| use crate::cmp; |
| use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen}; |
| use crate::ops::{ControlFlow, Try}; |
| |
| /// An iterator that only iterates over the first `n` iterations of `iter`. |
| /// |
| /// This `struct` is created by the [`take`] method on [`Iterator`]. See its |
| /// documentation for more. |
| /// |
| /// [`take`]: Iterator::take |
| /// [`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 Take<I> { |
| iter: I, |
| n: usize, |
| } |
| |
| impl<I> Take<I> { |
| pub(in crate::iter) fn new(iter: I, n: usize) -> Take<I> { |
| Take { iter, n } |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<I> Iterator for Take<I> |
| where |
| I: Iterator, |
| { |
| type Item = <I as Iterator>::Item; |
| |
| #[inline] |
| fn next(&mut self) -> Option<<I as Iterator>::Item> { |
| if self.n != 0 { |
| self.n -= 1; |
| self.iter.next() |
| } else { |
| None |
| } |
| } |
| |
| #[inline] |
| fn nth(&mut self, n: usize) -> Option<I::Item> { |
| if self.n > n { |
| self.n -= n + 1; |
| self.iter.nth(n) |
| } else { |
| if self.n > 0 { |
| self.iter.nth(self.n - 1); |
| self.n = 0; |
| } |
| None |
| } |
| } |
| |
| #[inline] |
| fn size_hint(&self) -> (usize, Option<usize>) { |
| if self.n == 0 { |
| return (0, Some(0)); |
| } |
| |
| let (lower, upper) = self.iter.size_hint(); |
| |
| let lower = cmp::min(lower, self.n); |
| |
| let upper = match upper { |
| Some(x) if x < self.n => Some(x), |
| _ => Some(self.n), |
| }; |
| |
| (lower, upper) |
| } |
| |
| #[inline] |
| fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R |
| where |
| Fold: FnMut(Acc, Self::Item) -> R, |
| R: Try<Output = Acc>, |
| { |
| fn check<'a, T, Acc, R: Try<Output = Acc>>( |
| n: &'a mut usize, |
| mut fold: impl FnMut(Acc, T) -> R + 'a, |
| ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a { |
| move |acc, x| { |
| *n -= 1; |
| let r = fold(acc, x); |
| if *n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) } |
| } |
| } |
| |
| if self.n == 0 { |
| try { init } |
| } else { |
| let n = &mut self.n; |
| self.iter.try_fold(init, check(n, fold)).into_try() |
| } |
| } |
| |
| impl_fold_via_try_fold! { fold -> try_fold } |
| |
| #[inline] |
| fn for_each<F: FnMut(Self::Item)>(mut self, f: F) { |
| // The default implementation would use a unit accumulator, so we can |
| // avoid a stateful closure by folding over the remaining number |
| // of items we wish to return instead. |
| fn check<'a, Item>( |
| mut action: impl FnMut(Item) + 'a, |
| ) -> impl FnMut(usize, Item) -> Option<usize> + 'a { |
| move |more, x| { |
| action(x); |
| more.checked_sub(1) |
| } |
| } |
| |
| let remaining = self.n; |
| if remaining > 0 { |
| self.iter.try_fold(remaining - 1, check(f)); |
| } |
| } |
| |
| #[inline] |
| #[rustc_inherit_overflow_checks] |
| fn advance_by(&mut self, n: usize) -> Result<(), usize> { |
| let min = self.n.min(n); |
| match self.iter.advance_by(min) { |
| Ok(_) => { |
| self.n -= min; |
| if min < n { Err(min) } else { Ok(()) } |
| } |
| ret @ Err(advanced) => { |
| self.n -= advanced; |
| ret |
| } |
| } |
| } |
| } |
| |
| #[unstable(issue = "none", feature = "inplace_iteration")] |
| unsafe impl<I> SourceIter for Take<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 Take<I> {} |
| |
| #[stable(feature = "double_ended_take_iterator", since = "1.38.0")] |
| impl<I> DoubleEndedIterator for Take<I> |
| where |
| I: DoubleEndedIterator + ExactSizeIterator, |
| { |
| #[inline] |
| fn next_back(&mut self) -> Option<Self::Item> { |
| if self.n == 0 { |
| None |
| } else { |
| let n = self.n; |
| self.n -= 1; |
| self.iter.nth_back(self.iter.len().saturating_sub(n)) |
| } |
| } |
| |
| #[inline] |
| fn nth_back(&mut self, n: usize) -> Option<Self::Item> { |
| let len = self.iter.len(); |
| if self.n > n { |
| let m = len.saturating_sub(self.n) + n; |
| self.n -= n + 1; |
| self.iter.nth_back(m) |
| } else { |
| if len > 0 { |
| self.iter.nth_back(len - 1); |
| } |
| None |
| } |
| } |
| |
| #[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<Output = Acc>, |
| { |
| if self.n == 0 { |
| try { init } |
| } else { |
| let len = self.iter.len(); |
| if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() { |
| try { init } |
| } else { |
| self.iter.try_rfold(init, fold) |
| } |
| } |
| } |
| |
| #[inline] |
| fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc |
| where |
| Self: Sized, |
| Fold: FnMut(Acc, Self::Item) -> Acc, |
| { |
| if self.n == 0 { |
| init |
| } else { |
| let len = self.iter.len(); |
| if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() { |
| init |
| } else { |
| self.iter.rfold(init, fold) |
| } |
| } |
| } |
| |
| #[inline] |
| #[rustc_inherit_overflow_checks] |
| fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { |
| // The amount by which the inner iterator needs to be shortened for it to be |
| // at most as long as the take() amount. |
| let trim_inner = self.iter.len().saturating_sub(self.n); |
| // The amount we need to advance inner to fulfill the caller's request. |
| // take(), advance_by() and len() all can be at most usize, so we don't have to worry |
| // about having to advance more than usize::MAX here. |
| let advance_by = trim_inner.saturating_add(n); |
| |
| let advanced = match self.iter.advance_back_by(advance_by) { |
| Ok(_) => advance_by - trim_inner, |
| Err(advanced) => advanced - trim_inner, |
| }; |
| self.n -= advanced; |
| return if advanced < n { Err(advanced) } else { Ok(()) }; |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {} |
| |
| #[stable(feature = "fused", since = "1.26.0")] |
| impl<I> FusedIterator for Take<I> where I: FusedIterator {} |
| |
| #[unstable(feature = "trusted_len", issue = "37572")] |
| unsafe impl<I: TrustedLen> TrustedLen for Take<I> {} |