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Diffstat (limited to 'rust/alloc/vec/drain_filter.rs')
| -rw-r--r-- | rust/alloc/vec/drain_filter.rs | 199 |
1 files changed, 199 insertions, 0 deletions
diff --git a/rust/alloc/vec/drain_filter.rs b/rust/alloc/vec/drain_filter.rs new file mode 100644 index 000000000..09efff090 --- /dev/null +++ b/rust/alloc/vec/drain_filter.rs @@ -0,0 +1,199 @@ +// SPDX-License-Identifier: Apache-2.0 OR MIT + +use crate::alloc::{Allocator, Global}; +use core::mem::{ManuallyDrop, SizedTypeProperties}; +use core::ptr; +use core::slice; + +use super::Vec; + +/// An iterator which uses a closure to determine if an element should be removed. +/// +/// This struct is created by [`Vec::drain_filter`]. +/// See its documentation for more. +/// +/// # Example +/// +/// ``` +/// #![feature(drain_filter)] +/// +/// let mut v = vec![0, 1, 2]; +/// let iter: std::vec::DrainFilter<'_, _, _> = v.drain_filter(|x| *x % 2 == 0); +/// ``` +#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] +#[derive(Debug)] +pub struct DrainFilter< + 'a, + T, + F, + #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global, +> where + F: FnMut(&mut T) -> bool, +{ + pub(super) vec: &'a mut Vec<T, A>, + /// The index of the item that will be inspected by the next call to `next`. + pub(super) idx: usize, + /// The number of items that have been drained (removed) thus far. + pub(super) del: usize, + /// The original length of `vec` prior to draining. + pub(super) old_len: usize, + /// The filter test predicate. + pub(super) pred: F, + /// A flag that indicates a panic has occurred in the filter test predicate. + /// This is used as a hint in the drop implementation to prevent consumption + /// of the remainder of the `DrainFilter`. Any unprocessed items will be + /// backshifted in the `vec`, but no further items will be dropped or + /// tested by the filter predicate. + pub(super) panic_flag: bool, +} + +impl<T, F, A: Allocator> DrainFilter<'_, T, F, A> +where + F: FnMut(&mut T) -> bool, +{ + /// Returns a reference to the underlying allocator. + #[unstable(feature = "allocator_api", issue = "32838")] + #[inline] + pub fn allocator(&self) -> &A { + self.vec.allocator() + } + + /// Keep unyielded elements in the source `Vec`. + /// + /// # Examples + /// + /// ``` + /// #![feature(drain_filter)] + /// #![feature(drain_keep_rest)] + /// + /// let mut vec = vec!['a', 'b', 'c']; + /// let mut drain = vec.drain_filter(|_| true); + /// + /// assert_eq!(drain.next().unwrap(), 'a'); + /// + /// // This call keeps 'b' and 'c' in the vec. + /// drain.keep_rest(); + /// + /// // If we wouldn't call `keep_rest()`, + /// // `vec` would be empty. + /// assert_eq!(vec, ['b', 'c']); + /// ``` + #[unstable(feature = "drain_keep_rest", issue = "101122")] + pub fn keep_rest(self) { + // At this moment layout looks like this: + // + // _____________________/-- old_len + // / \ + // [kept] [yielded] [tail] + // \_______/ ^-- idx + // \-- del + // + // Normally `Drop` impl would drop [tail] (via .for_each(drop), ie still calling `pred`) + // + // 1. Move [tail] after [kept] + // 2. Update length of the original vec to `old_len - del` + // a. In case of ZST, this is the only thing we want to do + // 3. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do + let mut this = ManuallyDrop::new(self); + + unsafe { + // ZSTs have no identity, so we don't need to move them around. + if !T::IS_ZST && this.idx < this.old_len && this.del > 0 { + let ptr = this.vec.as_mut_ptr(); + let src = ptr.add(this.idx); + let dst = src.sub(this.del); + let tail_len = this.old_len - this.idx; + src.copy_to(dst, tail_len); + } + + let new_len = this.old_len - this.del; + this.vec.set_len(new_len); + } + } +} + +#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] +impl<T, F, A: Allocator> Iterator for DrainFilter<'_, T, F, A> +where + F: FnMut(&mut T) -> bool, +{ + type Item = T; + + fn next(&mut self) -> Option<T> { + unsafe { + while self.idx < self.old_len { + let i = self.idx; + let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len); + self.panic_flag = true; + let drained = (self.pred)(&mut v[i]); + self.panic_flag = false; + // Update the index *after* the predicate is called. If the index + // is updated prior and the predicate panics, the element at this + // index would be leaked. + self.idx += 1; + if drained { + self.del += 1; + return Some(ptr::read(&v[i])); + } else if self.del > 0 { + let del = self.del; + let src: *const T = &v[i]; + let dst: *mut T = &mut v[i - del]; + ptr::copy_nonoverlapping(src, dst, 1); + } + } + None + } + } + + fn size_hint(&self) -> (usize, Option<usize>) { + (0, Some(self.old_len - self.idx)) + } +} + +#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] +impl<T, F, A: Allocator> Drop for DrainFilter<'_, T, F, A> +where + F: FnMut(&mut T) -> bool, +{ + fn drop(&mut self) { + struct BackshiftOnDrop<'a, 'b, T, F, A: Allocator> + where + F: FnMut(&mut T) -> bool, + { + drain: &'b mut DrainFilter<'a, T, F, A>, + } + + impl<'a, 'b, T, F, A: Allocator> Drop for BackshiftOnDrop<'a, 'b, T, F, A> + where + F: FnMut(&mut T) -> bool, + { + fn drop(&mut self) { + unsafe { + if self.drain.idx < self.drain.old_len && self.drain.del > 0 { + // This is a pretty messed up state, and there isn't really an + // obviously right thing to do. We don't want to keep trying + // to execute `pred`, so we just backshift all the unprocessed + // elements and tell the vec that they still exist. The backshift + // is required to prevent a double-drop of the last successfully + // drained item prior to a panic in the predicate. + let ptr = self.drain.vec.as_mut_ptr(); + let src = ptr.add(self.drain.idx); + let dst = src.sub(self.drain.del); + let tail_len = self.drain.old_len - self.drain.idx; + src.copy_to(dst, tail_len); + } + self.drain.vec.set_len(self.drain.old_len - self.drain.del); + } + } + } + + let backshift = BackshiftOnDrop { drain: self }; + + // Attempt to consume any remaining elements if the filter predicate + // has not yet panicked. We'll backshift any remaining elements + // whether we've already panicked or if the consumption here panics. + if !backshift.drain.panic_flag { + backshift.drain.for_each(drop); + } + } +} |