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

use super::Peekable;

/// An iterator adapter that places a separator between all elements.
///
/// This `struct` is created by [`Iterator::intersperse`]. See its documentation
/// for more information.
#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
#[derive(Debug, Clone)]
pub struct Intersperse<I: Iterator>
where
    I::Item: Clone,
{
    separator: I::Item,
    iter: Peekable<I>,
    needs_sep: bool,
}

impl<I: Iterator> Intersperse<I>
where
    I::Item: Clone,
{
    pub(in crate::iter) fn new(iter: I, separator: I::Item) -> Self {
        Self { iter: iter.peekable(), separator, needs_sep: false }
    }
}

#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
impl<I> Iterator for Intersperse<I>
where
    I: Iterator,
    I::Item: Clone,
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.needs_sep && self.iter.peek().is_some() {
            self.needs_sep = false;
            Some(self.separator.clone())
        } else {
            self.needs_sep = true;
            self.iter.next()
        }
    }

    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        let separator = self.separator;
        intersperse_fold(self.iter, init, f, move || separator.clone(), self.needs_sep)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        intersperse_size_hint(&self.iter, self.needs_sep)
    }
}

/// An iterator adapter that places a separator between all elements.
///
/// This `struct` is created by [`Iterator::intersperse_with`]. See its
/// documentation for more information.
#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
pub struct IntersperseWith<I, G>
where
    I: Iterator,
{
    separator: G,
    iter: Peekable<I>,
    needs_sep: bool,
}

#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
impl<I, G> crate::fmt::Debug for IntersperseWith<I, G>
where
    I: Iterator + crate::fmt::Debug,
    I::Item: crate::fmt::Debug,
    G: crate::fmt::Debug,
{
    fn fmt(&self, f: &mut crate::fmt::Formatter<'_>) -> crate::fmt::Result {
        f.debug_struct("IntersperseWith")
            .field("separator", &self.separator)
            .field("iter", &self.iter)
            .field("needs_sep", &self.needs_sep)
            .finish()
    }
}

#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
impl<I, G> crate::clone::Clone for IntersperseWith<I, G>
where
    I: Iterator + crate::clone::Clone,
    I::Item: crate::clone::Clone,
    G: Clone,
{
    fn clone(&self) -> Self {
        IntersperseWith {
            separator: self.separator.clone(),
            iter: self.iter.clone(),
            needs_sep: self.needs_sep.clone(),
        }
    }
}

impl<I, G> IntersperseWith<I, G>
where
    I: Iterator,
    G: FnMut() -> I::Item,
{
    pub(in crate::iter) fn new(iter: I, separator: G) -> Self {
        Self { iter: iter.peekable(), separator, needs_sep: false }
    }
}

#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
impl<I, G> Iterator for IntersperseWith<I, G>
where
    I: Iterator,
    G: FnMut() -> I::Item,
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.needs_sep && self.iter.peek().is_some() {
            self.needs_sep = false;
            Some((self.separator)())
        } else {
            self.needs_sep = true;
            self.iter.next()
        }
    }

    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        intersperse_fold(self.iter, init, f, self.separator, self.needs_sep)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        intersperse_size_hint(&self.iter, self.needs_sep)
    }
}

fn intersperse_size_hint<I>(iter: &I, needs_sep: bool) -> (usize, Option<usize>)
where
    I: Iterator,
{
    let (lo, hi) = iter.size_hint();
    let next_is_elem = !needs_sep;
    (
        lo.saturating_sub(next_is_elem as usize).saturating_add(lo),
        hi.and_then(|hi| hi.saturating_sub(next_is_elem as usize).checked_add(hi)),
    )
}

fn intersperse_fold<I, B, F, G>(
    mut iter: I,
    init: B,
    mut f: F,
    mut separator: G,
    needs_sep: bool,
) -> B
where
    I: Iterator,
    F: FnMut(B, I::Item) -> B,
    G: FnMut() -> I::Item,
{
    let mut accum = init;

    if !needs_sep {
        if let Some(x) = iter.next() {
            accum = f(accum, x);
        } else {
            return accum;
        }
    }

    iter.fold(accum, |mut accum, x| {
        accum = f(accum, separator());
        accum = f(accum, x);
        accum
    })
}