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

use crate::fmt;
use crate::iter::{adapters::SourceIter, InPlaceIterable};
use crate::ops::{ControlFlow, Try};

/// 一个仅在 `predicate` 返回 `Some(_)` 时才接受元素的迭代器。
///
/// 该 `struct` 是通过 [`Iterator`] 上的 [`map_while`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`map_while`]: Iterator::map_while
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
#[derive(Clone)]
pub struct MapWhile<I, P> {
    iter: I,
    predicate: P,
}

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

#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("MapWhile").field("iter", &self.iter).finish()
    }
}

#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
where
    P: FnMut(I::Item) -> Option<B>,
{
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        let x = self.iter.next()?;
        (self.predicate)(x)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // 由于谓词，无法知道下界
    }

    #[inline]
    fn 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>,
    {
        let Self { iter, predicate } = self;
        iter.try_fold(init, |acc, x| match predicate(x) {
            Some(item) => ControlFlow::from_try(fold(acc, item)),
            None => ControlFlow::Break(try { acc }),
        })
        .into_try()
    }

    #[inline]
    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        #[inline]
        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
            move |acc, x| Ok(f(acc, x))
        }

        self.try_fold(init, ok(fold)).unwrap()
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<S: Iterator, B, I: Iterator, P> SourceIter for MapWhile<I, P>
where
    P: FnMut(I::Item) -> Option<B>,
    I: SourceIter<Source = S>,
{
    type Source = S;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut S {
        // SAFETY: 将不安全的函数转发到具有相同要求的不安全的函数
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<B, I: InPlaceIterable, P> InPlaceIterable for MapWhile<I, P> where
    P: FnMut(I::Item) -> Option<B>
{
}