take_while.rs - source
Description: Source of the Rust file `library/core/src/iter/adapters/take_while.rs`.

#
core/iter/adapters/
take_while.rs

    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("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}