1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
use core::iter::FusedIterator;
use core::marker::PhantomData;
use core::mem::{self, SizedTypeProperties};
use core::ptr::NonNull;
use core::{fmt, ptr};

use crate::alloc::{Allocator, Global};

use super::VecDeque;

/// A draining iterator over the elements of a `VecDeque`.
///
/// This `struct` is created by the [`drain`] method on [`VecDeque`]. See its
/// documentation for more.
///
/// [`drain`]: VecDeque::drain
#[stable(feature = "drain", since = "1.6.0")]
pub struct Drain<
    'a,
    T: 'a,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
> {
    // We can't just use a &mut VecDeque<T, A>, as that would make Drain invariant over T
    // and we want it to be covariant instead
    deque: NonNull<VecDeque<T, A>>,
    // drain_start is stored in deque.len
    drain_len: usize,
    // index into the logical array, not the physical one (always lies in [0..deque.len))
    idx: usize,
    // number of elements after the drain range
    tail_len: usize,
    remaining: usize,
    // Needed to make Drain covariant over T
    _marker: PhantomData<&'a T>,
}

impl<'a, T, A: Allocator> Drain<'a, T, A> {
    pub(super) unsafe fn new(
        deque: &'a mut VecDeque<T, A>,
        drain_start: usize,
        drain_len: usize,
    ) -> Self {
        let orig_len = mem::replace(&mut deque.len, drain_start);
        let tail_len = orig_len - drain_start - drain_len;
        Drain {
            deque: NonNull::from(deque),
            drain_len,
            idx: drain_start,
            tail_len,
            remaining: drain_len,
            _marker: PhantomData,
        }
    }

    // Only returns pointers to the slices, as that's
    // all we need to drop them. May only be called if `self.remaining != 0`.
    unsafe fn as_slices(&self) -> (*mut [T], *mut [T]) {
        unsafe {
            let deque = self.deque.as_ref();
            // FIXME: This is doing almost exactly the same thing as the else branch in `VecDeque::slice_ranges`.
            // Unfortunately, we can't just call `slice_ranges` here, as the deque's `len` is currently
            // just `drain_start`, so the range check would (almost) always panic. Between temporarily
            // adjusting the deques `len` to call `slice_ranges`, and just copy pasting the `slice_ranges`
            // implementation, this seemed like the less hacky solution, though it might be good to
            // find a better one in the future.

            // because `self.remaining != 0`, we know that `self.idx < deque.original_len`, so it's a valid
            // logical index.
            let wrapped_start = deque.to_physical_idx(self.idx);

            let head_len = deque.capacity() - wrapped_start;

            let (a_range, b_range) = if head_len >= self.remaining {
                (wrapped_start..wrapped_start + self.remaining, 0..0)
            } else {
                let tail_len = self.remaining - head_len;
                (wrapped_start..deque.capacity(), 0..tail_len)
            };

            // SAFETY: the range `self.idx..self.idx+self.remaining` lies strictly inside
            // the range `0..deque.original_len`. because of this, and because of the fact
            // that we acquire `a_range` and `b_range` exactly like `slice_ranges` would,
            // it's guaranteed that `a_range` and `b_range` represent valid ranges into
            // the deques buffer.
            (deque.buffer_range(a_range), deque.buffer_range(b_range))
        }
    }
}

#[stable(feature = "collection_debug", since = "1.17.0")]
impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("Drain")
            .field(&self.drain_len)
            .field(&self.idx)
            .field(&self.tail_len)
            .field(&self.remaining)
            .finish()
    }
}

#[stable(feature = "drain", since = "1.6.0")]
unsafe impl<T: Sync, A: Allocator + Sync> Sync for Drain<'_, T, A> {}
#[stable(feature = "drain", since = "1.6.0")]
unsafe impl<T: Send, A: Allocator + Send> Send for Drain<'_, T, A> {}

#[stable(feature = "drain", since = "1.6.0")]
impl<T, A: Allocator> Drop for Drain<'_, T, A> {
    fn drop(&mut self) {
        struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);

        impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
            fn drop(&mut self) {
                if self.0.remaining != 0 {
                    unsafe {
                        // SAFETY: We just checked that `self.remaining != 0`.
                        let (front, back) = self.0.as_slices();
                        ptr::drop_in_place(front);
                        ptr::drop_in_place(back);
                    }
                }

                let source_deque = unsafe { self.0.deque.as_mut() };

                let drain_start = source_deque.len();
                let drain_len = self.0.drain_len;
                let drain_end = drain_start + drain_len;

                let orig_len = self.0.tail_len + drain_end;

                if T::IS_ZST {
                    // no need to copy around any memory if T is a ZST
                    source_deque.len = orig_len - drain_len;
                    return;
                }

                let head_len = drain_start;
                let tail_len = self.0.tail_len;

                match (head_len, tail_len) {
                    (0, 0) => {
                        source_deque.head = 0;
                        source_deque.len = 0;
                    }
                    (0, _) => {
                        source_deque.head = source_deque.to_physical_idx(drain_len);
                        source_deque.len = orig_len - drain_len;
                    }
                    (_, 0) => {
                        source_deque.len = orig_len - drain_len;
                    }
                    _ => unsafe {
                        if head_len <= tail_len {
                            source_deque.wrap_copy(
                                source_deque.head,
                                source_deque.to_physical_idx(drain_len),
                                head_len,
                            );
                            source_deque.head = source_deque.to_physical_idx(drain_len);
                            source_deque.len = orig_len - drain_len;
                        } else {
                            source_deque.wrap_copy(
                                source_deque.to_physical_idx(head_len + drain_len),
                                source_deque.to_physical_idx(head_len),
                                tail_len,
                            );
                            source_deque.len = orig_len - drain_len;
                        }
                    },
                }
            }
        }

        let guard = DropGuard(self);
        if guard.0.remaining != 0 {
            unsafe {
                // SAFETY: We just checked that `self.remaining != 0`.
                let (front, back) = guard.0.as_slices();
                // since idx is a logical index, we don't need to worry about wrapping.
                guard.0.idx += front.len();
                guard.0.remaining -= front.len();
                ptr::drop_in_place(front);
                guard.0.remaining = 0;
                ptr::drop_in_place(back);
            }
        }

        // Dropping `guard` handles moving the remaining elements into place.
    }
}

#[stable(feature = "drain", since = "1.6.0")]
impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<T> {
        if self.remaining == 0 {
            return None;
        }
        let wrapped_idx = unsafe { self.deque.as_ref().to_physical_idx(self.idx) };
        self.idx += 1;
        self.remaining -= 1;
        Some(unsafe { self.deque.as_mut().buffer_read(wrapped_idx) })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.remaining;
        (len, Some(len))
    }
}

#[stable(feature = "drain", since = "1.6.0")]
impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
    #[inline]
    fn next_back(&mut self) -> Option<T> {
        if self.remaining == 0 {
            return None;
        }
        self.remaining -= 1;
        let wrapped_idx = unsafe { self.deque.as_ref().to_physical_idx(self.idx + self.remaining) };
        Some(unsafe { self.deque.as_mut().buffer_read(wrapped_idx) })
    }
}

#[stable(feature = "drain", since = "1.6.0")]
impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}