1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3use crate::alloc::{Allocator, Global};
4use core::ptr;
5use core::slice;
6
7use super::Vec;
8
9/// An iterator which uses a closure to determine if an element should be removed.
10///
11/// This struct is created by [`Vec::extract_if`].
12/// See its documentation for more.
13///
14/// # Example
15///
16/// ```
17/// #![feature(extract_if)]
18///
19/// let mut v = vec![0, 1, 2];
20/// let iter: std::vec::ExtractIf<'_, _, _> = v.extract_if(|x| *x % 2 == 0);
21/// ```
22#[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
23#[derive(Debug)]
24#[must_use = "iterators are lazy and do nothing unless consumed"]
25pub struct ExtractIf<
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}
43
44impl<T, F, A: Allocator> ExtractIf<'_, T, F, A>
45where
46    F: FnMut(&mut T) -> bool,
47{
48    /// Returns a reference to the underlying allocator.
49    #[unstable(feature = "allocator_api", issue = "32838")]
50    #[inline]
51    pub fn allocator(&self) -> &A {
52        self.vec.allocator()
53    }
54}
55
56#[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
57impl<T, F, A: Allocator> Iterator for ExtractIf<'_, T, F, A>
58where
59    F: FnMut(&mut T) -> bool,
60{
61    type Item = T;
62
63    fn next(&mut self) -> Option<T> {
64        unsafe {
65            while self.idx < self.old_len {
66                let i = self.idx;
67                let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
68                let drained = (self.pred)(&mut v[i]);
69                // Update the index *after* the predicate is called. If the index
70                // is updated prior and the predicate panics, the element at this
71                // index would be leaked.
72                self.idx += 1;
73                if drained {
74                    self.del += 1;
75                    return Some(ptr::read(&v[i]));
76                } else if self.del > 0 {
77                    let del = self.del;
78                    let src: *const T = &v[i];
79                    let dst: *mut T = &mut v[i - del];
80                    ptr::copy_nonoverlapping(src, dst, 1);
81                }
82            }
83            None
84        }
85    }
86
87    fn size_hint(&self) -> (usize, Option<usize>) {
88        (0, Some(self.old_len - self.idx))
89    }
90}
91
92#[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
93impl<T, F, A: Allocator> Drop for ExtractIf<'_, T, F, A>
94where
95    F: FnMut(&mut T) -> bool,
96{
97    fn drop(&mut self) {
98        unsafe {
99            if self.idx < self.old_len && self.del > 0 {
100                // This is a pretty messed up state, and there isn't really an
101                // obviously right thing to do. We don't want to keep trying
102                // to execute `pred`, so we just backshift all the unprocessed
103                // elements and tell the vec that they still exist. The backshift
104                // is required to prevent a double-drop of the last successfully
105                // drained item prior to a panic in the predicate.
106                let ptr = self.vec.as_mut_ptr();
107                let src = ptr.add(self.idx);
108                let dst = src.sub(self.del);
109                let tail_len = self.old_len - self.idx;
110                src.copy_to(dst, tail_len);
111            }
112            self.vec.set_len(self.old_len - self.del);
113        }
114    }
115}
116