1 use crate::fmt;
2 use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
3 use crate::ops::{ControlFlow, Try};
4 
5 /// An iterator that only accepts elements while `predicate` returns `true`.
6 ///
7 /// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
8 /// documentation for more.
9 ///
10 /// [`take_while`]: Iterator::take_while
11 /// [`Iterator`]: trait.Iterator.html
12 #[must_use = "iterators are lazy and do nothing unless consumed"]
13 #[stable(feature = "rust1", since = "1.0.0")]
14 #[derive(Clone)]
15 pub struct TakeWhile<I, P> {
16     iter: I,
17     flag: bool,
18     predicate: P,
19 }
20 
21 impl<I, P> TakeWhile<I, P> {
new(iter: I, predicate: P) -> TakeWhile<I, P>22     pub(in crate::iter) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
23         TakeWhile { iter, flag: false, predicate }
24     }
25 }
26 
27 #[stable(feature = "core_impl_debug", since = "1.9.0")]
28 impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result29     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
30         f.debug_struct("TakeWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
31     }
32 }
33 
34 #[stable(feature = "rust1", since = "1.0.0")]
35 impl<I: Iterator, P> Iterator for TakeWhile<I, P>
36 where
37     P: FnMut(&I::Item) -> bool,
38 {
39     type Item = I::Item;
40 
41     #[inline]
next(&mut self) -> Option<I::Item>42     fn next(&mut self) -> Option<I::Item> {
43         if self.flag {
44             None
45         } else {
46             let x = self.iter.next()?;
47             if (self.predicate)(&x) {
48                 Some(x)
49             } else {
50                 self.flag = true;
51                 None
52             }
53         }
54     }
55 
56     #[inline]
size_hint(&self) -> (usize, Option<usize>)57     fn size_hint(&self) -> (usize, Option<usize>) {
58         if self.flag {
59             (0, Some(0))
60         } else {
61             let (_, upper) = self.iter.size_hint();
62             (0, upper) // can't know a lower bound, due to the predicate
63         }
64     }
65 
66     #[inline]
try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Output = Acc>,67     fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
68     where
69         Self: Sized,
70         Fold: FnMut(Acc, Self::Item) -> R,
71         R: Try<Output = Acc>,
72     {
73         fn check<'a, T, Acc, R: Try<Output = Acc>>(
74             flag: &'a mut bool,
75             p: &'a mut impl FnMut(&T) -> bool,
76             mut fold: impl FnMut(Acc, T) -> R + 'a,
77         ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
78             move |acc, x| {
79                 if p(&x) {
80                     ControlFlow::from_try(fold(acc, x))
81                 } else {
82                     *flag = true;
83                     ControlFlow::Break(try { acc })
84                 }
85             }
86         }
87 
88         if self.flag {
89             try { init }
90         } else {
91             let flag = &mut self.flag;
92             let p = &mut self.predicate;
93             self.iter.try_fold(init, check(flag, p, fold)).into_try()
94         }
95     }
96 
97     #[inline]
fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc where Self: Sized, Fold: FnMut(Acc, Self::Item) -> Acc,98     fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
99     where
100         Self: Sized,
101         Fold: FnMut(Acc, Self::Item) -> Acc,
102     {
103         #[inline]
104         fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
105             move |acc, x| Ok(f(acc, x))
106         }
107 
108         self.try_fold(init, ok(fold)).unwrap()
109     }
110 }
111 
112 #[stable(feature = "fused", since = "1.26.0")]
113 impl<I, P> FusedIterator for TakeWhile<I, P>
114 where
115     I: FusedIterator,
116     P: FnMut(&I::Item) -> bool,
117 {
118 }
119 
120 #[unstable(issue = "none", feature = "inplace_iteration")]
121 unsafe impl<P, I> SourceIter for TakeWhile<I, P>
122 where
123     I: SourceIter,
124 {
125     type Source = I::Source;
126 
127     #[inline]
as_inner(&mut self) -> &mut I::Source128     unsafe fn as_inner(&mut self) -> &mut I::Source {
129         // SAFETY: unsafe function forwarding to unsafe function with the same requirements
130         unsafe { SourceIter::as_inner(&mut self.iter) }
131     }
132 }
133 
134 #[unstable(issue = "none", feature = "inplace_iteration")]
135 unsafe impl<I: InPlaceIterable, F> InPlaceIterable for TakeWhile<I, F> where
136     F: FnMut(&I::Item) -> bool
137 {
138 }
139