1 use crate::fmt;
2 use crate::iter::{adapters::SourceIter, InPlaceIterable};
3 use crate::ops::{ControlFlow, Try};
4 
5 /// An iterator that only accepts elements while `predicate` returns `Some(_)`.
6 ///
7 /// This `struct` is created by the [`map_while`] method on [`Iterator`]. See its
8 /// documentation for more.
9 ///
10 /// [`map_while`]: Iterator::map_while
11 /// [`Iterator`]: trait.Iterator.html
12 #[must_use = "iterators are lazy and do nothing unless consumed"]
13 #[stable(feature = "iter_map_while", since = "1.57.0")]
14 #[derive(Clone)]
15 pub struct MapWhile<I, P> {
16     iter: I,
17     predicate: P,
18 }
19 
20 impl<I, P> MapWhile<I, P> {
new(iter: I, predicate: P) -> MapWhile<I, P>21     pub(in crate::iter) fn new(iter: I, predicate: P) -> MapWhile<I, P> {
22         MapWhile { iter, predicate }
23     }
24 }
25 
26 #[stable(feature = "iter_map_while", since = "1.57.0")]
27 impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result28     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
29         f.debug_struct("MapWhile").field("iter", &self.iter).finish()
30     }
31 }
32 
33 #[stable(feature = "iter_map_while", since = "1.57.0")]
34 impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
35 where
36     P: FnMut(I::Item) -> Option<B>,
37 {
38     type Item = B;
39 
40     #[inline]
next(&mut self) -> Option<B>41     fn next(&mut self) -> Option<B> {
42         let x = self.iter.next()?;
43         (self.predicate)(x)
44     }
45 
46     #[inline]
size_hint(&self) -> (usize, Option<usize>)47     fn size_hint(&self) -> (usize, Option<usize>) {
48         let (_, upper) = self.iter.size_hint();
49         (0, upper) // can't know a lower bound, due to the predicate
50     }
51 
52     #[inline]
try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Output = Acc>,53     fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R
54     where
55         Self: Sized,
56         Fold: FnMut(Acc, Self::Item) -> R,
57         R: Try<Output = Acc>,
58     {
59         let Self { iter, predicate } = self;
60         iter.try_fold(init, |acc, x| match predicate(x) {
61             Some(item) => ControlFlow::from_try(fold(acc, item)),
62             None => ControlFlow::Break(try { acc }),
63         })
64         .into_try()
65     }
66 
67     #[inline]
fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc where Self: Sized, Fold: FnMut(Acc, Self::Item) -> Acc,68     fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
69     where
70         Self: Sized,
71         Fold: FnMut(Acc, Self::Item) -> Acc,
72     {
73         #[inline]
74         fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
75             move |acc, x| Ok(f(acc, x))
76         }
77 
78         self.try_fold(init, ok(fold)).unwrap()
79     }
80 }
81 
82 #[unstable(issue = "none", feature = "inplace_iteration")]
83 unsafe impl<I, P> SourceIter for MapWhile<I, P>
84 where
85     I: SourceIter,
86 {
87     type Source = I::Source;
88 
89     #[inline]
as_inner(&mut self) -> &mut I::Source90     unsafe fn as_inner(&mut self) -> &mut I::Source {
91         // SAFETY: unsafe function forwarding to unsafe function with the same requirements
92         unsafe { SourceIter::as_inner(&mut self.iter) }
93     }
94 }
95 
96 #[unstable(issue = "none", feature = "inplace_iteration")]
97 unsafe impl<B, I: InPlaceIterable, P> InPlaceIterable for MapWhile<I, P> where
98     P: FnMut(I::Item) -> Option<B>
99 {
100 }
101