use super::size_hint;

use std::cell::RefCell;

use alloc::collections::VecDeque;

use alloc::rc::Rc;

/// Common buffer object for the two tee halves

#[derive(Debug)]

struct TeeBuffer<A, I> {

    backlog: VecDeque<A>,

    iter: I,

    /// The owner field indicates which id should read from the backlog

    owner: bool,

}

/// One half of an iterator pair where both return the same elements.

///

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

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

#[derive(Debug)]

pub struct Tee<I>

    where I: Iterator

{

    rcbuffer: Rc<RefCell<TeeBuffer<I::Item, I>>>,

    id: bool,

}

pub fn new<I>(iter: I) -> (Tee<I>, Tee<I>)

    where I: Iterator

{

    let buffer = TeeBuffer{backlog: VecDeque::new(), iter, owner: false};

    let t1 = Tee{rcbuffer: Rc::new(RefCell::new(buffer)), id: true};

    let t2 = Tee{rcbuffer: t1.rcbuffer.clone(), id: false};

    (t1, t2)

}

impl<I> Iterator for Tee<I>

    where I: Iterator,

          I::Item: Clone

{

    type Item = I::Item;

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

        // .borrow_mut may fail here -- but only if the user has tied some kind of weird

        // knot where the iterator refers back to itself.

        let mut buffer = self.rcbuffer.borrow_mut();

        if buffer.owner == self.id {

            match buffer.backlog.pop_front() {

                None => {}

                some_elt => return some_elt,

            }

        }

        match buffer.iter.next() {

            None => None,

            Some(elt) => {

                buffer.backlog.push_back(elt.clone());

                buffer.owner = !self.id;

                Some(elt)

            }

        }

    }

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

        let buffer = self.rcbuffer.borrow();

        let sh = buffer.iter.size_hint();

        if buffer.owner == self.id {

            let log_len = buffer.backlog.len();

            size_hint::add_scalar(sh, log_len)

        } else {

            sh

        }

    }

}

impl<I> ExactSizeIterator for Tee<I>

    where I: ExactSizeIterator,

          I::Item: Clone

{}