diff --git a/src/libcore/iter/adapters/mod.rs b/src/libcore/iter/adapters/mod.rs index 518442efe7417..c2edcd22f953b 100644 --- a/src/libcore/iter/adapters/mod.rs +++ b/src/libcore/iter/adapters/mod.rs @@ -1509,6 +1509,20 @@ impl DoubleEndedIterator for Skip where I: DoubleEndedIterator + ExactSize } } + #[inline] + fn nth_back(&mut self, n: usize) -> Option { + let len = self.len(); + if n < len { + self.iter.nth_back(n) + } else { + if len > 0 { + // consume the original iterator + self.iter.nth_back(len-1); + } + None + } + } + fn try_rfold(&mut self, init: Acc, mut fold: Fold) -> R where Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try { diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs index 7dfb1adad9eed..7e3190f8a20d6 100644 --- a/src/libcore/tests/iter.rs +++ b/src/libcore/tests/iter.rs @@ -2280,6 +2280,40 @@ fn test_skip_try_folds() { assert_eq!(iter.next_back(), Some(24)); } +#[test] +fn test_skip_nth_back() { + let xs = [0, 1, 2, 3, 4, 5]; + let mut it = xs.iter().skip(2); + assert_eq!(it.nth_back(0), Some(&5)); + assert_eq!(it.nth_back(1), Some(&3)); + assert_eq!(it.nth_back(0), Some(&2)); + assert_eq!(it.nth_back(0), None); + + let ys = [2, 3, 4, 5]; + let mut ity = ys.iter(); + let mut it = xs.iter().skip(2); + assert_eq!(it.nth_back(1), ity.nth_back(1)); + assert_eq!(it.clone().nth(0), ity.clone().nth(0)); + assert_eq!(it.nth_back(0), ity.nth_back(0)); + assert_eq!(it.clone().nth(0), ity.clone().nth(0)); + assert_eq!(it.nth_back(0), ity.nth_back(0)); + assert_eq!(it.clone().nth(0), ity.clone().nth(0)); + assert_eq!(it.nth_back(0), ity.nth_back(0)); + assert_eq!(it.clone().nth(0), ity.clone().nth(0)); + + let mut it = xs.iter().skip(2); + assert_eq!(it.nth_back(4), None); + assert_eq!(it.nth_back(0), None); + + let mut it = xs.iter(); + it.by_ref().skip(2).nth_back(3); + assert_eq!(it.next_back(), Some(&1)); + + let mut it = xs.iter(); + it.by_ref().skip(2).nth_back(10); + assert_eq!(it.next_back(), Some(&1)); +} + #[test] fn test_take_try_folds() { let f = &|acc, x| i32::checked_add(2*acc, x);