#[macro_export]
macro_rules! system {
    ($name:ident ( $($pat:tt)* ) { $($rest:tt)* } ) => {
        pub struct $name;
        impl<'a> specs::System<'a> for $name {
            type SystemData = args_to_systemdata!(($($pat)*,));
            fn run(&mut self, args_to_fn_pat!(($($pat)*)): Self::SystemData) {
                use specs::join::Join;
                for args_to_join_pat!(($($pat)*,)) in args_to_join!(($($pat)*,)).join() {
                    $($rest)*
                }
            }
        }
    };
    ($name:ident $pat:tt { $($rest:tt)* } finally { $($finally:tt)* }) => {
        pub struct $name;
        impl<'a> specs::System<'a> for $name {
            type SystemData = args_to_systemdata!($pat);
            fn run(&mut self, args_to_fn_pat!($pat): Self::SystemData) {
                use specs::join::Join;
                for args_to_join_pat!($pat) in args_to_join!($pat).join() {
                    $($rest)*
                }
                $($finally)*
            }
        }
    };
}

#[macro_export]
macro_rules! system_impl {
    ($name:ident ( $($pat:tt)* ) { $($rest:tt)* } ) => {
        pub struct $name;
        impl<'a> specs::System<'a> for $name {
            type SystemData = args_to_systemdata!(($($pat)*,));
            fn run(&mut self, args_to_fn_pat!(($($pat)*)): Self::SystemData) {
                use specs::join::Join;
                $($rest)*
            }
        }
    };
}

#[macro_export]
macro_rules! args_to_systemdata {
    ( ( $name:ident : Entity $(,)? ) ) => {
        ( specs::Entities<'a>, )
    };
    ( ( $name:ident : $ty:ty $(,)? ) ) => {
        ( specs::ReadStorage<'a, $ty> ,)
    };
    ( ( mut $name:ident : $ty:ty $(,)? ) ) => {
        ( specs::WriteStorage<'a, $ty> ,)
    };
    ( ( resource $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( resource mut $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( $name:ident : Entity , $($tok:tt)* ) ) => {
        ( specs::Entities<'a>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( specs::ReadStorage<'a, $ty>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
    ( ( mut $name:ident : $ty:ty , $( $tok:tt )* ) ) => {
        ( specs::WriteStorage<'a, $ty>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
    ( ( resource $name:ident : $ty:ty , $( $tok:tt )* ) ) => {
        ( specs::ReadExpect<'a, $ty>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
    ( ( resource mut $name:ident : $ty:ty , $( $tok:tt )* ) ) => {
        ( specs::WriteExpect<'a, $ty>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
}

#[macro_export]
macro_rules! args_to_fn_pat {
    ( ( $name:ident : $ty:ty $(,)? ) ) => {
        ( $name ,)
    };
    ( ( mut $name:ident : $ty:ty $(,)? ) ) => {
        ( mut $name ,)
    };
    ( ( resource $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( resource mut $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( $name, args_to_fn_pat!( ( $($tok)* ) ) )
    };
    ( ( mut $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( mut $name, args_to_fn_pat!( ( $($tok)* ) ) )
    };
    ( ( resource $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( $name, args_to_fn_pat!( ( $($tok)* ) ) )
    };
    ( ( resource mut $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( mut $name, args_to_fn_pat!( ( $($tok)* ) ) )
    };
}

#[macro_export]
macro_rules! args_to_join_pat {
    ( ( $name:ident : $ty:ty $(,)? ) ) => {
        ( $name ,)
    };
    ( ( mut $name:ident : $ty:ty $(,)? ) ) => {
        ( mut $name ,)
    };
    ( ( resource $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( resource mut $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( $name, args_to_join_pat!( ( $($tok)* ) ) )
    };
    ( ( mut $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( mut $name, args_to_join_pat!( ( $($tok)* ) ) )
    };
    ( ( resource $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        args_to_join_pat!( ( $($tok)* ) )
    };
    ( ( resource mut $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        args_to_join_pat!( ( $($tok)* ) )
    };
}

#[macro_export]
macro_rules! args_to_join {
    ( ( $name:ident : $ty:ty $(,)? ) ) => {
        ( & $name ,)
    };
    ( ( mut $name:ident : $ty:ty $(,)? ) ) => {
        ( &mut $name ,)
    };
    ( ( resource $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( resource mut $name:ident : $ty:ty $(,)? ) ) => {
        compile_error!("Resources cannot come at the end of the argument block.")
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( & $name, args_to_join!( ( $($tok)* ) ) )
    };
    ( ( mut $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( &mut $name, args_to_join!( ( $($tok)* ) ) )
    };
    ( ( resource $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        args_to_join!( ( $($tok)* ) )
    };
    ( ( resource mut $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        args_to_join!( ( $($tok)* ) )
    };
}

#[cfg(test)]
mod tests {

    pub struct Pos {
        x: usize,
    }
    impl specs::Component for Pos {
        type Storage = specs::VecStorage<Pos>;
    }

    pub struct Mov;
    impl specs::Component for Mov {
        type Storage = specs::VecStorage<Mov>;
    }

    system! {
        Foo (_x: Mov, mut y: Pos) {
            y.x += 1;
        }
    }

    system_impl! {
        FooAlt (x: Mov, mut y: Pos) {
            for (_, mut y) in (&x, &mut y).join() {
                y.x += 1;
            }
        }
    }

    system! {
        Bar (_x: Mov, mut y: Pos) {
            y.x += 1;
        } finally {
            println!("Done!");
        }
    }

    system_impl! {
        BarAlt (x: Mov, mut y: Pos) {
            for (_, mut y) in (&x, &mut y).join() {
                y.x += 1;
            }
            println!("Done!");
        }
    }

    system! {
        Baz (resource n: usize, _x: Mov, mut y: Pos) {
            y.x += *n;
        }
    }

    system_impl! {
        BazAlt (resource n: usize, x: Mov, mut y: Pos) {
            for (_, mut y) in (&x, &mut y).join() {
                y.x += *n;
            }
        }
    }

    system! {
        Quux (resource mut n: usize, _x: Mov, mut y: Pos) {
            y.x += *n;
            *n += 1;
        } finally {
            *n += 1;
        }
    }

    system_impl! {
        QuuxAlt (resource mut n: usize, x: Mov, mut y: Pos) {
            for (_, mut y) in (&x, &mut y).join() {
                y.x += *n;
            }
            *n += 1;
        }
    }

    system! {
        Pippo (_e: Entity, _x: Mov, mut y: Pos) {
            y.x += 1;
        }
    }

    system_impl! {
        PippoAlt (_e: Entity, x: Mov, mut y: Pos) {
            for (_, mut y) in (&x, &mut y).join() {
                y.x += 1;
            }
        }
    }

}