core/iter/adapters/
skip_while.rs

1use crate::fmt;
2use crate::iter::adapters::SourceIter;
3use crate::iter::{FusedIterator, InPlaceIterable, TrustedFused};
4use crate::num::NonZero;
5use crate::ops::Try;
6
7/// An iterator that rejects elements while `predicate` returns `true`.
8///
9/// This `struct` is created by the [`skip_while`] method on [`Iterator`]. See its
10/// documentation for more.
11///
12/// [`skip_while`]: Iterator::skip_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 SkipWhile<I, P> {
18    iter: I,
19    flag: bool,
20    predicate: P,
21}
22
23impl<I, P> SkipWhile<I, P> {
24    pub(in crate::iter) fn new(iter: I, predicate: P) -> SkipWhile<I, P> {
25        SkipWhile { 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 SkipWhile<I, P> {
31    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
32        f.debug_struct("SkipWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
33    }
34}
35
36#[stable(feature = "rust1", since = "1.0.0")]
37impl<I: Iterator, P> Iterator for SkipWhile<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        fn check<'a, T>(
46            flag: &'a mut bool,
47            pred: &'a mut impl FnMut(&T) -> bool,
48        ) -> impl FnMut(&T) -> bool + 'a {
49            move |x| {
50                if *flag || !pred(x) {
51                    *flag = true;
52                    true
53                } else {
54                    false
55                }
56            }
57        }
58
59        let flag = &mut self.flag;
60        let pred = &mut self.predicate;
61        self.iter.find(check(flag, pred))
62    }
63
64    #[inline]
65    fn size_hint(&self) -> (usize, Option<usize>) {
66        let (_, upper) = self.iter.size_hint();
67        (0, upper) // can't know a lower bound, due to the predicate
68    }
69
70    #[inline]
71    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
72    where
73        Self: Sized,
74        Fold: FnMut(Acc, Self::Item) -> R,
75        R: Try<Output = Acc>,
76    {
77        if !self.flag {
78            match self.next() {
79                Some(v) => init = fold(init, v)?,
80                None => return try { init },
81            }
82        }
83        self.iter.try_fold(init, fold)
84    }
85
86    #[inline]
87    fn fold<Acc, Fold>(mut self, mut init: Acc, mut fold: Fold) -> Acc
88    where
89        Fold: FnMut(Acc, Self::Item) -> Acc,
90    {
91        if !self.flag {
92            match self.next() {
93                Some(v) => init = fold(init, v),
94                None => return init,
95            }
96        }
97        self.iter.fold(init, fold)
98    }
99}
100
101#[stable(feature = "fused", since = "1.26.0")]
102impl<I, P> FusedIterator for SkipWhile<I, P>
103where
104    I: FusedIterator,
105    P: FnMut(&I::Item) -> bool,
106{
107}
108
109#[unstable(issue = "none", feature = "trusted_fused")]
110unsafe impl<I: TrustedFused, P> TrustedFused for SkipWhile<I, P> {}
111
112#[unstable(issue = "none", feature = "inplace_iteration")]
113unsafe impl<P, I> SourceIter for SkipWhile<I, P>
114where
115    I: SourceIter,
116{
117    type Source = I::Source;
118
119    #[inline]
120    unsafe fn as_inner(&mut self) -> &mut I::Source {
121        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
122        unsafe { SourceIter::as_inner(&mut self.iter) }
123    }
124}
125
126#[unstable(issue = "none", feature = "inplace_iteration")]
127unsafe impl<I: InPlaceIterable, F> InPlaceIterable for SkipWhile<I, F> {
128    const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
129    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
130}