| use crate::intrinsics::unlikely; |
| use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable}; |
| use crate::ops::{ControlFlow, Try}; |
| |
| /// An iterator that skips over `n` elements of `iter`. |
| /// |
| /// This `struct` is created by the [`skip`] method on [`Iterator`]. See its |
| /// documentation for more. |
| /// |
| /// [`skip`]: Iterator::skip |
| /// [`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 Skip<I> { |
| iter: I, |
| n: usize, |
| } |
| |
| impl<I> Skip<I> { |
| pub(in crate::iter) fn new(iter: I, n: usize) -> Skip<I> { |
| Skip { iter, n } |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<I> Iterator for Skip<I> |
| where |
| I: Iterator, |
| { |
| type Item = <I as Iterator>::Item; |
| |
| #[inline] |
| fn next(&mut self) -> Option<I::Item> { |
| if unlikely(self.n > 0) { |
| self.iter.nth(crate::mem::take(&mut self.n)) |
| } else { |
| self.iter.next() |
| } |
| } |
| |
| #[inline] |
| fn nth(&mut self, n: usize) -> Option<I::Item> { |
| if self.n > 0 { |
| let skip: usize = crate::mem::take(&mut self.n); |
| // Checked add to handle overflow case. |
| let n = match skip.checked_add(n) { |
| Some(nth) => nth, |
| None => { |
| // In case of overflow, load skip value, before loading `n`. |
| // Because the amount of elements to iterate is beyond `usize::MAX`, this |
| // is split into two `nth` calls where the `skip` `nth` call is discarded. |
| self.iter.nth(skip - 1)?; |
| n |
| } |
| }; |
| // Load nth element including skip. |
| self.iter.nth(n) |
| } else { |
| self.iter.nth(n) |
| } |
| } |
| |
| #[inline] |
| fn count(mut self) -> usize { |
| if self.n > 0 { |
| // nth(n) skips n+1 |
| if self.iter.nth(self.n - 1).is_none() { |
| return 0; |
| } |
| } |
| self.iter.count() |
| } |
| |
| #[inline] |
| fn last(mut self) -> Option<I::Item> { |
| if self.n > 0 { |
| // nth(n) skips n+1 |
| self.iter.nth(self.n - 1)?; |
| } |
| self.iter.last() |
| } |
| |
| #[inline] |
| fn size_hint(&self) -> (usize, Option<usize>) { |
| let (lower, upper) = self.iter.size_hint(); |
| |
| let lower = lower.saturating_sub(self.n); |
| let upper = match upper { |
| Some(x) => Some(x.saturating_sub(self.n)), |
| None => None, |
| }; |
| |
| (lower, upper) |
| } |
| |
| #[inline] |
| fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R |
| where |
| Self: Sized, |
| Fold: FnMut(Acc, Self::Item) -> R, |
| R: Try<Output = Acc>, |
| { |
| let n = self.n; |
| self.n = 0; |
| if n > 0 { |
| // nth(n) skips n+1 |
| if self.iter.nth(n - 1).is_none() { |
| return try { init }; |
| } |
| } |
| self.iter.try_fold(init, fold) |
| } |
| |
| #[inline] |
| fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc |
| where |
| Fold: FnMut(Acc, Self::Item) -> Acc, |
| { |
| if self.n > 0 { |
| // nth(n) skips n+1 |
| if self.iter.nth(self.n - 1).is_none() { |
| return init; |
| } |
| } |
| self.iter.fold(init, fold) |
| } |
| |
| #[inline] |
| #[rustc_inherit_overflow_checks] |
| fn advance_by(&mut self, n: usize) -> Result<(), usize> { |
| let mut rem = n; |
| let step_one = self.n.saturating_add(rem); |
| |
| match self.iter.advance_by(step_one) { |
| Ok(_) => { |
| rem -= step_one - self.n; |
| self.n = 0; |
| } |
| Err(advanced) => { |
| let advanced_without_skip = advanced.saturating_sub(self.n); |
| self.n = self.n.saturating_sub(advanced); |
| return if n == 0 { Ok(()) } else { Err(advanced_without_skip) }; |
| } |
| } |
| |
| // step_one calculation may have saturated |
| if unlikely(rem > 0) { |
| return match self.iter.advance_by(rem) { |
| ret @ Ok(_) => ret, |
| Err(advanced) => { |
| rem -= advanced; |
| Err(n - rem) |
| } |
| }; |
| } |
| |
| Ok(()) |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {} |
| |
| #[stable(feature = "double_ended_skip_iterator", since = "1.9.0")] |
| impl<I> DoubleEndedIterator for Skip<I> |
| where |
| I: DoubleEndedIterator + ExactSizeIterator, |
| { |
| fn next_back(&mut self) -> Option<Self::Item> { |
| if self.len() > 0 { self.iter.next_back() } else { None } |
| } |
| |
| #[inline] |
| fn nth_back(&mut self, n: usize) -> Option<I::Item> { |
| 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<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R |
| where |
| Self: Sized, |
| Fold: FnMut(Acc, Self::Item) -> R, |
| R: Try<Output = Acc>, |
| { |
| fn check<T, Acc, R: Try<Output = Acc>>( |
| mut n: usize, |
| mut fold: impl FnMut(Acc, T) -> R, |
| ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> { |
| move |acc, x| { |
| n -= 1; |
| let r = fold(acc, x); |
| if n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) } |
| } |
| } |
| |
| let n = self.len(); |
| if n == 0 { try { init } } else { self.iter.try_rfold(init, check(n, fold)).into_try() } |
| } |
| |
| impl_fold_via_try_fold! { rfold -> try_rfold } |
| |
| #[inline] |
| fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { |
| let min = crate::cmp::min(self.len(), n); |
| return match self.iter.advance_back_by(min) { |
| ret @ Ok(_) if n <= min => ret, |
| Ok(_) => Err(min), |
| _ => panic!("ExactSizeIterator contract violation"), |
| }; |
| } |
| } |
| |
| #[stable(feature = "fused", since = "1.26.0")] |
| impl<I> FusedIterator for Skip<I> where I: FusedIterator {} |
| |
| #[unstable(issue = "none", feature = "inplace_iteration")] |
| unsafe impl<I> SourceIter for Skip<I> |
| where |
| I: SourceIter, |
| { |
| type Source = I::Source; |
| |
| #[inline] |
| unsafe fn as_inner(&mut self) -> &mut I::Source { |
| // SAFETY: unsafe function forwarding to unsafe function with the same requirements |
| unsafe { SourceIter::as_inner(&mut self.iter) } |
| } |
| } |
| |
| #[unstable(issue = "none", feature = "inplace_iteration")] |
| unsafe impl<I: InPlaceIterable> InPlaceIterable for Skip<I> {} |