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Diffstat (limited to 'library/core/src/iter/adapters/take.rs')
| -rw-r--r-- | library/core/src/iter/adapters/take.rs | 244 |
1 files changed, 244 insertions, 0 deletions
diff --git a/library/core/src/iter/adapters/take.rs b/library/core/src/iter/adapters/take.rs new file mode 100644 index 000000000..2962e0104 --- /dev/null +++ b/library/core/src/iter/adapters/take.rs @@ -0,0 +1,244 @@ +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 + Self: Sized, + 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() + } + } + + #[inline] + fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc + where + Self: Sized, + Fold: FnMut(Acc, Self::Item) -> Acc, + { + #[inline] + fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> { + move |acc, x| Ok(f(acc, x)) + } + + self.try_fold(init, ok(fold)).unwrap() + } + + #[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> {} |