core/iter/traits/

double_ended.rs

use crate::num::NonZero;
use crate::ops::{ControlFlow, Try};

/// An iterator able to yield elements from both ends.
///
/// Something that implements `DoubleEndedIterator` has one extra capability
/// over something that implements [`Iterator`]: the ability to also take
/// `Item`s from the back, as well as the front.
///
/// It is important to note that both back and forth work on the same range,
/// and do not cross: iteration is over when they meet in the middle.
///
/// In a similar fashion to the [`Iterator`] protocol, once a
/// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it
/// again may or may not ever return [`Some`] again. [`next()`] and
/// [`next_back()`] are interchangeable for this purpose.
///
/// [`next_back()`]: DoubleEndedIterator::next_back
/// [`next()`]: Iterator::next
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let numbers = vec![1, 2, 3, 4, 5, 6];
///
/// let mut iter = numbers.iter();
///
/// assert_eq!(Some(&1), iter.next());
/// assert_eq!(Some(&6), iter.next_back());
/// assert_eq!(Some(&5), iter.next_back());
/// assert_eq!(Some(&2), iter.next());
/// assert_eq!(Some(&3), iter.next());
/// assert_eq!(Some(&4), iter.next());
/// assert_eq!(None, iter.next());
/// assert_eq!(None, iter.next_back());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "DoubleEndedIterator")]
pub trait DoubleEndedIterator: Iterator {
    /// Removes and returns an element from the end of the iterator.
    ///
    /// Returns `None` when there are no more elements.
    ///
    /// The [trait-level] docs contain more details.
    ///
    /// [trait-level]: DoubleEndedIterator
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let numbers = vec![1, 2, 3, 4, 5, 6];
    ///
    /// let mut iter = numbers.iter();
    ///
    /// assert_eq!(Some(&1), iter.next());
    /// assert_eq!(Some(&6), iter.next_back());
    /// assert_eq!(Some(&5), iter.next_back());
    /// assert_eq!(Some(&2), iter.next());
    /// assert_eq!(Some(&3), iter.next());
    /// assert_eq!(Some(&4), iter.next());
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next_back());
    /// ```
    ///
    /// # Remarks
    ///
    /// The elements yielded by `DoubleEndedIterator`'s methods may differ from
    /// the ones yielded by [`Iterator`]'s methods:
    ///
    /// ```
    /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
    /// let uniq_by_fst_comp = || {
    ///     let mut seen = std::collections::HashSet::new();
    ///     vec.iter().copied().filter(move |x| seen.insert(x.0))
    /// };
    ///
    /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
    /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
    ///
    /// assert_eq!(
    ///     uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
    ///     vec![(1, 'a'), (2, 'a')]
    /// );
    /// assert_eq!(
    ///     uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
    ///     vec![(2, 'b'), (1, 'c')]
    /// );
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn next_back(&mut self) -> Option<Self::Item>;

    /// Advances the iterator from the back by `n` elements.
    ///
    /// `advance_back_by` is the reverse version of [`advance_by`]. This method will
    /// eagerly skip `n` elements starting from the back by calling [`next_back`] up
    /// to `n` times until [`None`] is encountered.
    ///
    /// `advance_back_by(n)` will return `Ok(())` if the iterator successfully advances by
    /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered, where `k`
    /// is remaining number of steps that could not be advanced because the iterator ran out.
    /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
    /// Otherwise, `k` is always less than `n`.
    ///
    /// Calling `advance_back_by(0)` can do meaningful work, for example [`Flatten`] can advance its
    /// outer iterator until it finds an inner iterator that is not empty, which then often
    /// allows it to return a more accurate `size_hint()` than in its initial state.
    ///
    /// [`advance_by`]: Iterator::advance_by
    /// [`Flatten`]: crate::iter::Flatten
    /// [`next_back`]: DoubleEndedIterator::next_back
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_advance_by)]
    ///
    /// use std::num::NonZero;
    ///
    /// let a = [3, 4, 5, 6];
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.advance_back_by(2), Ok(()));
    /// assert_eq!(iter.next_back(), Some(&4));
    /// assert_eq!(iter.advance_back_by(0), Ok(()));
    /// assert_eq!(iter.advance_back_by(100), Err(NonZero::new(99).unwrap())); // only `&3` was skipped
    /// ```
    ///
    /// [`Ok(())`]: Ok
    /// [`Err(k)`]: Err
    #[inline]
    #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
        for i in 0..n {
            if self.next_back().is_none() {
                // SAFETY: `i` is always less than `n`.
                return Err(unsafe { NonZero::new_unchecked(n - i) });
            }
        }
        Ok(())
    }

    /// Returns the `n`th element from the end of the iterator.
    ///
    /// This is essentially the reversed version of [`Iterator::nth()`].
    /// Although like most indexing operations, the count starts from zero, so
    /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the
    /// second, and so on.
    ///
    /// Note that all elements between the end and the returned element will be
    /// consumed, including the returned element. This also means that calling
    /// `nth_back(0)` multiple times on the same iterator will return different
    /// elements.
    ///
    /// `nth_back()` will return [`None`] if `n` is greater than or equal to the
    /// length of the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().nth_back(2), Some(&1));
    /// ```
    ///
    /// Calling `nth_back()` multiple times doesn't rewind the iterator:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.nth_back(1), Some(&2));
    /// assert_eq!(iter.nth_back(1), None);
    /// ```
    ///
    /// Returning `None` if there are less than `n + 1` elements:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().nth_back(10), None);
    /// ```
    #[inline]
    #[stable(feature = "iter_nth_back", since = "1.37.0")]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        if self.advance_back_by(n).is_err() {
            return None;
        }
        self.next_back()
    }

    /// This is the reverse version of [`Iterator::try_fold()`]: it takes
    /// elements starting from the back of the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = ["1", "2", "3"];
    /// let sum = a.iter()
    ///     .map(|&s| s.parse::<i32>())
    ///     .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
    /// assert_eq!(sum, Ok(6));
    /// ```
    ///
    /// Short-circuiting:
    ///
    /// ```
    /// let a = ["1", "rust", "3"];
    /// let mut it = a.iter();
    /// let sum = it
    ///     .by_ref()
    ///     .map(|&s| s.parse::<i32>())
    ///     .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
    /// assert!(sum.is_err());
    ///
    /// // Because it short-circuited, the remaining elements are still
    /// // available through the iterator.
    /// assert_eq!(it.next_back(), Some(&"1"));
    /// ```
    #[inline]
    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        let mut accum = init;
        while let Some(x) = self.next_back() {
            accum = f(accum, x)?;
        }
        try { accum }
    }

    /// An iterator method that reduces the iterator's elements to a single,
    /// final value, starting from the back.
    ///
    /// This is the reverse version of [`Iterator::fold()`]: it takes elements
    /// starting from the back of the iterator.
    ///
    /// `rfold()` takes two arguments: an initial value, and a closure with two
    /// arguments: an 'accumulator', and an element. The closure returns the value that
    /// the accumulator should have for the next iteration.
    ///
    /// The initial value is the value the accumulator will have on the first
    /// call.
    ///
    /// After applying this closure to every element of the iterator, `rfold()`
    /// returns the accumulator.
    ///
    /// This operation is sometimes called 'reduce' or 'inject'.
    ///
    /// Folding is useful whenever you have a collection of something, and want
    /// to produce a single value from it.
    ///
    /// Note: `rfold()` combines elements in a *right-associative* fashion. For associative
    /// operators like `+`, the order the elements are combined in is not important, but for non-associative
    /// operators like `-` the order will affect the final result.
    /// For a *left-associative* version of `rfold()`, see [`Iterator::fold()`].
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// // the sum of all of the elements of a
    /// let sum = a.iter()
    ///            .rfold(0, |acc, &x| acc + x);
    ///
    /// assert_eq!(sum, 6);
    /// ```
    ///
    /// This example demonstrates the right-associative nature of `rfold()`:
    /// it builds a string, starting with an initial value
    /// and continuing with each element from the back until the front:
    ///
    /// ```
    /// let numbers = [1, 2, 3, 4, 5];
    ///
    /// let zero = "0".to_string();
    ///
    /// let result = numbers.iter().rfold(zero, |acc, &x| {
    ///     format!("({x} + {acc})")
    /// });
    ///
    /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
    /// ```
    #[doc(alias = "foldr")]
    #[inline]
    #[stable(feature = "iter_rfold", since = "1.27.0")]
    fn rfold<B, F>(mut self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        let mut accum = init;
        while let Some(x) = self.next_back() {
            accum = f(accum, x);
        }
        accum
    }

    /// Searches for an element of an iterator from the back that satisfies a predicate.
    ///
    /// `rfind()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, starting at the end, and if any
    /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
    /// `false`, it returns [`None`].
    ///
    /// `rfind()` is short-circuiting; in other words, it will stop processing
    /// as soon as the closure returns `true`.
    ///
    /// Because `rfind()` takes a reference, and many iterators iterate over
    /// references, this leads to a possibly confusing situation where the
    /// argument is a double reference. You can see this effect in the
    /// examples below, with `&&x`.
    ///
    /// [`Some(element)`]: Some
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
    ///
    /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next_back(), Some(&1));
    /// ```
    #[inline]
    #[stable(feature = "iter_rfind", since = "1.27.0")]
    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        #[inline]
        fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
            move |(), x| {
                if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
            }
        }

        self.try_rfold((), check(predicate)).break_value()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
    fn next_back(&mut self) -> Option<I::Item> {
        (**self).next_back()
    }
    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
        (**self).advance_back_by(n)
    }
    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
        (**self).nth_back(n)
    }
    fn rfold<B, F>(self, init: B, f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        self.spec_rfold(init, f)
    }
    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        self.spec_try_rfold(init, f)
    }
}

/// Helper trait to specialize `rfold` and `rtry_fold` for `&mut I where I: Sized`
trait DoubleEndedIteratorRefSpec: DoubleEndedIterator {
    fn spec_rfold<B, F>(self, init: B, f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B;

    fn spec_try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>;
}

impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIteratorRefSpec for &mut I {
    default fn spec_rfold<B, F>(self, init: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        let mut accum = init;
        while let Some(x) = self.next_back() {
            accum = f(accum, x);
        }
        accum
    }

    default fn spec_try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        let mut accum = init;
        while let Some(x) = self.next_back() {
            accum = f(accum, x)?;
        }
        try { accum }
    }
}

impl<I: DoubleEndedIterator> DoubleEndedIteratorRefSpec for &mut I {
    impl_fold_via_try_fold! { spec_rfold -> spec_try_rfold }

    fn spec_try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        (**self).try_rfold(init, f)
    }
}