting the raw pointer of the created lua instance back to a usable Rust struct
+    let lua: &mut mlua::Lua = &mut *lua;
+    // converting the c string into a `CStr` (which then can be converted to a `String`)
+    let to_execute = std::ffi::CStr::from_ptr(to_execute);
+    
+    // execute the input code via the lua interpreter
+    if let Err(err) = lua.load(&to_execute.to_string_lossy().to_string()).exec() {
+        // because emscripten wraps stderr, we are able to catch the error on the js
+        // side just fine
+        eprintln!("{}", err)
+    }
+}
+```
+
+Okay, this looks great! In theory. So let's head over to the next page to see how to compile the code to make it actually usable via Javascript.