itertools/
peek_nth.rs

1use crate::size_hint;
2use crate::PeekingNext;
3use alloc::collections::VecDeque;
4use std::iter::Fuse;
5
6/// See [`peek_nth()`] for more information.
7#[derive(Clone, Debug)]
8pub struct PeekNth<I>
9where
10    I: Iterator,
11{
12    iter: Fuse<I>,
13    buf: VecDeque<I::Item>,
14}
15
16/// A drop-in replacement for [`std::iter::Peekable`] which adds a `peek_nth`
17/// method allowing the user to `peek` at a value several iterations forward
18/// without advancing the base iterator.
19///
20/// This differs from `multipeek` in that subsequent calls to `peek` or
21/// `peek_nth` will always return the same value until `next` is called
22/// (making `reset_peek` unnecessary).
23pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter>
24where
25    I: IntoIterator,
26{
27    PeekNth {
28        iter: iterable.into_iter().fuse(),
29        buf: VecDeque::new(),
30    }
31}
32
33impl<I> PeekNth<I>
34where
35    I: Iterator,
36{
37    /// Works exactly like the `peek` method in `std::iter::Peekable`
38    pub fn peek(&mut self) -> Option<&I::Item> {
39        self.peek_nth(0)
40    }
41
42    /// Returns a reference to the `nth` value without advancing the iterator.
43    ///
44    /// # Examples
45    ///
46    /// Basic usage:
47    ///
48    /// ```rust
49    /// use itertools::peek_nth;
50    ///
51    /// let xs = vec![1,2,3];
52    /// let mut iter = peek_nth(xs.iter());
53    ///
54    /// assert_eq!(iter.peek_nth(0), Some(&&1));
55    /// assert_eq!(iter.next(), Some(&1));
56    ///
57    /// // The iterator does not advance even if we call `peek_nth` multiple times
58    /// assert_eq!(iter.peek_nth(0), Some(&&2));
59    /// assert_eq!(iter.peek_nth(1), Some(&&3));
60    /// assert_eq!(iter.next(), Some(&2));
61    ///
62    /// // Calling `peek_nth` past the end of the iterator will return `None`
63    /// assert_eq!(iter.peek_nth(1), None);
64    /// ```
65    pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> {
66        let unbuffered_items = (n + 1).saturating_sub(self.buf.len());
67
68        self.buf.extend(self.iter.by_ref().take(unbuffered_items));
69
70        self.buf.get(n)
71    }
72}
73
74impl<I> Iterator for PeekNth<I>
75where
76    I: Iterator,
77{
78    type Item = I::Item;
79
80    fn next(&mut self) -> Option<Self::Item> {
81        self.buf.pop_front().or_else(|| self.iter.next())
82    }
83
84    fn size_hint(&self) -> (usize, Option<usize>) {
85        size_hint::add_scalar(self.iter.size_hint(), self.buf.len())
86    }
87}
88
89impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {}
90
91impl<I> PeekingNext for PeekNth<I>
92where
93    I: Iterator,
94{
95    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
96    where
97        F: FnOnce(&Self::Item) -> bool,
98    {
99        self.peek().filter(|item| accept(item))?;
100        self.next()
101    }
102}
itertools/peek_nth.rs

itertools/
peek_nth.rs
