//! Iterator utilities.

/// Extra methods for iterators.
pub trait IteratorExtras: Iterator {
    /// Create an iterator that produces adjacent pairs of elements from the iterator.
    fn adjacent_pairs(mut self) -> AdjacentPairs<Self>
        where Self: Sized,
              Self::Item: Clone
    {
        let elem = self.next();
        AdjacentPairs {
            iter: self,
            elem: elem,
        }
    }
}

impl<T> IteratorExtras for T where T: Iterator {}

/// Adjacent pairs iterator returned by `adjacent_pairs()`.
///
/// This wraps another iterator and produces a sequence of adjacent pairs of elements.
pub struct AdjacentPairs<I>
    where I: Iterator,
          I::Item: Clone
{
    iter: I,
    elem: Option<I::Item>,
}

impl<I> Iterator for AdjacentPairs<I>
    where I: Iterator,
          I::Item: Clone
{
    type Item = (I::Item, I::Item);

    fn next(&mut self) -> Option<Self::Item> {
        self.elem
            .take()
            .and_then(|e| {
                          self.elem = self.iter.next();
                          self.elem.clone().map(|n| (e, n))
                      })
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn adjpairs() {
        use super::IteratorExtras;

        assert_eq!([1, 2, 3, 4]
                       .iter()
                       .cloned()
                       .adjacent_pairs()
                       .collect::<Vec<_>>(),
                   vec![(1, 2), (2, 3), (3, 4)]);
        assert_eq!([2, 3, 4]
                       .iter()
                       .cloned()
                       .adjacent_pairs()
                       .collect::<Vec<_>>(),
                   vec![(2, 3), (3, 4)]);
        assert_eq!([2, 3, 4]
                       .iter()
                       .cloned()
                       .adjacent_pairs()
                       .collect::<Vec<_>>(),
                   vec![(2, 3), (3, 4)]);
        assert_eq!([3, 4]
                       .iter()
                       .cloned()
                       .adjacent_pairs()
                       .collect::<Vec<_>>(),
                   vec![(3, 4)]);
        assert_eq!([4]
                       .iter()
                       .cloned()
                       .adjacent_pairs()
                       .collect::<Vec<_>>(),
                   vec![]);
        assert_eq!([]
                       .iter()
                       .cloned()
                       .adjacent_pairs()
                       .collect::<Vec<(i32, i32)>>(),
                   vec![]);
    }
}