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use self::ChainState::{Buffered, Linked};
use crate::Chain;
use std::error::Error as StdError;
use std::vec;

#[derive(Clone)]
pub(crate) enum ChainState<'a> {
    Linked {
        next: Option<&'a (dyn StdError + 'static)>,
    },
    Buffered {
        rest: vec::IntoIter<&'a (dyn StdError + 'static)>,
    },
}

impl<'a> Iterator for Chain<'a> {
    type Item = &'a (dyn StdError + 'static);

    fn next(&mut self) -> Option<Self::Item> {
        match &mut self.state {
            Linked { next } => {
                let error = (*next)?;
                *next = error.source();
                Some(error)
            }
            Buffered { rest } => rest.next(),
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.len();
        (len, Some(len))
    }
}

impl DoubleEndedIterator for Chain<'_> {
    fn next_back(&mut self) -> Option<Self::Item> {
        match &mut self.state {
            Linked { mut next } => {
                let mut rest = Vec::new();
                while let Some(cause) = next {
                    next = cause.source();
                    rest.push(cause);
                }
                let mut rest = rest.into_iter();
                let last = rest.next_back();
                self.state = Buffered { rest };
                last
            }
            Buffered { rest } => rest.next_back(),
        }
    }
}

impl ExactSizeIterator for Chain<'_> {
    fn len(&self) -> usize {
        match &self.state {
            Linked { mut next } => {
                let mut len = 0;
                while let Some(cause) = next {
                    next = cause.source();
                    len += 1;
                }
                len
            }
            Buffered { rest } => rest.len(),
        }
    }
}