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| 1 | +// SPDX-License-Identifier: Apache-2.0 OR MIT |
| 2 | + |
| 3 | +use crate::alloc::{Allocator, Global}; |
| 4 | +use core::mem::{ManuallyDrop, SizedTypeProperties}; |
| 5 | +use core::ptr; |
| 6 | +use core::slice; |
| 7 | + |
| 8 | +use super::Vec; |
| 9 | + |
| 10 | +/// An iterator which uses a closure to determine if an element should be removed. |
| 11 | +/// |
| 12 | +/// This struct is created by [`Vec::drain_filter`]. |
| 13 | +/// See its documentation for more. |
| 14 | +/// |
| 15 | +/// # Example |
| 16 | +/// |
| 17 | +/// ``` |
| 18 | +/// #![feature(drain_filter)] |
| 19 | +/// |
| 20 | +/// let mut v = vec![0, 1, 2]; |
| 21 | +/// let iter: std::vec::DrainFilter<'_, _, _> = v.drain_filter(|x| *x % 2 == 0); |
| 22 | +/// ``` |
| 23 | +#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] |
| 24 | +#[derive(Debug)] |
| 25 | +pub struct DrainFilter< |
| 26 | + 'a, |
| 27 | + T, |
| 28 | + F, |
| 29 | + #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global, |
| 30 | +> where |
| 31 | + F: FnMut(&mut T) -> bool, |
| 32 | +{ |
| 33 | + pub(super) vec: &'a mut Vec<T, A>, |
| 34 | + /// The index of the item that will be inspected by the next call to `next`. |
| 35 | + pub(super) idx: usize, |
| 36 | + /// The number of items that have been drained (removed) thus far. |
| 37 | + pub(super) del: usize, |
| 38 | + /// The original length of `vec` prior to draining. |
| 39 | + pub(super) old_len: usize, |
| 40 | + /// The filter test predicate. |
| 41 | + pub(super) pred: F, |
| 42 | + /// A flag that indicates a panic has occurred in the filter test predicate. |
| 43 | + /// This is used as a hint in the drop implementation to prevent consumption |
| 44 | + /// of the remainder of the `DrainFilter`. Any unprocessed items will be |
| 45 | + /// backshifted in the `vec`, but no further items will be dropped or |
| 46 | + /// tested by the filter predicate. |
| 47 | + pub(super) panic_flag: bool, |
| 48 | +} |
| 49 | + |
| 50 | +impl<T, F, A: Allocator> DrainFilter<'_, T, F, A> |
| 51 | +where |
| 52 | + F: FnMut(&mut T) -> bool, |
| 53 | +{ |
| 54 | + /// Returns a reference to the underlying allocator. |
| 55 | + #[unstable(feature = "allocator_api", issue = "32838")] |
| 56 | + #[inline] |
| 57 | + pub fn allocator(&self) -> &A { |
| 58 | + self.vec.allocator() |
| 59 | + } |
| 60 | + |
| 61 | + /// Keep unyielded elements in the source `Vec`. |
| 62 | + /// |
| 63 | + /// # Examples |
| 64 | + /// |
| 65 | + /// ``` |
| 66 | + /// #![feature(drain_filter)] |
| 67 | + /// #![feature(drain_keep_rest)] |
| 68 | + /// |
| 69 | + /// let mut vec = vec!['a', 'b', 'c']; |
| 70 | + /// let mut drain = vec.drain_filter(|_| true); |
| 71 | + /// |
| 72 | + /// assert_eq!(drain.next().unwrap(), 'a'); |
| 73 | + /// |
| 74 | + /// // This call keeps 'b' and 'c' in the vec. |
| 75 | + /// drain.keep_rest(); |
| 76 | + /// |
| 77 | + /// // If we wouldn't call `keep_rest()`, |
| 78 | + /// // `vec` would be empty. |
| 79 | + /// assert_eq!(vec, ['b', 'c']); |
| 80 | + /// ``` |
| 81 | + #[unstable(feature = "drain_keep_rest", issue = "101122")] |
| 82 | + pub fn keep_rest(self) { |
| 83 | + // At this moment layout looks like this: |
| 84 | + // |
| 85 | + // _____________________/-- old_len |
| 86 | + // / \ |
| 87 | + // [kept] [yielded] [tail] |
| 88 | + // \_______/ ^-- idx |
| 89 | + // \-- del |
| 90 | + // |
| 91 | + // Normally `Drop` impl would drop [tail] (via .for_each(drop), ie still calling `pred`) |
| 92 | + // |
| 93 | + // 1. Move [tail] after [kept] |
| 94 | + // 2. Update length of the original vec to `old_len - del` |
| 95 | + // a. In case of ZST, this is the only thing we want to do |
| 96 | + // 3. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do |
| 97 | + let mut this = ManuallyDrop::new(self); |
| 98 | + |
| 99 | + unsafe { |
| 100 | + // ZSTs have no identity, so we don't need to move them around. |
| 101 | + if !T::IS_ZST && this.idx < this.old_len && this.del > 0 { |
| 102 | + let ptr = this.vec.as_mut_ptr(); |
| 103 | + let src = ptr.add(this.idx); |
| 104 | + let dst = src.sub(this.del); |
| 105 | + let tail_len = this.old_len - this.idx; |
| 106 | + src.copy_to(dst, tail_len); |
| 107 | + } |
| 108 | + |
| 109 | + let new_len = this.old_len - this.del; |
| 110 | + this.vec.set_len(new_len); |
| 111 | + } |
| 112 | + } |
| 113 | +} |
| 114 | + |
| 115 | +#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] |
| 116 | +impl<T, F, A: Allocator> Iterator for DrainFilter<'_, T, F, A> |
| 117 | +where |
| 118 | + F: FnMut(&mut T) -> bool, |
| 119 | +{ |
| 120 | + type Item = T; |
| 121 | + |
| 122 | + fn next(&mut self) -> Option<T> { |
| 123 | + unsafe { |
| 124 | + while self.idx < self.old_len { |
| 125 | + let i = self.idx; |
| 126 | + let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len); |
| 127 | + self.panic_flag = true; |
| 128 | + let drained = (self.pred)(&mut v[i]); |
| 129 | + self.panic_flag = false; |
| 130 | + // Update the index *after* the predicate is called. If the index |
| 131 | + // is updated prior and the predicate panics, the element at this |
| 132 | + // index would be leaked. |
| 133 | + self.idx += 1; |
| 134 | + if drained { |
| 135 | + self.del += 1; |
| 136 | + return Some(ptr::read(&v[i])); |
| 137 | + } else if self.del > 0 { |
| 138 | + let del = self.del; |
| 139 | + let src: *const T = &v[i]; |
| 140 | + let dst: *mut T = &mut v[i - del]; |
| 141 | + ptr::copy_nonoverlapping(src, dst, 1); |
| 142 | + } |
| 143 | + } |
| 144 | + None |
| 145 | + } |
| 146 | + } |
| 147 | + |
| 148 | + fn size_hint(&self) -> (usize, Option<usize>) { |
| 149 | + (0, Some(self.old_len - self.idx)) |
| 150 | + } |
| 151 | +} |
| 152 | + |
| 153 | +#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] |
| 154 | +impl<T, F, A: Allocator> Drop for DrainFilter<'_, T, F, A> |
| 155 | +where |
| 156 | + F: FnMut(&mut T) -> bool, |
| 157 | +{ |
| 158 | + fn drop(&mut self) { |
| 159 | + struct BackshiftOnDrop<'a, 'b, T, F, A: Allocator> |
| 160 | + where |
| 161 | + F: FnMut(&mut T) -> bool, |
| 162 | + { |
| 163 | + drain: &'b mut DrainFilter<'a, T, F, A>, |
| 164 | + } |
| 165 | + |
| 166 | + impl<'a, 'b, T, F, A: Allocator> Drop for BackshiftOnDrop<'a, 'b, T, F, A> |
| 167 | + where |
| 168 | + F: FnMut(&mut T) -> bool, |
| 169 | + { |
| 170 | + fn drop(&mut self) { |
| 171 | + unsafe { |
| 172 | + if self.drain.idx < self.drain.old_len && self.drain.del > 0 { |
| 173 | + // This is a pretty messed up state, and there isn't really an |
| 174 | + // obviously right thing to do. We don't want to keep trying |
| 175 | + // to execute `pred`, so we just backshift all the unprocessed |
| 176 | + // elements and tell the vec that they still exist. The backshift |
| 177 | + // is required to prevent a double-drop of the last successfully |
| 178 | + // drained item prior to a panic in the predicate. |
| 179 | + let ptr = self.drain.vec.as_mut_ptr(); |
| 180 | + let src = ptr.add(self.drain.idx); |
| 181 | + let dst = src.sub(self.drain.del); |
| 182 | + let tail_len = self.drain.old_len - self.drain.idx; |
| 183 | + src.copy_to(dst, tail_len); |
| 184 | + } |
| 185 | + self.drain.vec.set_len(self.drain.old_len - self.drain.del); |
| 186 | + } |
| 187 | + } |
| 188 | + } |
| 189 | + |
| 190 | + let backshift = BackshiftOnDrop { drain: self }; |
| 191 | + |
| 192 | + // Attempt to consume any remaining elements if the filter predicate |
| 193 | + // has not yet panicked. We'll backshift any remaining elements |
| 194 | + // whether we've already panicked or if the consumption here panics. |
| 195 | + if !backshift.drain.panic_flag { |
| 196 | + backshift.drain.for_each(drop); |
| 197 | + } |
| 198 | + } |
| 199 | +} |
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