1use crate::fmt;
2use crate::iter::adapters::SourceIter;
3use crate::iter::{FusedIterator, InPlaceIterable, TrustedFused};
4use crate::num::NonZero;
5use crate::ops::{ControlFlow, Try};
6
7/// An iterator that only accepts elements while `predicate` returns `true`.
8///
9/// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
10/// documentation for more.
11///
12/// [`take_while`]: Iterator::take_while
13/// [`Iterator`]: trait.Iterator.html
14#[must_use = "iterators are lazy and do nothing unless consumed"]
15#[stable(feature = "rust1", since = "1.0.0")]
16#[derive(Clone)]
17pub struct TakeWhile<I, P> {
18 iter: I,
19 flag: bool,
20 predicate: P,
21}
22
23impl<I, P> TakeWhile<I, P> {
24 pub(in crate::iter) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
25 TakeWhile { iter, flag: false, predicate }
26 }
27}
28
29#[stable(feature = "core_impl_debug", since = "1.9.0")]
30impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
31 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
32 f.debug_struct("TakeWhile").field("iter", &self.iter).field(name:"flag", &self.flag).finish()
33 }
34}
35
36#[stable(feature = "rust1", since = "1.0.0")]
37impl<I: Iterator, P> Iterator for TakeWhile<I, P>
38where
39 P: FnMut(&I::Item) -> bool,
40{
41 type Item = I::Item;
42
43 #[inline]
44 fn next(&mut self) -> Option<I::Item> {
45 if self.flag {
46 None
47 } else {
48 let x = self.iter.next()?;
49 if (self.predicate)(&x) {
50 Some(x)
51 } else {
52 self.flag = true;
53 None
54 }
55 }
56 }
57
58 #[inline]
59 fn size_hint(&self) -> (usize, Option<usize>) {
60 if self.flag {
61 (0, Some(0))
62 } else {
63 let (_, upper) = self.iter.size_hint();
64 (0, upper) // can't know a lower bound, due to the predicate
65 }
66 }
67
68 #[inline]
69 fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
70 where
71 Self: Sized,
72 Fold: FnMut(Acc, Self::Item) -> R,
73 R: Try<Output = Acc>,
74 {
75 fn check<'a, T, Acc, R: Try<Output = Acc>>(
76 flag: &'a mut bool,
77 p: &'a mut impl FnMut(&T) -> bool,
78 mut fold: impl FnMut(Acc, T) -> R + 'a,
79 ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
80 move |acc, x| {
81 if p(&x) {
82 ControlFlow::from_try(fold(acc, x))
83 } else {
84 *flag = true;
85 ControlFlow::Break(try { acc })
86 }
87 }
88 }
89
90 if self.flag {
91 try { init }
92 } else {
93 let flag = &mut self.flag;
94 let p = &mut self.predicate;
95 self.iter.try_fold(init, check(flag, p, fold)).into_try()
96 }
97 }
98
99 impl_fold_via_try_fold! { fold -> try_fold }
100}
101
102#[stable(feature = "fused", since = "1.26.0")]
103impl<I, P> FusedIterator for TakeWhile<I, P>
104where
105 I: FusedIterator,
106 P: FnMut(&I::Item) -> bool,
107{
108}
109
110#[unstable(issue = "none", feature = "trusted_fused")]
111unsafe impl<I: TrustedFused, P> TrustedFused for TakeWhile<I, P> {}
112
113#[unstable(issue = "none", feature = "inplace_iteration")]
114unsafe impl<P, I> SourceIter for TakeWhile<I, P>
115where
116 I: SourceIter,
117{
118 type Source = I::Source;
119
120 #[inline]
121 unsafe fn as_inner(&mut self) -> &mut I::Source {
122 // SAFETY: unsafe function forwarding to unsafe function with the same requirements
123 unsafe { SourceIter::as_inner(&mut self.iter) }
124 }
125}
126
127#[unstable(issue = "none", feature = "inplace_iteration")]
128unsafe impl<I: InPlaceIterable, F> InPlaceIterable for TakeWhile<I, F> {
129 const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
130 const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
131}
132