1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130

use crate::fmt;
use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedFused};
use crate::num::NonZeroUsize;
use crate::ops::{ControlFlow, Try};

/// An iterator that only accepts elements while `predicate` returns `true`.
///
/// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`take_while`]: Iterator::take_while
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct TakeWhile<I, P> {
    iter: I,
    flag: bool,
    predicate: P,
}

impl<I, P> TakeWhile<I, P> {
    pub(in crate::iter) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
        TakeWhile { iter, flag: false, predicate }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TakeWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for TakeWhile<I, P>
where
    P: FnMut(&I::Item) -> bool,
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.flag {
            None
        } else {
            let x = self.iter.next()?;
            if (self.predicate)(&x) {
                Some(x)
            } else {
                self.flag = true;
                None
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.flag {
            (0, Some(0))
        } else {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }
    }

    #[inline]
    fn 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>,
    {
        fn check<'a, T, Acc, R: Try<Output = Acc>>(
            flag: &'a mut bool,
            p: &'a mut impl FnMut(&T) -> bool,
            mut fold: impl FnMut(Acc, T) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
            move |acc, x| {
                if p(&x) {
                    ControlFlow::from_try(fold(acc, x))
                } else {
                    *flag = true;
                    ControlFlow::Break(try { acc })
                }
            }
        }

        if self.flag {
            try { init }
        } else {
            let flag = &mut self.flag;
            let p = &mut self.predicate;
            self.iter.try_fold(init, check(flag, p, fold)).into_try()
        }
    }

    impl_fold_via_try_fold! { fold -> try_fold }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I, P> FusedIterator for TakeWhile<I, P>
where
    I: FusedIterator,
    P: FnMut(&I::Item) -> bool,
{
}

#[unstable(issue = "none", feature = "trusted_fused")]
unsafe impl<I: TrustedFused, P> TrustedFused for TakeWhile<I, P> {}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<P, I> SourceIter for TakeWhile<I, P>
where
    I: SourceIter,
{
    type Source = I::Source;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut I::Source {
        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I: InPlaceIterable, F> InPlaceIterable for TakeWhile<I, F> {
    const EXPAND_BY: Option<NonZeroUsize> = I::EXPAND_BY;
    const MERGE_BY: Option<NonZeroUsize> = I::MERGE_BY;
}