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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)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
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)
}
/// Works exactly like the `peek_mut` method in [`std::iter::Peekable`].
pub fn peek_mut(&mut self) -> Option<&mut I::Item> {
self.peek_nth_mut(0)
}
/// Returns a reference to the `nth` value without advancing the iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use itertools::peek_nth;
///
/// let xs = vec![1, 2, 3];
/// let mut iter = peek_nth(xs.into_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)
}
/// Returns a mutable reference to the `nth` value without advancing the iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use itertools::peek_nth;
///
/// let xs = vec![1, 2, 3, 4, 5];
/// let mut iter = peek_nth(xs.into_iter());
///
/// assert_eq!(iter.peek_nth_mut(0), Some(&mut 1));
/// assert_eq!(iter.next(), Some(1));
///
/// // The iterator does not advance even if we call `peek_nth_mut` multiple times
/// assert_eq!(iter.peek_nth_mut(0), Some(&mut 2));
/// assert_eq!(iter.peek_nth_mut(1), Some(&mut 3));
/// assert_eq!(iter.next(), Some(2));
///
/// // Peek into the iterator and set the value behind the mutable reference.
/// if let Some(p) = iter.peek_nth_mut(1) {
/// assert_eq!(*p, 4);
/// *p = 9;
/// }
///
/// // The value we put in reappears as the iterator continues.
/// assert_eq!(iter.next(), Some(3));
/// assert_eq!(iter.next(), Some(9));
///
/// // Calling `peek_nth_mut` past the end of the iterator will return `None`
/// assert_eq!(iter.peek_nth_mut(1), None);
/// ```
pub fn peek_nth_mut(&mut self, n: usize) -> Option<&mut 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_mut(n)
}
/// Works exactly like the `next_if` method in [`std::iter::Peekable`].
pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {
match self.next() {
Some(item) if func(&item) => Some(item),
Some(item) => {
self.buf.push_front(item);
None
}
_ => None,
}
}
/// Works exactly like the `next_if_eq` method in [`std::iter::Peekable`].
pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>
where
T: ?Sized,
I::Item: PartialEq<T>,
{
self.next_if(|next| next == expected)
}
}
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())
}
fn fold<B, F>(self, mut init: B, mut f: F) -> B
where
F: FnMut(B, Self::Item) -> B,
{
init = self.buf.into_iter().fold(init, &mut f);
self.iter.fold(init, f)
}
}
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()
}
}