| use crate::iter::{FusedIterator, TrustedLen}; |
| use crate::mem::ManuallyDrop; |
| |
| /// Creates a new iterator that repeats a single element a given number of times. |
| /// |
| /// The `repeat_n()` function repeats a single value exactly `n` times. |
| /// |
| /// This is very similar to using [`repeat()`] with [`Iterator::take()`], |
| /// but there are two differences: |
| /// - `repeat_n()` can return the original value, rather than always cloning. |
| /// - `repeat_n()` produces an [`ExactSizeIterator`]. |
| /// |
| /// [`repeat()`]: crate::iter::repeat |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// #![feature(iter_repeat_n)] |
| /// use std::iter; |
| /// |
| /// // four of the number four: |
| /// let mut four_fours = iter::repeat_n(4, 4); |
| /// |
| /// assert_eq!(Some(4), four_fours.next()); |
| /// assert_eq!(Some(4), four_fours.next()); |
| /// assert_eq!(Some(4), four_fours.next()); |
| /// assert_eq!(Some(4), four_fours.next()); |
| /// |
| /// // no more fours |
| /// assert_eq!(None, four_fours.next()); |
| /// ``` |
| /// |
| /// For non-`Copy` types, |
| /// |
| /// ``` |
| /// #![feature(iter_repeat_n)] |
| /// use std::iter; |
| /// |
| /// let v: Vec<i32> = Vec::with_capacity(123); |
| /// let mut it = iter::repeat_n(v, 5); |
| /// |
| /// for i in 0..4 { |
| /// // It starts by cloning things |
| /// let cloned = it.next().unwrap(); |
| /// assert_eq!(cloned.len(), 0); |
| /// assert_eq!(cloned.capacity(), 0); |
| /// } |
| /// |
| /// // ... but the last item is the original one |
| /// let last = it.next().unwrap(); |
| /// assert_eq!(last.len(), 0); |
| /// assert_eq!(last.capacity(), 123); |
| /// |
| /// // ... and now we're done |
| /// assert_eq!(None, it.next()); |
| /// ``` |
| #[inline] |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| #[doc(hidden)] // waiting on ACP#120 to decide whether to expose publicly |
| pub fn repeat_n<T: Clone>(element: T, count: usize) -> RepeatN<T> { |
| let mut element = ManuallyDrop::new(element); |
| |
| if count == 0 { |
| // SAFETY: we definitely haven't dropped it yet, since we only just got |
| // passed it in, and because the count is zero the instance we're about |
| // to create won't drop it, so to avoid leaking we need to now. |
| unsafe { ManuallyDrop::drop(&mut element) }; |
| } |
| |
| RepeatN { element, count } |
| } |
| |
| /// An iterator that repeats an element an exact number of times. |
| /// |
| /// This `struct` is created by the [`repeat_n()`] function. |
| /// See its documentation for more. |
| #[derive(Clone, Debug)] |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| #[doc(hidden)] // waiting on ACP#120 to decide whether to expose publicly |
| pub struct RepeatN<A> { |
| count: usize, |
| // Invariant: has been dropped iff count == 0. |
| element: ManuallyDrop<A>, |
| } |
| |
| impl<A> RepeatN<A> { |
| /// If we haven't already dropped the element, return it in an option. |
| /// |
| /// Clears the count so it won't be dropped again later. |
| #[inline] |
| fn take_element(&mut self) -> Option<A> { |
| if self.count > 0 { |
| self.count = 0; |
| // SAFETY: We just set count to zero so it won't be dropped again, |
| // and it used to be non-zero so it hasn't already been dropped. |
| unsafe { Some(ManuallyDrop::take(&mut self.element)) } |
| } else { |
| None |
| } |
| } |
| } |
| |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| impl<A> Drop for RepeatN<A> { |
| fn drop(&mut self) { |
| self.take_element(); |
| } |
| } |
| |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| impl<A: Clone> Iterator for RepeatN<A> { |
| type Item = A; |
| |
| #[inline] |
| fn next(&mut self) -> Option<A> { |
| if self.count == 0 { |
| return None; |
| } |
| |
| self.count -= 1; |
| Some(if self.count == 0 { |
| // SAFETY: the check above ensured that the count used to be non-zero, |
| // so element hasn't been dropped yet, and we just lowered the count to |
| // zero so it won't be dropped later, and thus it's okay to take it here. |
| unsafe { ManuallyDrop::take(&mut self.element) } |
| } else { |
| A::clone(&mut self.element) |
| }) |
| } |
| |
| #[inline] |
| fn size_hint(&self) -> (usize, Option<usize>) { |
| let len = self.len(); |
| (len, Some(len)) |
| } |
| |
| #[inline] |
| fn advance_by(&mut self, skip: usize) -> Result<(), usize> { |
| let len = self.count; |
| |
| if skip >= len { |
| self.take_element(); |
| } |
| |
| if skip > len { |
| Err(len) |
| } else { |
| self.count = len - skip; |
| Ok(()) |
| } |
| } |
| |
| #[inline] |
| fn last(mut self) -> Option<A> { |
| self.take_element() |
| } |
| |
| #[inline] |
| fn count(self) -> usize { |
| self.len() |
| } |
| } |
| |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| impl<A: Clone> ExactSizeIterator for RepeatN<A> { |
| fn len(&self) -> usize { |
| self.count |
| } |
| } |
| |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| impl<A: Clone> DoubleEndedIterator for RepeatN<A> { |
| #[inline] |
| fn next_back(&mut self) -> Option<A> { |
| self.next() |
| } |
| |
| #[inline] |
| fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { |
| self.advance_by(n) |
| } |
| |
| #[inline] |
| fn nth_back(&mut self, n: usize) -> Option<A> { |
| self.nth(n) |
| } |
| } |
| |
| #[unstable(feature = "iter_repeat_n", issue = "104434")] |
| impl<A: Clone> FusedIterator for RepeatN<A> {} |
| |
| #[unstable(feature = "trusted_len", issue = "37572")] |
| unsafe impl<A: Clone> TrustedLen for RepeatN<A> {} |