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implement specialized nth_back() for Bytes, Fuse and Enumerate #60023

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Apr 19, 2019
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26 changes: 26 additions & 0 deletions src/libcore/iter/adapters/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -980,6 +980,16 @@ impl<I> DoubleEndedIterator for Enumerate<I> where
})
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
self.iter.nth_back(n).map(|a| {
let len = self.iter.len();
// Can safely add, `ExactSizeIterator` promises that the number of
// elements fits into a `usize`.
(self.count + len, a)
})
}

#[inline]
fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
Expand Down Expand Up @@ -1789,6 +1799,17 @@ impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
}
}

#[inline]
default fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
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if self.done {
None
} else {
let nth = self.iter.nth_back(n);
self.done = nth.is_none();
nth
}
}

#[inline]
default 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>
Expand Down Expand Up @@ -1877,6 +1898,11 @@ impl<I> DoubleEndedIterator for Fuse<I>
self.iter.next_back()
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
self.iter.nth_back(n)
}

#[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>
Expand Down
5 changes: 5 additions & 0 deletions src/libcore/str/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -795,6 +795,11 @@ impl DoubleEndedIterator for Bytes<'_> {
self.0.next_back()
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
self.0.nth_back(n)
}

#[inline]
fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item> where
P: FnMut(&Self::Item) -> bool
Expand Down
18 changes: 18 additions & 0 deletions src/libcore/tests/iter.rs
Original file line number Diff line number Diff line change
Expand Up @@ -389,6 +389,24 @@ fn test_iterator_enumerate_nth() {
assert_eq!(i, 3);
}

#[test]
fn test_iterator_enumerate_nth_back() {
let xs = [0, 1, 2, 3, 4, 5];
let mut it = xs.iter().enumerate();
while let Some((i, &x)) = it.nth_back(0) {
assert_eq!(i, x);
}

let mut it = xs.iter().enumerate();
while let Some((i, &x)) = it.nth_back(1) {
assert_eq!(i, x);
}

let (i, &x) = xs.iter().enumerate().nth_back(3).unwrap();
assert_eq!(i, x);
assert_eq!(i, 2);
}

#[test]
fn test_iterator_enumerate_count() {
let xs = [0, 1, 2, 3, 4, 5];
Expand Down