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use std::ops::DerefMut;

pub fn merge3<F, A: ?Sized, B: ?Sized, C: ?Sized, R>(
    a_original: &A,
    a: Option<A::Owned>,
    b_original: &B,
    b: Option<B::Owned>,
    c_original: &C,
    c: Option<C::Owned>,
    f: F,
) -> Option<R>
where
    A: ToOwned,
    B: ToOwned,
    C: ToOwned,
    F: FnOnce(A::Owned, B::Owned, C::Owned) -> R,
{
    let a_b = merge(a_original, a, b_original, b, |a, b| (a, b));
    merge_fn(
        &(a_original, b_original),
        |_| (a_original.to_owned(), b_original.to_owned()),
        a_b,
        c_original,
        C::to_owned,
        c,
        |(a, b), c| f(a, b, c),
    )
}

/// Merges two values using `f` if either or both them is `Some(..)`.
/// If both are `None`, `None` is returned.
pub fn merge<F, A: ?Sized, B: ?Sized, R>(
    a_original: &A,
    a: Option<A::Owned>,
    b_original: &B,
    b: Option<B::Owned>,
    f: F,
) -> Option<R>
where
    A: ToOwned,
    B: ToOwned,
    F: FnOnce(A::Owned, B::Owned) -> R,
{
    merge_fn(a_original, A::to_owned, a, b_original, B::to_owned, b, f)
}

pub fn merge_fn<'a, 'b, F, G, H, A: ?Sized, B: ?Sized, A1, B1, R>(
    a_original: &'a A,
    g: G,
    a: Option<A1>,
    b_original: &'b B,
    h: H,
    b: Option<B1>,
    merger: F,
) -> Option<R>
where
    F: FnOnce(A1, B1) -> R,
    G: FnOnce(&'a A) -> A1,
    H: FnOnce(&'b B) -> B1,
{
    match (a, b) {
        (Some(a), Some(b)) => Some(merger(a, b)),
        (Some(a), None) => Some(merger(a, h(b_original))),
        (None, Some(b)) => Some(merger(g(a_original), b)),
        (None, None) => None,
    }
}

pub fn merge_tuple_iter<'a, I, F, T, R>(types: I, mut f: F) -> Option<R>
where
    I: IntoIterator<Item = (&'a T, &'a T)>,
    F: FnMut(&'a T, &'a T) -> Option<T>,
    T: Clone + 'a,
    R: Default + Extend<T> + DerefMut<Target = [T]>,
{
    merge_iter(types, |(l, r)| f(l, r), |(l, _)| l.clone())
}

pub fn merge_iter<I, F, G, U, R>(types: I, mut action: F, mut converter: G) -> Option<R>
where
    I: IntoIterator,
    F: FnMut(I::Item) -> Option<U>,
    G: FnMut(I::Item) -> U,
    I::Item: Copy,
    R: Default + Extend<U> + DerefMut<Target = [U]>,
{
    let mut out = R::default();
    merge_iter_(
        types.into_iter(),
        false,
        &mut out,
        &mut action,
        &mut converter,
    );
    if out.is_empty() {
        None
    } else {
        out.reverse();
        Some(out)
    }
}

fn merge_iter_<I, F, G, U, R>(
    mut types: I,
    replaced: bool,
    output: &mut R,
    f: &mut F,
    converter: &mut G,
) where
    I: Iterator,
    F: FnMut(I::Item) -> Option<U>,
    G: FnMut(I::Item) -> U,
    I::Item: Copy,
    R: Default + Extend<U> + DerefMut<Target = [U]>,
{
    if let Some(l) = types.next() {
        let new = f(l);
        merge_iter_(types, replaced || new.is_some(), output, f, converter);
        match new {
            Some(typ) => {
                output.extend(Some(typ));
            }
            None if replaced || !output[..].is_empty() => {
                output.extend(Some(converter(l)));
            }
            None => (),
        }
    }
}