/// Result returned by [`Comparator::pick_smaller`].
#[derive(Debug, PartialEq)]
pub enum Comparison {
    L,
    /// Compared values were equal.
    E,
    R,
}

/// Returns [`Comparison`] saying which value is smaller.
///
/// ```rust
/// # use radn_rs::flow::comparator::*;
/// assert_eq!(DefaultComparator.pick_smaller(&1, &3), Comparison::L);
/// assert_eq!(DefaultComparator.pick_smaller(&2, &2), Comparison::E);
/// assert_eq!(DefaultComparator.pick_smaller(&3, &1), Comparison::R);
/// ```
pub trait Comparator<A> {
    fn pick_smaller(&self, kl: &A, kr: &A) -> Comparison;
}

/// Implementation of a [Comparator] relying on [`PartialOrd`].
pub struct DefaultComparator;

impl<A: PartialOrd> Comparator<A> for DefaultComparator {
    fn pick_smaller(&self, kl: &A, kr: &A) -> Comparison {
        if kl < kr {
            Comparison::L
        } else if kr < kl {
            Comparison::R
        } else {
            Comparison::E
        }
    }
}

pub struct RcComparator<C>(C);

mod rc_comparator_impl {
    use super::*;
    use std::rc::Rc;

    impl<A, C: Comparator<A>> Comparator<Rc<A>> for RcComparator<C> {
        fn pick_smaller(&self, kl: &Rc<A>, kr: &Rc<A>) -> Comparison {
            self.0.pick_smaller(kl, kr)
        }
    }
}