take.rs - source
Description: Source of the Rust file `library/core/src/iter/adapters/take.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::cmp;
    use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
    use crate::ops::{ControlFlow, Try};

    /// An iterator that only iterates over the first `n` iterations of `iter`.
    ///
    /// This `struct` is created by the [`take`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`take`]: Iterator::take
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Take<I> {
        iter: I,
        n: usize,
    }

    impl<I> Take<I> {
        pub(in crate::iter) fn new(iter: I, n: usize) -> Take<I> {
            Take { iter, n }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Take<I>
    where
        I: Iterator,
    {
        type Item = <I as Iterator>::Item;

        #[inline]
        fn next(&mut self) -> Option<<I as Iterator>::Item> {
            if self.n != 0 {
                self.n -= 1;
                self.iter.next()
            } else {
                None
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            if self.n > n {
                self.n -= n + 1;
                self.iter.nth(n)
            } else {
                if self.n > 0 {
                    self.iter.nth(self.n - 1);
                    self.n = 0;
                }
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.n == 0 {
                return (0, Some(0));
            }

            let (lower, upper) = self.iter.size_hint();

            let lower = cmp::min(lower, self.n);

            let upper = match upper {
                Some(x) if x < self.n => Some(x),
                _ => Some(self.n),
            };

            (lower, upper)
        }

        #[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<Ok = Acc>,
        {
            fn check<'a, T, Acc, R: Try<Ok = Acc>>(
                n: &'a mut usize,
                mut fold: impl FnMut(Acc, T) -> R + 'a,
            ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
                move |acc, x| {
                    *n -= 1;
                    let r = fold(acc, x);
                    if *n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) }
                }
            }

            if self.n == 0 {
                try { init }
            } else {
                let n = &mut self.n;
                self.iter.try_fold(init, check(n, fold)).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, I: Iterator> SourceIter for Take<I>
    where
        I: SourceIter<Source = S>,
    {
        type Source = S;

        #[inline]
        unsafe fn as_inner(&mut self) -> &mut S {
            // 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> InPlaceIterable for Take<I> {}

    #[stable(feature = "double_ended_take_iterator", since = "1.38.0")]
    impl<I> DoubleEndedIterator for Take<I>
    where
        I: DoubleEndedIterator + ExactSizeIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.n == 0 {
                None
            } else {
                let n = self.n;
                self.n -= 1;
                self.iter.nth_back(self.iter.len().saturating_sub(n))
            }
        }

        #[inline]
        fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
            let len = self.iter.len();
            if self.n > n {
                let m = len.saturating_sub(self.n) + n;
                self.n -= n + 1;
                self.iter.nth_back(m)
            } else {
                if len > 0 {
                    self.iter.nth_back(len - 1);
                }
                None
            }
        }

        #[inline]
        fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            if self.n == 0 {
                try { init }
            } else {
                let len = self.iter.len();
                if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
                    try { init }
                } else {
                    self.iter.try_rfold(init, fold)
                }
            }
        }

        #[inline]
        fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            if self.n == 0 {
                init
            } else {
                let len = self.iter.len();
                if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
                    init
                } else {
                    self.iter.rfold(init, fold)
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<I> FusedIterator for Take<I> where I: FusedIterator {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<I: TrustedLen> TrustedLen for Take<I> {}