//! Module which defines the function registration mechanism.

use crate::any::{Any, Dynamic};
use crate::engine::{Engine, FnCallArgs};
use crate::parser::Position;
use crate::result::EvalAltResult;
use std::any::TypeId;

/// A trait to register custom functions with the `Engine`.
///
/// # Example
///
/// ```rust
/// # fn main() -> Result<(), rhai::EvalAltResult> {
/// use rhai::{Engine, RegisterFn};
///
/// // Normal function
/// fn add(x: i64, y: i64) -> i64 {
///     x + y
/// }
///
/// let mut engine = Engine::new();
///
/// // You must use the trait rhai::RegisterFn to get this method.
/// engine.register_fn("add", add);
///
/// let result = engine.eval::<i64>("add(40, 2)")?;
///
/// println!("Answer: {}", result);  // prints 42
/// # Ok(())
/// # }
/// ```
pub trait RegisterFn<FN, ARGS, RET> {
    /// Register a custom function with the `Engine`.
    fn register_fn(&mut self, name: &str, f: FN);
}

/// A trait to register custom functions that return `Dynamic` values with the `Engine`.
///
/// # Example
///
/// ```rust
/// # fn main() -> Result<(), rhai::EvalAltResult> {
/// use rhai::{Engine, Dynamic, RegisterDynamicFn};
///
/// // Function that returns a Dynamic value
/// fn get_an_any(x: i64) -> Dynamic {
///     Box::new(x)
/// }
///
/// let mut engine = Engine::new();
///
/// // You must use the trait rhai::RegisterDynamicFn to get this method.
/// engine.register_dynamic_fn("get_an_any", get_an_any);
///
/// let result = engine.eval::<i64>("get_an_any(42)")?;
///
/// println!("Answer: {}", result);  // prints 42
/// # Ok(())
/// # }
/// ```
pub trait RegisterDynamicFn<FN, ARGS> {
    /// Register a custom function returning `Dynamic` values with the `Engine`.
    fn register_dynamic_fn(&mut self, name: &str, f: FN);
}

/// A trait to register fallible custom functions returning Result<_, EvalAltResult> with the `Engine`.
///
/// # Example
///
/// ```rust
/// # fn main() -> Result<(), rhai::EvalAltResult> {
/// use rhai::{Engine, RegisterFn};
///
/// // Normal function
/// fn add(x: i64, y: i64) -> i64 {
///     x + y
/// }
///
/// let mut engine = Engine::new();
///
/// // You must use the trait rhai::RegisterFn to get this method.
/// engine.register_fn("add", add);
///
/// let result = engine.eval::<i64>("add(40, 2)")?;
///
/// println!("Answer: {}", result);  // prints 42
/// # Ok(())
/// # }
/// ```
pub trait RegisterResultFn<FN, ARGS, RET> {
    /// Register a custom function with the `Engine`.
    fn register_result_fn(&mut self, name: &str, f: FN);
}

pub struct Ref<A>(A);
pub struct Mut<A>(A);

macro_rules! count_args {
    () => { 0_usize };
    ( $head:ident $($tail:ident)* ) => { 1_usize + count_args!($($tail)*) };
}

macro_rules! def_register {
    () => {
        def_register!(imp);
    };
    (imp $($par:ident => $mark:ty => $param:ty => $clone:expr),*) => {
        impl<
            $($par: Any + Clone,)*
            FN: Fn($($param),*) -> RET + 'static,
            RET: Any
        > RegisterFn<FN, ($($mark,)*), RET> for Engine<'_>
        {
            fn register_fn(&mut self, name: &str, f: FN) {
                let fn_name = name.to_string();

                let fun = move |mut args: FnCallArgs, pos: Position| {
                    // Check for length at the beginning to avoid per-element bound checks.
                    const NUM_ARGS: usize = count_args!($($par)*);

                    if args.len() != NUM_ARGS {
                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, args.len(), pos))
                    } else {
                        #[allow(unused_variables, unused_mut)]
                        let mut drain = args.drain(..);
                        $(
                        // Downcast every element, return in case of a type mismatch
                        let $par = drain.next().unwrap().downcast_mut::<$par>().unwrap();
                        )*

                        // Call the user-supplied function using ($clone) to
                        // potentially clone the value, otherwise pass the reference.
                        let r = f($(($clone)($par)),*);
                        Ok(Box::new(r) as Dynamic)
                    }
                };
                self.register_fn_raw(name, Some(vec![$(TypeId::of::<$par>()),*]), Box::new(fun));
            }
        }

        impl<
            $($par: Any + Clone,)*
            FN: Fn($($param),*) -> Dynamic + 'static,
        > RegisterDynamicFn<FN, ($($mark,)*)> for Engine<'_>
        {
            fn register_dynamic_fn(&mut self, name: &str, f: FN) {
                let fn_name = name.to_string();

                let fun = move |mut args: FnCallArgs, pos: Position| {
                    // Check for length at the beginning to avoid per-element bound checks.
                    const NUM_ARGS: usize = count_args!($($par)*);

                    if args.len() != NUM_ARGS {
                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, args.len(), pos))
                    } else {
                        #[allow(unused_variables, unused_mut)]
                        let mut drain = args.drain(..);
                        $(
                        // Downcast every element, return in case of a type mismatch
                        let $par = drain.next().unwrap().downcast_mut::<$par>().unwrap();
                        )*

                        // Call the user-supplied function using ($clone) to
                        // potentially clone the value, otherwise pass the reference.
                        Ok(f($(($clone)($par)),*))
                    }
                };
                self.register_fn_raw(name, Some(vec![$(TypeId::of::<$par>()),*]), Box::new(fun));
            }
        }

        impl<
            $($par: Any + Clone,)*
            FN: Fn($($param),*) -> Result<RET, EvalAltResult> + 'static,
            RET: Any
        > RegisterResultFn<FN, ($($mark,)*), RET> for Engine<'_>
        {
            fn register_result_fn(&mut self, name: &str, f: FN) {
                let fn_name = name.to_string();

                let fun = move |mut args: FnCallArgs, pos: Position| {
                    // Check for length at the beginning to avoid per-element bound checks.
                    const NUM_ARGS: usize = count_args!($($par)*);

                    if args.len() != NUM_ARGS {
                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, args.len(), pos))
                    } else {
                        #[allow(unused_variables, unused_mut)]
                        let mut drain = args.drain(..);
                        $(
                        // Downcast every element, return in case of a type mismatch
                        let $par = drain.next().unwrap().downcast_mut::<$par>().unwrap();
                        )*

                        // Call the user-supplied function using ($clone) to
                        // potentially clone the value, otherwise pass the reference.
                        match f($(($clone)($par)),*) {
                            Ok(r) => Ok(Box::new(r) as Dynamic),
                            Err(mut err) => {
                                err.set_position(pos);
                                Err(err)
                            }
                        }
                    }
                };
                self.register_fn_raw(name, Some(vec![$(TypeId::of::<$par>()),*]), Box::new(fun));
            }
        }

        //def_register!(imp_pop $($par => $mark => $param),*);
    };
    ($p0:ident $(, $p:ident)*) => {
        def_register!(imp $p0 => $p0 => $p0 => Clone::clone $(, $p => $p => $p => Clone::clone)*);
        def_register!(imp $p0 => Ref<$p0> => &$p0 => |x| { x } $(, $p => $p => $p => Clone::clone)*);
        def_register!(imp $p0 => Mut<$p0> => &mut $p0 => |x| { x } $(, $p => $p => $p => Clone::clone)*);

        def_register!($($p),*);
    };
//    (imp_pop) => {};
//    (imp_pop $head:ident => $head_mark:ty => $head_param:ty $(,$tail:ident => $tail_mark:ty => $tp:ty)*) => {
//        def_register!(imp $($tail => $tail_mark => $tp),*);
//    };
}

#[cfg_attr(rustfmt, rustfmt_skip)]
def_register!(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T);