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

use super::size_hint;

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

///

/// 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())

    }

}

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),

        }

    }

}

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

{}