1use crate::fmt;
2use crate::iter::InPlaceIterable;
3use crate::iter::adapters::SourceIter;
4use crate::num::NonZero;
5use crate::ops::{ControlFlow, Try};
6
7/// An iterator that only accepts elements while `predicate` returns `Some(_)`.
8///
9/// This `struct` is created by the [`map_while`] method on [`Iterator`]. See its
10/// documentation for more.
11///
12/// [`map_while`]: Iterator::map_while
13/// [`Iterator`]: trait.Iterator.html
14#[must_use = "iterators are lazy and do nothing unless consumed"]
15#[stable(feature = "iter_map_while", since = "1.57.0")]
16#[derive(Clone)]
17pub struct MapWhile<I, P> {
18 iter: I,
19 predicate: P,
20}
21
22impl<I, P> MapWhile<I, P> {
23 pub(in crate::iter) fn new(iter: I, predicate: P) -> MapWhile<I, P> {
24 MapWhile { iter, predicate }
25 }
26}
27
28#[stable(feature = "iter_map_while", since = "1.57.0")]
29impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
30 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
31 f.debug_struct("MapWhile").field(name:"iter", &self.iter).finish()
32 }
33}
34
35#[stable(feature = "iter_map_while", since = "1.57.0")]
36impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
37where
38 P: FnMut(I::Item) -> Option<B>,
39{
40 type Item = B;
41
42 #[inline]
43 fn next(&mut self) -> Option<B> {
44 let x = self.iter.next()?;
45 (self.predicate)(x)
46 }
47
48 #[inline]
49 fn size_hint(&self) -> (usize, Option<usize>) {
50 let (_, upper) = self.iter.size_hint();
51 (0, upper) // can't know a lower bound, due to the predicate
52 }
53
54 #[inline]
55 fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R
56 where
57 Self: Sized,
58 Fold: FnMut(Acc, Self::Item) -> R,
59 R: Try<Output = Acc>,
60 {
61 let Self { iter, predicate } = self;
62 iter.try_fold(init, |acc, x| match predicate(x) {
63 Some(item) => ControlFlow::from_try(fold(acc, item)),
64 None => ControlFlow::Break(try { acc }),
65 })
66 .into_try()
67 }
68
69 impl_fold_via_try_fold! { fold -> try_fold }
70}
71
72#[unstable(issue = "none", feature = "inplace_iteration")]
73unsafe impl<I, P> SourceIter for MapWhile<I, P>
74where
75 I: SourceIter,
76{
77 type Source = I::Source;
78
79 #[inline]
80 unsafe fn as_inner(&mut self) -> &mut I::Source {
81 // SAFETY: unsafe function forwarding to unsafe function with the same requirements
82 unsafe { SourceIter::as_inner(&mut self.iter) }
83 }
84}
85
86#[unstable(issue = "none", feature = "inplace_iteration")]
87unsafe impl<I: InPlaceIterable, P> InPlaceIterable for MapWhile<I, P> {
88 const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
89 const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
90}
91