use super::size_hint;

use std::cmp::Ordering::{Equal, Greater, Less};

use std::iter::{Fuse, FusedIterator};

use crate::either_or_both::EitherOrBoth;

// ZipLongest originally written by SimonSapin,

// and dedicated to itertools https://github.com/rust-lang/rust/pull/19283

/// An iterator which iterates two other iterators simultaneously

/// and wraps the elements in [`EitherOrBoth`].

///

/// This iterator is *fused*.

///

/// See [`.zip_longest()`](crate::Itertools::zip_longest) for more information.

#[derive(Clone, Debug)]

#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]

pub struct ZipLongest<T, U> {

    a: Fuse<T>,

    b: Fuse<U>,

}

/// Create a new `ZipLongest` iterator.

pub fn zip_longest<T, U>(a: T, b: U) -> ZipLongest<T, U>

where

    T: Iterator,

    U: Iterator,

{

    ZipLongest {

        a: a.fuse(),

        b: b.fuse(),

    }

}

impl<T, U> Iterator for ZipLongest<T, U>

where

    T: Iterator,

    U: Iterator,

{

    type Item = EitherOrBoth<T::Item, U::Item>;

    #[inline]

    fn next(&mut self) -> Option<Self::Item> {

        match (self.a.next(), self.b.next()) {

            (None, None) => None,

            (Some(a), None) => Some(EitherOrBoth::Left(a)),

            (None, Some(b)) => Some(EitherOrBoth::Right(b)),

            (Some(a), Some(b)) => Some(EitherOrBoth::Both(a, b)),

        }

    }

    #[inline]

    fn size_hint(&self) -> (usize, Option<usize>) {

        size_hint::max(self.a.size_hint(), self.b.size_hint())

    }

    #[inline]

    fn fold<B, F>(self, init: B, mut f: F) -> B

    where

        Self: Sized,

        F: FnMut(B, Self::Item) -> B,

    {

        let Self { mut a, mut b } = self;

        let res = a.try_fold(init, |init, a| match b.next() {

            Some(b) => Ok(f(init, EitherOrBoth::Both(a, b))),

            None => Err(f(init, EitherOrBoth::Left(a))),

        });

        match res {

            Ok(acc) => b.map(EitherOrBoth::Right).fold(acc, f),

            Err(acc) => a.map(EitherOrBoth::Left).fold(acc, f),

        }

    }

}

impl<T, U> DoubleEndedIterator for ZipLongest<T, U>

where

    T: DoubleEndedIterator + ExactSizeIterator,

    U: DoubleEndedIterator + ExactSizeIterator,

{

    #[inline]

    fn next_back(&mut self) -> Option<Self::Item> {

        match self.a.len().cmp(&self.b.len()) {

            Equal => match (self.a.next_back(), self.b.next_back()) {

                (None, None) => None,

                (Some(a), Some(b)) => Some(EitherOrBoth::Both(a, b)),

                // These can only happen if .len() is inconsistent with .next_back()

                (Some(a), None) => Some(EitherOrBoth::Left(a)),

                (None, Some(b)) => Some(EitherOrBoth::Right(b)),

            },

            Greater => self.a.next_back().map(EitherOrBoth::Left),

            Less => self.b.next_back().map(EitherOrBoth::Right),

        }

    }

    fn rfold<B, F>(self, mut init: B, mut f: F) -> B

    where

        F: FnMut(B, Self::Item) -> B,

    {

        let Self { mut a, mut b } = self;

        let a_len = a.len();

        let b_len = b.len();

        match a_len.cmp(&b_len) {

            Equal => {}

            Greater => {

                init = a

                    .by_ref()

                    .rev()

                    .take(a_len - b_len)

                    .map(EitherOrBoth::Left)

                    .fold(init, &mut f)

            }

            Less => {

                init = b

                    .by_ref()

                    .rev()

                    .take(b_len - a_len)

                    .map(EitherOrBoth::Right)

                    .fold(init, &mut f)

            }

        }

        a.rfold(init, |acc, item_a| {

            f(acc, EitherOrBoth::Both(item_a, b.next_back().unwrap()))

        })

    }

}

impl<T, U> ExactSizeIterator for ZipLongest<T, U>

where

    T: ExactSizeIterator,

    U: ExactSizeIterator,

{

}

impl<T, U> FusedIterator for ZipLongest<T, U>

where

    T: Iterator,

    U: Iterator,

{

}