1use alloc::vec::Vec;
2
3use crate::size_hint;
4
5/// An iterator adaptor that allows putting multiple
6/// items in front of the iterator.
7///
8/// Iterator element type is `I::Item`.
9#[derive(Debug, Clone)]
10#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
11pub struct PutBackN<I: Iterator> {
12 top: Vec<I::Item>,
13 iter: I,
14}
15
16/// Create an iterator where you can put back multiple values to the front
17/// of the iteration.
18///
19/// Iterator element type is `I::Item`.
20pub fn put_back_n<I>(iterable: I) -> PutBackN<I::IntoIter>
21where
22 I: IntoIterator,
23{
24 PutBackN {
25 top: Vec::new(),
26 iter: iterable.into_iter(),
27 }
28}
29
30impl<I: Iterator> PutBackN<I> {
31 /// Puts x in front of the iterator.
32 /// The values are yielded in order of the most recently put back
33 /// values first.
34 ///
35 /// ```rust
36 /// use itertools::put_back_n;
37 ///
38 /// let mut it = put_back_n(1..5);
39 /// it.next();
40 /// it.put_back(1);
41 /// it.put_back(0);
42 ///
43 /// assert!(itertools::equal(it, 0..5));
44 /// ```
45 #[inline]
46 pub fn put_back(&mut self, x: I::Item) {
47 self.top.push(x);
48 }
49}
50
51impl<I: Iterator> Iterator for PutBackN<I> {
52 type Item = I::Item;
53 #[inline]
54 fn next(&mut self) -> Option<Self::Item> {
55 self.top.pop().or_else(|| self.iter.next())
56 }
57
58 #[inline]
59 fn size_hint(&self) -> (usize, Option<usize>) {
60 size_hint::add_scalar(self.iter.size_hint(), self.top.len())
61 }
62
63 fn fold<B, F>(self, mut init: B, mut f: F) -> B
64 where
65 F: FnMut(B, Self::Item) -> B,
66 {
67 init = self.top.into_iter().rfold(init, &mut f);
68 self.iter.fold(init, f)
69 }
70}
71