elua/src/function.rs

use std::ffi::CStr;

use crate::{LuaRef, FromLuaStack, State, PushToLuaStack};

use mlua_sys as lua;

pub struct Function<'a> {
    state: &'a State,
    lref: LuaRef
}

impl<'a> FromLuaStack<'a> for Function<'a> {
    unsafe fn from_lua_stack(state: &'a crate::State) -> crate::Result<Self> {
        Ok(Self {
            state,
            lref: LuaRef::from_stack(state)?,
        })
    }
}

impl<'a> PushToLuaStack for Function<'a> {
    unsafe fn push_to_lua_stack(&self, state: &State) -> crate::Result<()> {
        self.lref.push_to_lua_stack(state)
    }
}

impl<'a> Function<'a> {
    pub fn exec<A, R>(&self, args: A) -> crate::Result<R>
    where
        A: FunctionArgs,
        R: FunctionResults<'a>,
    {
        unsafe {
            self.push_to_lua_stack(self.state)?;
            args.push_args_to_lua_stack(self.state)?;

            let arg_len = args.len() as i32;
            let res_len = R::len() as i32;
            let s = self.state.state_ptr();
            if lua::lua_pcall(s, arg_len, res_len, 0) != 0 {
                let s = lua::lua_tostring(s, -1);
                let s = CStr::from_ptr(s);
                let s = s.to_str().unwrap();

                return Err(crate::Error::runtime(s));
            }

            R::results_from_lua_stack(self.state)
        }
    }
}

pub trait FunctionArgs {
    fn len(&self) -> usize;
    fn push_args_to_lua_stack(&self, state: &State) -> crate::Result<()>;
}

impl<'a, T> FunctionArgs for T
where
    T: PushToLuaStack,
{
    fn len(&self) -> usize {
        1
    }

    fn push_args_to_lua_stack(&self, state: &State) -> crate::Result<()> {
        unsafe {
            self.push_to_lua_stack(state)?;
        }

        Ok(())
    }
}

pub trait FunctionResults<'a>: Sized {
    fn len() -> usize;
    fn results_from_lua_stack(state: &'a State) -> crate::Result<Self>;
}

impl<'a> FunctionResults<'a> for () {
    fn len() -> usize {
        0
    }

    fn results_from_lua_stack(_state: &'a State) -> crate::Result<Self> {
        Ok(())
    }
}

impl<'a, T: FromLuaStack<'a>> FunctionResults<'a> for T {
    fn len() -> usize {
        1
    }

    fn results_from_lua_stack(state: &'a State) -> crate::Result<Self> {
        unsafe { T::from_lua_stack(state) }
    }
}

macro_rules! impl_function_arg_tuple {
    ( $count: expr, $first: tt, $( $name: tt ),+ ) => (
        #[allow(non_snake_case)]
        impl<$first: PushToLuaStack, $($name: PushToLuaStack,)+> FunctionArgs for ($first, $($name,)+) {
            fn len(&self) -> usize {
                // this will end up generating $count - 1 - 1 - 1... hopefully the compiler will
                // optimize that out
                $count
            }

            fn push_args_to_lua_stack(&self, state: &State) -> crate::Result<()> {
                let ($first, $($name,)+) = self;
        
                unsafe {
                    $first.push_to_lua_stack(state)?;
                    $( $name.push_to_lua_stack(state)?; )+
                }
        
                Ok(())
            }
        }

        impl<'a, $first: FromLuaStack<'a>, $($name: FromLuaStack<'a>,)+> FunctionResults<'a> for ($first, $($name,)+) {
            fn len() -> usize {
                $count
            }
        
            fn results_from_lua_stack(state: &'a State) -> crate::Result<Self> {
                Ok(unsafe { ( $first::from_lua_stack(state)?, $( $name::from_lua_stack(state)?, )+ ) })
            }
        }

        impl_function_arg_tuple!( $count - 1, $( $name ),+ );
    );

    // implements FunctionArgs and FunctionResults for a tuple with a single element
    ( $count: expr, $only: tt ) => {
        #[allow(non_snake_case)]
        impl<$only: PushToLuaStack> FunctionArgs for ($only,) {
            fn len(&self) -> usize {
                1
            }

            fn push_args_to_lua_stack(&self, state: &State) -> crate::Result<()> {
                let (a,) = self;
        
                unsafe {
                    a.push_to_lua_stack(state)?;
                }
        
                Ok(())
            }
        }

        impl<'a, $only: FromLuaStack<'a>> FunctionResults<'a> for ($only,) {
            fn len() -> usize {
                1
            }
        
            fn results_from_lua_stack(state: &'a State) -> crate::Result<Self> {
                Ok(unsafe { ( $only::from_lua_stack(state)?, ) })
            }
        }
    };

}

// hopefully 16 is more than enough
// if you have 16 function results, and need more than 16, you NEED help
impl_function_arg_tuple! { 16, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16 }