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
use crate::size_hint;
use std::{
    fmt,
    iter::{DoubleEndedIterator, FusedIterator},
};

pub fn flatten_ok<I, T, E>(iter: I) -> FlattenOk<I, T, E>
where
    I: Iterator<Item = Result<T, E>>,
    T: IntoIterator,
{
    FlattenOk {
        iter,
        inner_front: None,
        inner_back: None,
    }
}

/// An iterator adaptor that flattens `Result::Ok` values and
/// allows `Result::Err` values through unchanged.
///
/// See [`.flatten_ok()`](crate::Itertools::flatten_ok) for more information.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct FlattenOk<I, T, E>
where
    I: Iterator<Item = Result<T, E>>,
    T: IntoIterator,
{
    iter: I,
    inner_front: Option<T::IntoIter>,
    inner_back: Option<T::IntoIter>,
}

impl<I, T, E> Iterator for FlattenOk<I, T, E>
where
    I: Iterator<Item = Result<T, E>>,
    T: IntoIterator,
{
    type Item = Result<T::Item, E>;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            // Handle the front inner iterator.
            if let Some(inner) = &mut self.inner_front {
                if let Some(item) = inner.next() {
                    return Some(Ok(item));
                } else {
                    // This is necessary for the iterator to implement `FusedIterator`
                    // with only the orginal iterator being fused.
                    self.inner_front = None;
                }
            }

            match self.iter.next() {
                Some(Ok(ok)) => self.inner_front = Some(ok.into_iter()),
                Some(Err(e)) => return Some(Err(e)),
                None => {
                    // Handle the back inner iterator.
                    if let Some(inner) = &mut self.inner_back {
                        if let Some(item) = inner.next() {
                            return Some(Ok(item));
                        } else {
                            // This is necessary for the iterator to implement `FusedIterator`
                            // with only the orginal iterator being fused.
                            self.inner_back = None;
                        }
                    } else {
                        return None;
                    }
                }
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let inner_hint = |inner: &Option<T::IntoIter>| {
            inner
                .as_ref()
                .map(Iterator::size_hint)
                .unwrap_or((0, Some(0)))
        };
        let inner_front = inner_hint(&self.inner_front);
        let inner_back = inner_hint(&self.inner_back);
        // The outer iterator `Ok` case could be (0, None) as we don't know its size_hint yet.
        let outer = match self.iter.size_hint() {
            (0, Some(0)) => (0, Some(0)),
            _ => (0, None),
        };

        size_hint::add(size_hint::add(inner_front, inner_back), outer)
    }
}

impl<I, T, E> DoubleEndedIterator for FlattenOk<I, T, E>
where
    I: DoubleEndedIterator<Item = Result<T, E>>,
    T: IntoIterator,
    T::IntoIter: DoubleEndedIterator,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        loop {
            // Handle the back inner iterator.
            if let Some(inner) = &mut self.inner_back {
                if let Some(item) = inner.next_back() {
                    return Some(Ok(item));
                } else {
                    // This is necessary for the iterator to implement `FusedIterator`
                    // with only the orginal iterator being fused.
                    self.inner_back = None;
                }
            }

            match self.iter.next_back() {
                Some(Ok(ok)) => self.inner_back = Some(ok.into_iter()),
                Some(Err(e)) => return Some(Err(e)),
                None => {
                    // Handle the front inner iterator.
                    if let Some(inner) = &mut self.inner_front {
                        if let Some(item) = inner.next_back() {
                            return Some(Ok(item));
                        } else {
                            // This is necessary for the iterator to implement `FusedIterator`
                            // with only the orginal iterator being fused.
                            self.inner_front = None;
                        }
                    } else {
                        return None;
                    }
                }
            }
        }
    }
}

impl<I, T, E> Clone for FlattenOk<I, T, E>
where
    I: Iterator<Item = Result<T, E>> + Clone,
    T: IntoIterator,
    T::IntoIter: Clone,
{
    #[inline]
    clone_fields!(iter, inner_front, inner_back);
}

impl<I, T, E> fmt::Debug for FlattenOk<I, T, E>
where
    I: Iterator<Item = Result<T, E>> + fmt::Debug,
    T: IntoIterator,
    T::IntoIter: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FlattenOk")
            .field("iter", &self.iter)
            .field("inner_front", &self.inner_front)
            .field("inner_back", &self.inner_back)
            .finish()
    }
}

/// Only the iterator being flattened needs to implement [`FusedIterator`].
impl<I, T, E> FusedIterator for FlattenOk<I, T, E>
where
    I: FusedIterator<Item = Result<T, E>>,
    T: IntoIterator,
{
}