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// Rust Bitcoin Library
// Written in 2014 by
//     Andrew Poelstra <apoelstra@wpsoftware.net>
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to
// the public domain worldwide. This software is distributed without
// any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software.
// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
//

//! # Iterator adaptors
//!
//! Iterator adaptors needed by Bitcoin but not provided by the Rust
//! standard library.

/// An iterator that returns pairs of elements
pub struct Pair<I>
    where I: Iterator
{
    iter: I,
    last_elem: Option<I::Item>
}

impl<I: Iterator> Iterator for Pair<I> {
    type Item = (I::Item, I::Item);

    #[inline]
    fn next(&mut self) -> Option<(I::Item, I::Item)> {
        let elem1 = self.iter.next();
        if elem1.is_none() {
            None
        } else {
            let elem2 = self.iter.next();
            if elem2.is_none() {
                self.last_elem = elem1;
                None
            } else {
                Some((elem1.unwrap(), elem2.unwrap()))
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        match self.iter.size_hint() {
            (n, None) => (n/2, None),
            (n, Some(m)) => (n/2, Some(m/2))
        }
    }
}

impl<I: Iterator> Pair<I> {
    /// Returns the last element of the iterator if there were an odd
    /// number of elements remaining before it was Pair-ified.
    #[inline]
    pub fn remainder(self) -> Option<I::Item> {
        self.last_elem
    }
}

/// Returns an iterator that returns elements of the original iterator 2 at a time
pub trait Pairable : Sized + Iterator {
    /// Returns an iterator that returns elements of the original iterator 2 at a time
    fn pair(self) -> Pair<Self>;
}

impl<I: Iterator> Pairable for I {
    /// Creates an iterator that yields pairs ef elements from the underlying
    /// iterator, yielding `None` when there are fewer than two elements to
    /// return.
    #[inline]
    fn pair(self) -> Pair<I> {
        Pair {iter: self, last_elem: None }
    }
}