stacks_common/util/
pair.rs

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// Copyright (C) 2013-2020 Blockstack PBC, a public benefit corporation
// Copyright (C) 2020 Stacks Open Internet Foundation
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

// Borrowed from Andrew Poelstra's rust-bitcoin 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 let Some(elem1) = elem1 {
            if let Some(elem2) = self.iter.next() {
                Some((elem1, elem2))
            } else {
                self.last_elem = Some(elem1);
                None
            }
        } else {
            None
        }
    }

    #[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 of 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,
        }
    }
}