core/iter/adapters/
chain.rs

1use crate::iter::{FusedIterator, TrustedLen};
2use crate::num::NonZero;
3use crate::ops::Try;
4
5/// An iterator that links two iterators together, in a chain.
6///
7/// This `struct` is created by [`chain`] or [`Iterator::chain`]. See their
8/// documentation for more.
9///
10/// # Examples
11///
12/// ```
13/// use std::iter::Chain;
14/// use std::slice::Iter;
15///
16/// let a1 = [1, 2, 3];
17/// let a2 = [4, 5, 6];
18/// let iter: Chain<Iter<'_, _>, Iter<'_, _>> = a1.iter().chain(a2.iter());
19/// ```
20#[derive(Clone, Debug)]
21#[must_use = "iterators are lazy and do nothing unless consumed"]
22#[stable(feature = "rust1", since = "1.0.0")]
23pub struct Chain<A, B> {
24    // These are "fused" with `Option` so we don't need separate state to track which part is
25    // already exhausted, and we may also get niche layout for `None`. We don't use the real `Fuse`
26    // adapter because its specialization for `FusedIterator` unconditionally descends into the
27    // iterator, and that could be expensive to keep revisiting stuff like nested chains. It also
28    // hurts compiler performance to add more iterator layers to `Chain`.
29    //
30    // Only the "first" iterator is actually set `None` when exhausted, depending on whether you
31    // iterate forward or backward. If you mix directions, then both sides may be `None`.
32    a: Option<A>,
33    b: Option<B>,
34}
35impl<A, B> Chain<A, B> {
36    pub(in super::super) fn new(a: A, b: B) -> Chain<A, B> {
37        Chain { a: Some(a), b: Some(b) }
38    }
39}
40
41/// Converts the arguments to iterators and links them together, in a chain.
42///
43/// See the documentation of [`Iterator::chain`] for more.
44///
45/// # Examples
46///
47/// ```
48/// use std::iter::chain;
49///
50/// let a = [1, 2, 3];
51/// let b = [4, 5, 6];
52///
53/// let mut iter = chain(a, b);
54///
55/// assert_eq!(iter.next(), Some(1));
56/// assert_eq!(iter.next(), Some(2));
57/// assert_eq!(iter.next(), Some(3));
58/// assert_eq!(iter.next(), Some(4));
59/// assert_eq!(iter.next(), Some(5));
60/// assert_eq!(iter.next(), Some(6));
61/// assert_eq!(iter.next(), None);
62/// ```
63#[stable(feature = "iter_chain", since = "CURRENT_RUSTC_VERSION")]
64pub fn chain<A, B>(a: A, b: B) -> Chain<A::IntoIter, B::IntoIter>
65where
66    A: IntoIterator,
67    B: IntoIterator<Item = A::Item>,
68{
69    Chain::new(a.into_iter(), b.into_iter())
70}
71
72#[stable(feature = "rust1", since = "1.0.0")]
73impl<A, B> Iterator for Chain<A, B>
74where
75    A: Iterator,
76    B: Iterator<Item = A::Item>,
77{
78    type Item = A::Item;
79
80    #[inline]
81    fn next(&mut self) -> Option<A::Item> {
82        and_then_or_clear(&mut self.a, Iterator::next).or_else(|| self.b.as_mut()?.next())
83    }
84
85    #[inline]
86    #[rustc_inherit_overflow_checks]
87    fn count(self) -> usize {
88        let a_count = match self.a {
89            Some(a) => a.count(),
90            None => 0,
91        };
92        let b_count = match self.b {
93            Some(b) => b.count(),
94            None => 0,
95        };
96        a_count + b_count
97    }
98
99    fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
100    where
101        Self: Sized,
102        F: FnMut(Acc, Self::Item) -> R,
103        R: Try<Output = Acc>,
104    {
105        if let Some(ref mut a) = self.a {
106            acc = a.try_fold(acc, &mut f)?;
107            self.a = None;
108        }
109        if let Some(ref mut b) = self.b {
110            acc = b.try_fold(acc, f)?;
111            // we don't fuse the second iterator
112        }
113        try { acc }
114    }
115
116    fn fold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
117    where
118        F: FnMut(Acc, Self::Item) -> Acc,
119    {
120        if let Some(a) = self.a {
121            acc = a.fold(acc, &mut f);
122        }
123        if let Some(b) = self.b {
124            acc = b.fold(acc, f);
125        }
126        acc
127    }
128
129    #[inline]
130    fn advance_by(&mut self, mut n: usize) -> Result<(), NonZero<usize>> {
131        if let Some(ref mut a) = self.a {
132            n = match a.advance_by(n) {
133                Ok(()) => return Ok(()),
134                Err(k) => k.get(),
135            };
136            self.a = None;
137        }
138
139        if let Some(ref mut b) = self.b {
140            return b.advance_by(n);
141            // we don't fuse the second iterator
142        }
143
144        NonZero::new(n).map_or(Ok(()), Err)
145    }
146
147    #[inline]
148    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
149        if let Some(ref mut a) = self.a {
150            n = match a.advance_by(n) {
151                Ok(()) => match a.next() {
152                    None => 0,
153                    x => return x,
154                },
155                Err(k) => k.get(),
156            };
157
158            self.a = None;
159        }
160
161        self.b.as_mut()?.nth(n)
162    }
163
164    #[inline]
165    fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
166    where
167        P: FnMut(&Self::Item) -> bool,
168    {
169        and_then_or_clear(&mut self.a, |a| a.find(&mut predicate))
170            .or_else(|| self.b.as_mut()?.find(predicate))
171    }
172
173    #[inline]
174    fn last(self) -> Option<A::Item> {
175        // Must exhaust a before b.
176        let a_last = self.a.and_then(Iterator::last);
177        let b_last = self.b.and_then(Iterator::last);
178        b_last.or(a_last)
179    }
180
181    #[inline]
182    fn size_hint(&self) -> (usize, Option<usize>) {
183        match self {
184            Chain { a: Some(a), b: Some(b) } => {
185                let (a_lower, a_upper) = a.size_hint();
186                let (b_lower, b_upper) = b.size_hint();
187
188                let lower = a_lower.saturating_add(b_lower);
189
190                let upper = match (a_upper, b_upper) {
191                    (Some(x), Some(y)) => x.checked_add(y),
192                    _ => None,
193                };
194
195                (lower, upper)
196            }
197            Chain { a: Some(a), b: None } => a.size_hint(),
198            Chain { a: None, b: Some(b) } => b.size_hint(),
199            Chain { a: None, b: None } => (0, Some(0)),
200        }
201    }
202}
203
204#[stable(feature = "rust1", since = "1.0.0")]
205impl<A, B> DoubleEndedIterator for Chain<A, B>
206where
207    A: DoubleEndedIterator,
208    B: DoubleEndedIterator<Item = A::Item>,
209{
210    #[inline]
211    fn next_back(&mut self) -> Option<A::Item> {
212        and_then_or_clear(&mut self.b, |b| b.next_back()).or_else(|| self.a.as_mut()?.next_back())
213    }
214
215    #[inline]
216    fn advance_back_by(&mut self, mut n: usize) -> Result<(), NonZero<usize>> {
217        if let Some(ref mut b) = self.b {
218            n = match b.advance_back_by(n) {
219                Ok(()) => return Ok(()),
220                Err(k) => k.get(),
221            };
222            self.b = None;
223        }
224
225        if let Some(ref mut a) = self.a {
226            return a.advance_back_by(n);
227            // we don't fuse the second iterator
228        }
229
230        NonZero::new(n).map_or(Ok(()), Err)
231    }
232
233    #[inline]
234    fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
235        if let Some(ref mut b) = self.b {
236            n = match b.advance_back_by(n) {
237                Ok(()) => match b.next_back() {
238                    None => 0,
239                    x => return x,
240                },
241                Err(k) => k.get(),
242            };
243
244            self.b = None;
245        }
246
247        self.a.as_mut()?.nth_back(n)
248    }
249
250    #[inline]
251    fn rfind<P>(&mut self, mut predicate: P) -> Option<Self::Item>
252    where
253        P: FnMut(&Self::Item) -> bool,
254    {
255        and_then_or_clear(&mut self.b, |b| b.rfind(&mut predicate))
256            .or_else(|| self.a.as_mut()?.rfind(predicate))
257    }
258
259    fn try_rfold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
260    where
261        Self: Sized,
262        F: FnMut(Acc, Self::Item) -> R,
263        R: Try<Output = Acc>,
264    {
265        if let Some(ref mut b) = self.b {
266            acc = b.try_rfold(acc, &mut f)?;
267            self.b = None;
268        }
269        if let Some(ref mut a) = self.a {
270            acc = a.try_rfold(acc, f)?;
271            // we don't fuse the second iterator
272        }
273        try { acc }
274    }
275
276    fn rfold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
277    where
278        F: FnMut(Acc, Self::Item) -> Acc,
279    {
280        if let Some(b) = self.b {
281            acc = b.rfold(acc, &mut f);
282        }
283        if let Some(a) = self.a {
284            acc = a.rfold(acc, f);
285        }
286        acc
287    }
288}
289
290// Note: *both* must be fused to handle double-ended iterators.
291#[stable(feature = "fused", since = "1.26.0")]
292impl<A, B> FusedIterator for Chain<A, B>
293where
294    A: FusedIterator,
295    B: FusedIterator<Item = A::Item>,
296{
297}
298
299#[unstable(feature = "trusted_len", issue = "37572")]
300unsafe impl<A, B> TrustedLen for Chain<A, B>
301where
302    A: TrustedLen,
303    B: TrustedLen<Item = A::Item>,
304{
305}
306
307#[stable(feature = "default_iters", since = "1.70.0")]
308impl<A: Default, B: Default> Default for Chain<A, B> {
309    /// Creates a `Chain` from the default values for `A` and `B`.
310    ///
311    /// ```
312    /// # use core::iter::Chain;
313    /// # use core::slice;
314    /// # use std::collections::{btree_set, BTreeSet};
315    /// # use std::mem;
316    /// struct Foo<'a>(Chain<slice::Iter<'a, u8>, btree_set::Iter<'a, u8>>);
317    ///
318    /// let set = BTreeSet::<u8>::new();
319    /// let slice: &[u8] = &[];
320    /// let mut foo = Foo(slice.iter().chain(set.iter()));
321    ///
322    /// // take requires `Default`
323    /// let _: Chain<_, _> = mem::take(&mut foo.0);
324    /// ```
325    fn default() -> Self {
326        Chain::new(Default::default(), Default::default())
327    }
328}
329
330#[inline]
331fn and_then_or_clear<T, U>(opt: &mut Option<T>, f: impl FnOnce(&mut T) -> Option<U>) -> Option<U> {
332    let x = f(opt.as_mut()?);
333    if x.is_none() {
334        *opt = None;
335    }
336    x
337}
core/iter/adapters/chain.rs

core/iter/adapters/
chain.rs
