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
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
use crate::iter::{DoubleEndedIterator, FusedIterator, Iterator, TrustedLen};
use crate::num::NonZeroUsize;
use crate::ops::Try;

/// An iterator that links two iterators together, in a chain.
///
/// This `struct` is created by [`Iterator::chain`]. See its documentation
/// for more.
///
/// # Examples
///
/// ```
/// use std::iter::Chain;
/// use std::slice::Iter;
///
/// let a1 = [1, 2, 3];
/// let a2 = [4, 5, 6];
/// let iter: Chain<Iter<'_, _>, Iter<'_, _>> = a1.iter().chain(a2.iter());
/// ```
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Chain<A, B> {
    // These are "fused" with `Option` so we don't need separate state to track which part is
    // already exhausted, and we may also get niche layout for `None`. We don't use the real `Fuse`
    // adapter because its specialization for `FusedIterator` unconditionally descends into the
    // iterator, and that could be expensive to keep revisiting stuff like nested chains. It also
    // hurts compiler performance to add more iterator layers to `Chain`.
    //
    // Only the "first" iterator is actually set `None` when exhausted, depending on whether you
    // iterate forward or backward. If you mix directions, then both sides may be `None`.
    a: Option<A>,
    b: Option<B>,
}
impl<A, B> Chain<A, B> {
    pub(in super::super) fn new(a: A, b: B) -> Chain<A, B> {
        Chain { a: Some(a), b: Some(b) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A, B> Iterator for Chain<A, B>
where
    A: Iterator,
    B: Iterator<Item = A::Item>,
{
    type Item = A::Item;

    #[inline]
    fn next(&mut self) -> Option<A::Item> {
        and_then_or_clear(&mut self.a, Iterator::next).or_else(|| self.b.as_mut()?.next())
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn count(self) -> usize {
        let a_count = match self.a {
            Some(a) => a.count(),
            None => 0,
        };
        let b_count = match self.b {
            Some(b) => b.count(),
            None => 0,
        };
        a_count + b_count
    }

    fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        if let Some(ref mut a) = self.a {
            acc = a.try_fold(acc, &mut f)?;
            self.a = None;
        }
        if let Some(ref mut b) = self.b {
            acc = b.try_fold(acc, f)?;
            // we don't fuse the second iterator
        }
        try { acc }
    }

    fn fold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        if let Some(a) = self.a {
            acc = a.fold(acc, &mut f);
        }
        if let Some(b) = self.b {
            acc = b.fold(acc, f);
        }
        acc
    }

    #[inline]
    fn advance_by(&mut self, mut n: usize) -> Result<(), NonZeroUsize> {
        if let Some(ref mut a) = self.a {
            n = match a.advance_by(n) {
                Ok(()) => return Ok(()),
                Err(k) => k.get(),
            };
            self.a = None;
        }

        if let Some(ref mut b) = self.b {
            return b.advance_by(n);
            // we don't fuse the second iterator
        }

        NonZeroUsize::new(n).map_or(Ok(()), Err)
    }

    #[inline]
    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
        if let Some(ref mut a) = self.a {
            n = match a.advance_by(n) {
                Ok(()) => match a.next() {
                    None => 0,
                    x => return x,
                },
                Err(k) => k.get(),
            };

            self.a = None;
        }

        self.b.as_mut()?.nth(n)
    }

    #[inline]
    fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        and_then_or_clear(&mut self.a, |a| a.find(&mut predicate))
            .or_else(|| self.b.as_mut()?.find(predicate))
    }

    #[inline]
    fn last(self) -> Option<A::Item> {
        // Must exhaust a before b.
        let a_last = self.a.and_then(Iterator::last);
        let b_last = self.b.and_then(Iterator::last);
        b_last.or(a_last)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        match self {
            Chain { a: Some(a), b: Some(b) } => {
                let (a_lower, a_upper) = a.size_hint();
                let (b_lower, b_upper) = b.size_hint();

                let lower = a_lower.saturating_add(b_lower);

                let upper = match (a_upper, b_upper) {
                    (Some(x), Some(y)) => x.checked_add(y),
                    _ => None,
                };

                (lower, upper)
            }
            Chain { a: Some(a), b: None } => a.size_hint(),
            Chain { a: None, b: Some(b) } => b.size_hint(),
            Chain { a: None, b: None } => (0, Some(0)),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A, B> DoubleEndedIterator for Chain<A, B>
where
    A: DoubleEndedIterator,
    B: DoubleEndedIterator<Item = A::Item>,
{
    #[inline]
    fn next_back(&mut self) -> Option<A::Item> {
        and_then_or_clear(&mut self.b, |b| b.next_back()).or_else(|| self.a.as_mut()?.next_back())
    }

    #[inline]
    fn advance_back_by(&mut self, mut n: usize) -> Result<(), NonZeroUsize> {
        if let Some(ref mut b) = self.b {
            n = match b.advance_back_by(n) {
                Ok(()) => return Ok(()),
                Err(k) => k.get(),
            };
            self.b = None;
        }

        if let Some(ref mut a) = self.a {
            return a.advance_back_by(n);
            // we don't fuse the second iterator
        }

        NonZeroUsize::new(n).map_or(Ok(()), Err)
    }

    #[inline]
    fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
        if let Some(ref mut b) = self.b {
            n = match b.advance_back_by(n) {
                Ok(()) => match b.next_back() {
                    None => 0,
                    x => return x,
                },
                Err(k) => k.get(),
            };

            self.b = None;
        }

        self.a.as_mut()?.nth_back(n)
    }

    #[inline]
    fn rfind<P>(&mut self, mut predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        and_then_or_clear(&mut self.b, |b| b.rfind(&mut predicate))
            .or_else(|| self.a.as_mut()?.rfind(predicate))
    }

    fn try_rfold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        if let Some(ref mut b) = self.b {
            acc = b.try_rfold(acc, &mut f)?;
            self.b = None;
        }
        if let Some(ref mut a) = self.a {
            acc = a.try_rfold(acc, f)?;
            // we don't fuse the second iterator
        }
        try { acc }
    }

    fn rfold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        if let Some(b) = self.b {
            acc = b.rfold(acc, &mut f);
        }
        if let Some(a) = self.a {
            acc = a.rfold(acc, f);
        }
        acc
    }
}

// Note: *both* must be fused to handle double-ended iterators.
#[stable(feature = "fused", since = "1.26.0")]
impl<A, B> FusedIterator for Chain<A, B>
where
    A: FusedIterator,
    B: FusedIterator<Item = A::Item>,
{
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A, B> TrustedLen for Chain<A, B>
where
    A: TrustedLen,
    B: TrustedLen<Item = A::Item>,
{
}

#[stable(feature = "default_iters", since = "1.70.0")]
impl<A: Default, B: Default> Default for Chain<A, B> {
    /// Creates a `Chain` from the default values for `A` and `B`.
    ///
    /// ```
    /// # use core::iter::Chain;
    /// # use core::slice;
    /// # use std::collections::{btree_set, BTreeSet};
    /// # use std::mem;
    /// struct Foo<'a>(Chain<slice::Iter<'a, u8>, btree_set::Iter<'a, u8>>);
    ///
    /// let set = BTreeSet::<u8>::new();
    /// let slice: &[u8] = &[];
    /// let mut foo = Foo(slice.iter().chain(set.iter()));
    ///
    /// // take requires `Default`
    /// let _: Chain<_, _> = mem::take(&mut foo.0);
    fn default() -> Self {
        Chain::new(Default::default(), Default::default())
    }
}

#[inline]
fn and_then_or_clear<T, U>(opt: &mut Option<T>, f: impl FnOnce(&mut T) -> Option<U>) -> Option<U> {
    let x = f(opt.as_mut()?);
    if x.is_none() {
        *opt = None;
    }
    x
}
This documentation is an old archive. Please see https://rust.docs.kernel.org instead.