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 ///
33 /// The values are yielded in order of the most recently put back
34 /// values first.
35 ///
36 /// ```rust
37 /// use itertools::put_back_n;
38 ///
39 /// let mut it = put_back_n(1..5);
40 /// it.next();
41 /// it.put_back(1);
42 /// it.put_back(0);
43 ///
44 /// assert!(itertools::equal(it, 0..5));
45 /// ```
46 #[inline]
47 pub fn put_back(&mut self, x: I::Item) {
48 self.top.push(x);
49 }
50}
51
52impl<I: Iterator> Iterator for PutBackN<I> {
53 type Item = I::Item;
54 #[inline]
55 fn next(&mut self) -> Option<Self::Item> {
56 self.top.pop().or_else(|| self.iter.next())
57 }
58
59 #[inline]
60 fn size_hint(&self) -> (usize, Option<usize>) {
61 size_hint::add_scalar(self.iter.size_hint(), self.top.len())
62 }
63
64 fn fold<B, F>(self, mut init: B, mut f: F) -> B
65 where
66 F: FnMut(B, Self::Item) -> B,
67 {
68 init = self.top.into_iter().rfold(init, &mut f);
69 self.iter.fold(init, f)
70 }
71}
72