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

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    use crate::iter::{DoubleEndedIterator, FusedIterator, Iterator, TrustedLen};
    use crate::ops::Try;
    use crate::usize;

    /// An iterator that links two iterators together, in a chain.
    ///
    /// This `struct` is created by the [`chain`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`chain`]: trait.Iterator.html#method.chain
    /// [`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 Chain<A, B> {
        // These are "fused" with `Option` so we don't need separate state to track which part is
        // already exhausted, and we may also get niche layout for `None`. We don't use the real `Fuse`
        // adapter because its specialization for `FusedIterator` unconditionally descends into the
        // iterator, and that could be expensive to keep revisiting stuff like nested chains. It also
        // hurts compiler performance to add more iterator layers to `Chain`.
        //
        // Only the "first" iterator is actually set `None` when exhausted, depending on whether you
        // iterate forward or backward. If you mix directions, then both sides may be `None`.
        a: Option<A>,
        b: Option<B>,
    }
    impl<A, B> Chain<A, B> {
        pub(in super::super) fn new(a: A, b: B) -> Chain<A, B> {
            Chain { a: Some(a), b: Some(b) }
        }
    }

    /// Fuse the iterator if the expression is `None`.
    macro_rules! fuse {
        ($self:ident . $iter:ident . $($call:tt)+) => {
            match $self.$iter {
                Some(ref mut iter) => match iter.$($call)+ {
                    None => {
                        $self.$iter = None;
                        None
                    }
                    item => item,
                },
                None => None,
            }
        };
    }

    /// Try an iterator method without fusing,
    /// like an inline `.as_mut().and_then(...)`
    macro_rules! maybe {
        ($self:ident . $iter:ident . $($call:tt)+) => {
            match $self.$iter {
                Some(ref mut iter) => iter.$($call)+,
                None => None,
            }
        };
    }

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

        #[inline]
        fn next(&mut self) -> Option<A::Item> {
            match fuse!(self.a.next()) {
                None => maybe!(self.b.next()),
                item => item,
            }
        }

        #[inline]
        #[rustc_inherit_overflow_checks]
        fn count(self) -> usize {
            let a_count = match self.a {
                Some(a) => a.count(),
                None => 0,
            };
            let b_count = match self.b {
                Some(b) => b.count(),
                None => 0,
            };
            a_count + b_count
        }

        fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            if let Some(ref mut a) = self.a {
                acc = a.try_fold(acc, &mut f)?;
                self.a = None;
            }
            if let Some(ref mut b) = self.b {
                acc = b.try_fold(acc, f)?;
                // we don't fuse the second iterator
            }
            Try::from_ok(acc)
        }

        fn fold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
        where
            F: FnMut(Acc, Self::Item) -> Acc,
        {
            if let Some(a) = self.a {
                acc = a.fold(acc, &mut f);
            }
            if let Some(b) = self.b {
                acc = b.fold(acc, f);
            }
            acc
        }

        #[inline]
        fn nth(&mut self, mut n: usize) -> Option<A::Item> {
            if let Some(ref mut a) = self.a {
                while let Some(x) = a.next() {
                    if n == 0 {
                        return Some(x);
                    }
                    n -= 1;
                }
                self.a = None;
            }
            maybe!(self.b.nth(n))
        }

        #[inline]
        fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
        where
            P: FnMut(&Self::Item) -> bool,
        {
            match fuse!(self.a.find(&mut predicate)) {
                None => maybe!(self.b.find(predicate)),
                item => item,
            }
        }

        #[inline]
        fn last(self) -> Option<A::Item> {
            // Must exhaust a before b.
            let a_last = match self.a {
                Some(a) => a.last(),
                None => None,
            };
            let b_last = match self.b {
                Some(b) => b.last(),
                None => None,
            };
            b_last.or(a_last)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            match self {
                Chain { a: Some(a), b: Some(b) } => {
                    let (a_lower, a_upper) = a.size_hint();
                    let (b_lower, b_upper) = b.size_hint();

                    let lower = a_lower.saturating_add(b_lower);

                    let upper = match (a_upper, b_upper) {
                        (Some(x), Some(y)) => x.checked_add(y),
                        _ => None,
                    };

                    (lower, upper)
                }
                Chain { a: Some(a), b: None } => a.size_hint(),
                Chain { a: None, b: Some(b) } => b.size_hint(),
                Chain { a: None, b: None } => (0, Some(0)),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> DoubleEndedIterator for Chain<A, B>
    where
        A: DoubleEndedIterator,
        B: DoubleEndedIterator<Item = A::Item>,
    {
        #[inline]
        fn next_back(&mut self) -> Option<A::Item> {
            match fuse!(self.b.next_back()) {
                None => maybe!(self.a.next_back()),
                item => item,
            }
        }

        #[inline]
        fn nth_back(&mut self, mut n: usize) -> Option<A::Item> {
            if let Some(ref mut b) = self.b {
                while let Some(x) = b.next_back() {
                    if n == 0 {
                        return Some(x);
                    }
                    n -= 1;
                }
                self.b = None;
            }
            maybe!(self.a.nth_back(n))
        }

        #[inline]
        fn rfind<P>(&mut self, mut predicate: P) -> Option<Self::Item>
        where
            P: FnMut(&Self::Item) -> bool,
        {
            match fuse!(self.b.rfind(&mut predicate)) {
                None => maybe!(self.a.rfind(predicate)),
                item => item,
            }
        }

        fn try_rfold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            if let Some(ref mut b) = self.b {
                acc = b.try_rfold(acc, &mut f)?;
                self.b = None;
            }
            if let Some(ref mut a) = self.a {
                acc = a.try_rfold(acc, f)?;
                // we don't fuse the second iterator
            }
            Try::from_ok(acc)
        }

        fn rfold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
        where
            F: FnMut(Acc, Self::Item) -> Acc,
        {
            if let Some(b) = self.b {
                acc = b.rfold(acc, &mut f);
            }
            if let Some(a) = self.a {
                acc = a.rfold(acc, f);
            }
            acc
        }
    }

    // Note: *both* must be fused to handle double-ended iterators.
    #[stable(feature = "fused", since = "1.26.0")]
    impl<A, B> FusedIterator for Chain<A, B>
    where
        A: FusedIterator,
        B: FusedIterator<Item = A::Item>,
    {
    }

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A, B> TrustedLen for Chain<A, B>
    where
        A: TrustedLen,
        B: TrustedLen<Item = A::Item>,
    {
    }