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}