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use crate::size_hint;
use crate::PeekingNext;
use alloc::collections::VecDeque;
use std::iter::Fuse;
/// See [`peek_nth()`] for more information.
#[derive(Clone, Debug)]
pub struct PeekNth<I>
where
I: Iterator,
{
iter: Fuse<I>,
buf: VecDeque<I::Item>,
}
/// A drop-in replacement for [`std::iter::Peekable`] which adds a `peek_nth`
/// method allowing the user to `peek` at a value several iterations forward
/// without advancing the base iterator.
///
/// This differs from `multipeek` in that subsequent calls to `peek` or
/// `peek_nth` will always return the same value until `next` is called
/// (making `reset_peek` unnecessary).
pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter>
where
I: IntoIterator,
{
PeekNth {
iter: iterable.into_iter().fuse(),
buf: VecDeque::new(),
}
}
impl<I> PeekNth<I>
where
I: Iterator,
{
/// Works exactly like the `peek` method in `std::iter::Peekable`
pub fn peek(&mut self) -> Option<&I::Item> {
self.peek_nth(0)
}
/// Returns a reference to the `nth` value without advancing the iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```rust
/// use itertools::peek_nth;
///
/// let xs = vec![1,2,3];
/// let mut iter = peek_nth(xs.iter());
///
/// assert_eq!(iter.peek_nth(0), Some(&&1));
/// assert_eq!(iter.next(), Some(&1));
///
/// // The iterator does not advance even if we call `peek_nth` multiple times
/// assert_eq!(iter.peek_nth(0), Some(&&2));
/// assert_eq!(iter.peek_nth(1), Some(&&3));
/// assert_eq!(iter.next(), Some(&2));
///
/// // Calling `peek_nth` past the end of the iterator will return `None`
/// assert_eq!(iter.peek_nth(1), None);
/// ```
pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> {
let unbuffered_items = (n + 1).saturating_sub(self.buf.len());
self.buf.extend(self.iter.by_ref().take(unbuffered_items));
self.buf.get(n)
}
}
impl<I> Iterator for PeekNth<I>
where
I: Iterator,
{
type Item = I::Item;
fn next(&mut self) -> Option<Self::Item> {
self.buf.pop_front().or_else(|| self.iter.next())
}
fn size_hint(&self) -> (usize, Option<usize>) {
size_hint::add_scalar(self.iter.size_hint(), self.buf.len())
}
}
impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {}
impl<I> PeekingNext for PeekNth<I>
where
I: Iterator,
{
fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
where
F: FnOnce(&Self::Item) -> bool,
{
self.peek().filter(|item| accept(item))?;
self.next()
}
}