day01.rs

macro_rules! monadic {
    (
        [$($e:tt)*] | [$head:ident : $tail:ident <- tails $origin:ident,
        $($heads:ident : $tails:ident <- tails $origins:ident),*], $condition:expr
    ) => {{
        let mut f = move || {
            monadic![@INNER [$($e)*] | [$head : $tail <- tails $origin $(,$heads : $tails <- tails $origins)*], $condition ];
            None
        };
        f()
    }};
    (
        @INNER [$($e:tt)*] | [$head:ident : $tail:ident <- tails $origin:ident,
        $($heads:ident : $tails:ident <- tails $origins:ident),*], $condition:expr
    ) => {

        while let Some(($head, mut $tail)) = $origin.split_first() {
            $origin = $tail;
            monadic![@INNER [$($e)*] | [$($heads : $tails <- tails $origins),*], $condition];
        }
    };
    (
        @INNER [$($e:tt)*] | [$head:ident : $tail:ident <- tails $origin:ident], $condition:expr
    ) => {
        while let Some(($head, $tail)) = $origin.split_first() {
            $origin = $tail;
            if ($condition) {
                return Some($($e)*)
            }
        }
    }
}

//[x * y * z | x : xs <- tails inpt, y : ys <- tails xs, z <- ys, x + y + z == 2020]
fn solve1(s: impl Deref<Target = [usize]>) -> Option<usize> {
    let mut s: &[_] = &*s;

    monadic![[x * y * z] | [x : xs <- tails s, y : ys <- tails xs, z : zs <- tails ys], x + y + z == 2020]
}