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

use crate::alloc::{Allocator, Global};
use core::ptr::{self};
use core::slice::{self};

use super::{Drain, Vec};

/// A splicing iterator for `Vec`.
///
/// This struct is created by [`Vec::splice()`].
/// See its documentation for more.
///
/// # Example
///
/// ```
/// let mut v = vec![0, 1, 2];
/// let new = [7, 8];
/// let iter: std::vec::Splice<_> = v.splice(1.., new);
/// ```
#[derive(Debug)]
#[stable(feature = "vec_splice", since = "1.21.0")]
pub struct Splice<
    'a,
    I: Iterator + 'a,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + 'a = Global,
> {
    pub(super) drain: Drain<'a, I::Item, A>,
    pub(super) replace_with: I,
}

#[stable(feature = "vec_splice", since = "1.21.0")]
impl<I: Iterator, A: Allocator> Iterator for Splice<'_, I, A> {
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        self.drain.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.drain.size_hint()
    }
}

#[stable(feature = "vec_splice", since = "1.21.0")]
impl<I: Iterator, A: Allocator> DoubleEndedIterator for Splice<'_, I, A> {
    fn next_back(&mut self) -> Option<Self::Item> {
        self.drain.next_back()
    }
}

#[stable(feature = "vec_splice", since = "1.21.0")]
impl<I: Iterator, A: Allocator> ExactSizeIterator for Splice<'_, I, A> {}

#[stable(feature = "vec_splice", since = "1.21.0")]
impl<I: Iterator, A: Allocator> Drop for Splice<'_, I, A> {
    fn drop(&mut self) {
        self.drain.by_ref().for_each(drop);

        unsafe {
            if self.drain.tail_len == 0 {
                self.drain.vec.as_mut().extend(self.replace_with.by_ref());
                return;
            }

            // First fill the range left by drain().
            if !self.drain.fill(&mut self.replace_with) {
                return;
            }

            // There may be more elements. Use the lower bound as an estimate.
            // FIXME: Is the upper bound a better guess? Or something else?
            let (lower_bound, _upper_bound) = self.replace_with.size_hint();
            if lower_bound > 0 {
                self.drain.move_tail(lower_bound);
                if !self.drain.fill(&mut self.replace_with) {
                    return;
                }
            }

            // Collect any remaining elements.
            // This is a zero-length vector which does not allocate if `lower_bound` was exact.
            let mut collected = self.replace_with.by_ref().collect::<Vec<I::Item>>().into_iter();
            // Now we have an exact count.
            if collected.len() > 0 {
                self.drain.move_tail(collected.len());
                let filled = self.drain.fill(&mut collected);
                debug_assert!(filled);
                debug_assert_eq!(collected.len(), 0);
            }
        }
        // Let `Drain::drop` move the tail back if necessary and restore `vec.len`.
    }
}

/// Private helper methods for `Splice::drop`
impl<T, A: Allocator> Drain<'_, T, A> {
    /// The range from `self.vec.len` to `self.tail_start` contains elements
    /// that have been moved out.
    /// Fill that range as much as possible with new elements from the `replace_with` iterator.
    /// Returns `true` if we filled the entire range. (`replace_with.next()` didn’t return `None`.)
    unsafe fn fill<I: Iterator<Item = T>>(&mut self, replace_with: &mut I) -> bool {
        let vec = unsafe { self.vec.as_mut() };
        let range_start = vec.len;
        let range_end = self.tail_start;
        let range_slice = unsafe {
            slice::from_raw_parts_mut(vec.as_mut_ptr().add(range_start), range_end - range_start)
        };

        for place in range_slice {
            if let Some(new_item) = replace_with.next() {
                unsafe { ptr::write(place, new_item) };
                vec.len += 1;
            } else {
                return false;
            }
        }
        true
    }

    /// Makes room for inserting more elements before the tail.
    unsafe fn move_tail(&mut self, additional: usize) {
        let vec = unsafe { self.vec.as_mut() };
        let len = self.tail_start + self.tail_len;
        vec.buf.reserve(len, additional);

        let new_tail_start = self.tail_start + additional;
        unsafe {
            let src = vec.as_ptr().add(self.tail_start);
            let dst = vec.as_mut_ptr().add(new_tail_start);
            ptr::copy(src, dst, self.tail_len);
        }
        self.tail_start = new_tail_start;
    }
}