#[cfg(feature = "use_std")]
use std::error::Error;
use std::fmt::{Debug, Display, Formatter, Result as FmtResult};

use std::iter::ExactSizeIterator;

use either::Either;

use crate::size_hint;

/// Iterator returned for the error case of `Itertools::exactly_one()`
/// This iterator yields exactly the same elements as the input iterator.
///
/// During the execution of `exactly_one` the iterator must be mutated.  This wrapper
/// effectively "restores" the state of the input iterator when it's handed back.
///
/// This is very similar to `PutBackN` except this iterator only supports 0-2 elements and does not
/// use a `Vec`.
#[derive(Clone)]
pub struct ExactlyOneError<I>
where
    I: Iterator,
{
    first_two: Option<Either<[I::Item; 2], I::Item>>,
    inner: I,
}

impl<I> ExactlyOneError<I>
where
    I: Iterator,
{
    /// Creates a new `ExactlyOneErr` iterator.
    pub(crate) fn new(first_two: Option<Either<[I::Item; 2], I::Item>>, inner: I) -> Self {
        Self { first_two, inner }
    }

    fn additional_len(&self) -> usize {
        match self.first_two {
            Some(Either::Left(_)) => 2,
            Some(Either::Right(_)) => 1,
            None => 0,
        }
    }
}

impl<I> Iterator for ExactlyOneError<I>
where
    I: Iterator,
{
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        match self.first_two.take() {
            Some(Either::Left([first, second])) => {
                self.first_two = Some(Either::Right(second));
                Some(first)
            }
            Some(Either::Right(second)) => Some(second),
            None => self.inner.next(),
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        size_hint::add_scalar(self.inner.size_hint(), self.additional_len())
    }

    fn fold<B, F>(self, mut init: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        match self.first_two {
            Some(Either::Left([first, second])) => {
                init = f(init, first);
                init = f(init, second);
            }
            Some(Either::Right(second)) => init = f(init, second),
            None => {}
        }
        self.inner.fold(init, f)
    }
}

impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}

impl<I> Display for ExactlyOneError<I>
where
    I: Iterator,
{
    fn fmt(&self, f: &mut Formatter) -> FmtResult {
        let additional = self.additional_len();
        if additional > 0 {
            write!(f, "got at least 2 elements when exactly one was expected")
        } else {
            write!(f, "got zero elements when exactly one was expected")
        }
    }
}

impl<I> Debug for ExactlyOneError<I>
where
    I: Iterator + Debug,
    I::Item: Debug,
{
    fn fmt(&self, f: &mut Formatter) -> FmtResult {
        let mut dbg = f.debug_struct("ExactlyOneError");
        match &self.first_two {
            Some(Either::Left([first, second])) => {
                dbg.field("first", first).field("second", second);
            }
            Some(Either::Right(second)) => {
                dbg.field("second", second);
            }
            None => {}
        }
        dbg.field("inner", &self.inner).finish()
    }
}

#[cfg(feature = "use_std")]
impl<I> Error for ExactlyOneError<I>
where
    I: Iterator + Debug,
    I::Item: Debug,
{
}