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    //! An iterator that returns `(start, end)` tuples from the walk.

    /// An iterator that returns `(start, end)` tuples from the walk.
    ///
    /// All the algorithms in this crate should have a `steps()` function associated with them to turn
    /// them into a [`Steps`] iterator.
    ///
    /// Example using [`WalkGrid`]:
    ///
    /// ```
    /// extern crate line_drawing;
    /// use line_drawing::WalkGrid;
    ///
    /// fn main() {
    ///     for (start, end) in WalkGrid::new((0, 0), (5, 3)).steps() {
    ///         println!("{:?} -> {:?}", start, end);
    ///     }
    /// }
    /// ```
    ///
    /// ```text
    /// (0, 0) -> (1, 0)
    /// (1, 0) -> (1, 1)
    /// (1, 1) -> (2, 1)
    /// (2, 1) -> (2, 2)
    /// (2, 2) -> (3, 2)
    /// (3, 2) -> (4, 2)
    /// (4, 2) -> (4, 3)
    /// (4, 3) -> (5, 3)
    /// ```
    ///
    /// [`Steps`]: struct.Steps.html
    /// [`WalkGrid`]: ../struct.WalkGrid.html
    pub struct Steps<T, I> {
        iterator: I,
        prev: Option<T>,
    }

    impl<T: Copy, I: Iterator<Item = T>> Steps<T, I> {
        #[inline]
        pub fn new(mut iterator: I) -> Self {
            Self {
                prev: iterator.next(),
                iterator,
            }
        }
    }

    impl<T: Copy, I: Iterator<Item = T>> Iterator for Steps<T, I> {
        type Item = (T, T);

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            self.iterator.next().and_then(|next| {
                self.prev.map(|prev| {
                    self.prev = Some(next);
                    (prev, next)
                })
            })
        }
    }

    #[test]
    fn steps() {
        use Midpoint;

        assert_eq!(
            Midpoint::new((0.0, 0.0), (3.0, 4.0))
                .steps()
                .collect::<Vec<_>>(),
            [
                ((0, 0), (1, 1)),
                ((1, 1), (2, 2)),
                ((2, 2), (2, 3)),
                ((2, 3), (3, 4))
            ]
        );
    }

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