366 lines
12 KiB
Rust
366 lines
12 KiB
Rust
use crate::ops::{ControlFlow, Try};
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/// An iterator able to yield elements from both ends.
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///
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/// Something that implements `DoubleEndedIterator` has one extra capability
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/// over something that implements [`Iterator`]: the ability to also take
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/// `Item`s from the back, as well as the front.
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///
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/// It is important to note that both back and forth work on the same range,
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/// and do not cross: iteration is over when they meet in the middle.
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///
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/// In a similar fashion to the [`Iterator`] protocol, once a
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/// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it
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/// again may or may not ever return [`Some`] again. [`next()`] and
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/// [`next_back()`] are interchangeable for this purpose.
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///
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/// [`next_back()`]: DoubleEndedIterator::next_back
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/// [`next()`]: Iterator::next
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// let numbers = vec![1, 2, 3, 4, 5, 6];
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///
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/// let mut iter = numbers.iter();
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///
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/// assert_eq!(Some(&1), iter.next());
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/// assert_eq!(Some(&6), iter.next_back());
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/// assert_eq!(Some(&5), iter.next_back());
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/// assert_eq!(Some(&2), iter.next());
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/// assert_eq!(Some(&3), iter.next());
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/// assert_eq!(Some(&4), iter.next());
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/// assert_eq!(None, iter.next());
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/// assert_eq!(None, iter.next_back());
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait DoubleEndedIterator: Iterator {
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/// Removes and returns an element from the end of the iterator.
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///
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/// Returns `None` when there are no more elements.
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///
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/// The [trait-level] docs contain more details.
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///
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/// [trait-level]: DoubleEndedIterator
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// let numbers = vec![1, 2, 3, 4, 5, 6];
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///
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/// let mut iter = numbers.iter();
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///
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/// assert_eq!(Some(&1), iter.next());
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/// assert_eq!(Some(&6), iter.next_back());
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/// assert_eq!(Some(&5), iter.next_back());
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/// assert_eq!(Some(&2), iter.next());
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/// assert_eq!(Some(&3), iter.next());
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/// assert_eq!(Some(&4), iter.next());
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/// assert_eq!(None, iter.next());
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/// assert_eq!(None, iter.next_back());
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/// ```
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///
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/// # Remarks
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///
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/// The elements yielded by `DoubleEndedIterator`'s methods may differ from
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/// the ones yielded by [`Iterator`]'s methods:
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///
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/// ```
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/// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
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/// let uniq_by_fst_comp = || {
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/// let mut seen = std::collections::HashSet::new();
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/// vec.iter().copied().filter(move |x| seen.insert(x.0))
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/// };
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///
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/// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
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/// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
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///
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/// assert_eq!(
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/// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
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/// vec![(1, 'a'), (2, 'a')]
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/// );
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/// assert_eq!(
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/// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
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/// vec![(2, 'b'), (1, 'c')]
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/// );
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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fn next_back(&mut self) -> Option<Self::Item>;
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/// Advances the iterator from the back by `n` elements.
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///
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/// `advance_back_by` is the reverse version of [`advance_by`]. This method will
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/// eagerly skip `n` elements starting from the back by calling [`next_back`] up
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/// to `n` times until [`None`] is encountered.
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///
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/// `advance_back_by(n)` will return [`Ok(())`] if the iterator successfully advances by
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/// `n` elements, or [`Err(k)`] if [`None`] is encountered, where `k` is the number of
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/// elements the iterator is advanced by before running out of elements (i.e. the length
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/// of the iterator). Note that `k` is always less than `n`.
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///
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/// Calling `advance_back_by(0)` does not consume any elements and always returns [`Ok(())`].
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///
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/// [`advance_by`]: Iterator::advance_by
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/// [`next_back`]: DoubleEndedIterator::next_back
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// #![feature(iter_advance_by)]
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///
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/// let a = [3, 4, 5, 6];
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/// let mut iter = a.iter();
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///
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/// assert_eq!(iter.advance_back_by(2), Ok(()));
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/// assert_eq!(iter.next_back(), Some(&4));
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/// assert_eq!(iter.advance_back_by(0), Ok(()));
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/// assert_eq!(iter.advance_back_by(100), Err(1)); // only `&3` was skipped
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/// ```
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///
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/// [`Ok(())`]: Ok
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/// [`Err(k)`]: Err
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#[inline]
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#[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
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fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
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for i in 0..n {
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self.next_back().ok_or(i)?;
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}
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Ok(())
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}
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/// Returns the `n`th element from the end of the iterator.
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///
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/// This is essentially the reversed version of [`Iterator::nth()`].
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/// Although like most indexing operations, the count starts from zero, so
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/// `nth_back(0)` returns the first value from the end, `nth_back(1)` the
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/// second, and so on.
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///
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/// Note that all elements between the end and the returned element will be
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/// consumed, including the returned element. This also means that calling
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/// `nth_back(0)` multiple times on the same iterator will return different
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/// elements.
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///
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/// `nth_back()` will return [`None`] if `n` is greater than or equal to the
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/// length of the iterator.
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// let a = [1, 2, 3];
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/// assert_eq!(a.iter().nth_back(2), Some(&1));
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/// ```
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///
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/// Calling `nth_back()` multiple times doesn't rewind the iterator:
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///
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/// ```
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/// let a = [1, 2, 3];
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///
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/// let mut iter = a.iter();
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///
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/// assert_eq!(iter.nth_back(1), Some(&2));
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/// assert_eq!(iter.nth_back(1), None);
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/// ```
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///
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/// Returning `None` if there are less than `n + 1` elements:
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///
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/// ```
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/// let a = [1, 2, 3];
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/// assert_eq!(a.iter().nth_back(10), None);
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/// ```
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#[inline]
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#[stable(feature = "iter_nth_back", since = "1.37.0")]
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fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
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self.advance_back_by(n).ok()?;
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self.next_back()
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}
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/// This is the reverse version of [`Iterator::try_fold()`]: it takes
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/// elements starting from the back of the iterator.
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// let a = ["1", "2", "3"];
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/// let sum = a.iter()
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/// .map(|&s| s.parse::<i32>())
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/// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
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/// assert_eq!(sum, Ok(6));
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/// ```
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///
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/// Short-circuiting:
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///
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/// ```
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/// let a = ["1", "rust", "3"];
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/// let mut it = a.iter();
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/// let sum = it
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/// .by_ref()
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/// .map(|&s| s.parse::<i32>())
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/// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
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/// assert!(sum.is_err());
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///
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/// // Because it short-circuited, the remaining elements are still
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/// // available through the iterator.
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/// assert_eq!(it.next_back(), Some(&"1"));
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/// ```
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#[inline]
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#[stable(feature = "iterator_try_fold", since = "1.27.0")]
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fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
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where
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Self: Sized,
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F: FnMut(B, Self::Item) -> R,
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R: Try<Ok = B>,
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{
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let mut accum = init;
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while let Some(x) = self.next_back() {
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accum = f(accum, x)?;
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}
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Try::from_ok(accum)
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}
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/// An iterator method that reduces the iterator's elements to a single,
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/// final value, starting from the back.
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///
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/// This is the reverse version of [`Iterator::fold()`]: it takes elements
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/// starting from the back of the iterator.
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///
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/// `rfold()` takes two arguments: an initial value, and a closure with two
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/// arguments: an 'accumulator', and an element. The closure returns the value that
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/// the accumulator should have for the next iteration.
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///
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/// The initial value is the value the accumulator will have on the first
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/// call.
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///
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/// After applying this closure to every element of the iterator, `rfold()`
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/// returns the accumulator.
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///
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/// This operation is sometimes called 'reduce' or 'inject'.
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///
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/// Folding is useful whenever you have a collection of something, and want
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/// to produce a single value from it.
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// let a = [1, 2, 3];
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///
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/// // the sum of all of the elements of a
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/// let sum = a.iter()
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/// .rfold(0, |acc, &x| acc + x);
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///
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/// assert_eq!(sum, 6);
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/// ```
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///
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/// This example builds a string, starting with an initial value
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/// and continuing with each element from the back until the front:
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///
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/// ```
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/// let numbers = [1, 2, 3, 4, 5];
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///
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/// let zero = "0".to_string();
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///
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/// let result = numbers.iter().rfold(zero, |acc, &x| {
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/// format!("({} + {})", x, acc)
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/// });
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///
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/// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
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/// ```
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#[inline]
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#[stable(feature = "iter_rfold", since = "1.27.0")]
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fn rfold<B, F>(mut self, init: B, mut f: F) -> B
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where
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Self: Sized,
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F: FnMut(B, Self::Item) -> B,
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{
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let mut accum = init;
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while let Some(x) = self.next_back() {
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accum = f(accum, x);
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}
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accum
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}
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/// Searches for an element of an iterator from the back that satisfies a predicate.
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///
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/// `rfind()` takes a closure that returns `true` or `false`. It applies
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/// this closure to each element of the iterator, starting at the end, and if any
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/// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
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/// `false`, it returns [`None`].
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///
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/// `rfind()` is short-circuiting; in other words, it will stop processing
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/// as soon as the closure returns `true`.
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///
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/// Because `rfind()` takes a reference, and many iterators iterate over
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/// references, this leads to a possibly confusing situation where the
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/// argument is a double reference. You can see this effect in the
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/// examples below, with `&&x`.
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///
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/// [`Some(element)`]: Some
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// let a = [1, 2, 3];
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///
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/// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
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///
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/// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
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/// ```
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///
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/// Stopping at the first `true`:
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///
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/// ```
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/// let a = [1, 2, 3];
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///
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/// let mut iter = a.iter();
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///
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/// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
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///
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/// // we can still use `iter`, as there are more elements.
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/// assert_eq!(iter.next_back(), Some(&1));
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/// ```
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#[inline]
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#[stable(feature = "iter_rfind", since = "1.27.0")]
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fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
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where
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Self: Sized,
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P: FnMut(&Self::Item) -> bool,
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{
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#[inline]
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fn check<T>(
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mut predicate: impl FnMut(&T) -> bool,
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) -> impl FnMut((), T) -> ControlFlow<(), T> {
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move |(), x| {
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if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
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}
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}
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self.try_rfold((), check(predicate)).break_value()
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
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fn next_back(&mut self) -> Option<I::Item> {
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(**self).next_back()
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}
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fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
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(**self).advance_back_by(n)
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}
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fn nth_back(&mut self, n: usize) -> Option<I::Item> {
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(**self).nth_back(n)
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}
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}
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