1 use crate::iter::{DoubleEndedIterator, FusedIterator, Iterator, TrustedLen};
2 use crate::{ops::Try, usize};
4 /// An iterator that links two iterators together, in a chain.
6 /// This `struct` is created by [`Iterator::chain`]. See its documentation
12 /// use std::iter::Chain;
13 /// use std::slice::Iter;
15 /// let a1 = [1, 2, 3];
16 /// let a2 = [4, 5, 6];
17 /// let iter: Chain<Iter<_>, Iter<_>> = a1.iter().chain(a2.iter());
19 #[derive(Clone, Debug)]
20 #[must_use = "iterators are lazy and do nothing unless consumed"]
21 #[stable(feature = "rust1", since = "1.0.0")]
22 pub struct Chain<A, B> {
23 // These are "fused" with `Option` so we don't need separate state to track which part is
24 // already exhausted, and we may also get niche layout for `None`. We don't use the real `Fuse`
25 // adapter because its specialization for `FusedIterator` unconditionally descends into the
26 // iterator, and that could be expensive to keep revisiting stuff like nested chains. It also
27 // hurts compiler performance to add more iterator layers to `Chain`.
29 // Only the "first" iterator is actually set `None` when exhausted, depending on whether you
30 // iterate forward or backward. If you mix directions, then both sides may be `None`.
34 impl<A, B> Chain<A, B> {
35 pub(in super::super) fn new(a: A, b: B) -> Chain<A, B> {
36 Chain { a: Some(a), b: Some(b) }
40 /// Fuse the iterator if the expression is `None`.
42 ($self:ident . $iter:ident . $($call:tt)+) => {
44 Some(ref mut iter) => match iter.$($call)+ {
56 /// Try an iterator method without fusing,
57 /// like an inline `.as_mut().and_then(...)`
59 ($self:ident . $iter:ident . $($call:tt)+) => {
61 Some(ref mut iter) => iter.$($call)+,
67 #[stable(feature = "rust1", since = "1.0.0")]
68 impl<A, B> Iterator for Chain<A, B>
71 B: Iterator<Item = A::Item>,
76 fn next(&mut self) -> Option<A::Item> {
77 match fuse!(self.a.next()) {
78 None => maybe!(self.b.next()),
84 #[rustc_inherit_overflow_checks]
85 fn count(self) -> usize {
86 let a_count = match self.a {
90 let b_count = match self.b {
97 fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
100 F: FnMut(Acc, Self::Item) -> R,
103 if let Some(ref mut a) = self.a {
104 acc = a.try_fold(acc, &mut f)?;
107 if let Some(ref mut b) = self.b {
108 acc = b.try_fold(acc, f)?;
109 // we don't fuse the second iterator
114 fn fold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
116 F: FnMut(Acc, Self::Item) -> Acc,
118 if let Some(a) = self.a {
119 acc = a.fold(acc, &mut f);
121 if let Some(b) = self.b {
122 acc = b.fold(acc, f);
128 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
131 if let Some(ref mut a) = self.a {
132 match a.advance_by(rem) {
133 Ok(()) => return Ok(()),
139 if let Some(ref mut b) = self.b {
140 match b.advance_by(rem) {
141 Ok(()) => return Ok(()),
144 // we don't fuse the second iterator
147 if rem == 0 { Ok(()) } else { Err(n - rem) }
151 fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
152 if let Some(ref mut a) = self.a {
153 match a.advance_by(n) {
154 Ok(()) => match a.next() {
164 maybe!(self.b.nth(n))
168 fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
170 P: FnMut(&Self::Item) -> bool,
172 match fuse!(self.a.find(&mut predicate)) {
173 None => maybe!(self.b.find(predicate)),
179 fn last(self) -> Option<A::Item> {
180 // Must exhaust a before b.
181 let a_last = match self.a {
185 let b_last = match self.b {
193 fn size_hint(&self) -> (usize, Option<usize>) {
195 Chain { a: Some(a), b: Some(b) } => {
196 let (a_lower, a_upper) = a.size_hint();
197 let (b_lower, b_upper) = b.size_hint();
199 let lower = a_lower.saturating_add(b_lower);
201 let upper = match (a_upper, b_upper) {
202 (Some(x), Some(y)) => x.checked_add(y),
208 Chain { a: Some(a), b: None } => a.size_hint(),
209 Chain { a: None, b: Some(b) } => b.size_hint(),
210 Chain { a: None, b: None } => (0, Some(0)),
215 #[stable(feature = "rust1", since = "1.0.0")]
216 impl<A, B> DoubleEndedIterator for Chain<A, B>
218 A: DoubleEndedIterator,
219 B: DoubleEndedIterator<Item = A::Item>,
222 fn next_back(&mut self) -> Option<A::Item> {
223 match fuse!(self.b.next_back()) {
224 None => maybe!(self.a.next_back()),
230 fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
233 if let Some(ref mut b) = self.b {
234 match b.advance_back_by(rem) {
235 Ok(()) => return Ok(()),
241 if let Some(ref mut a) = self.a {
242 match a.advance_back_by(rem) {
243 Ok(()) => return Ok(()),
246 // we don't fuse the second iterator
249 if rem == 0 { Ok(()) } else { Err(n - rem) }
253 fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
254 if let Some(ref mut b) = self.b {
255 match b.advance_back_by(n) {
256 Ok(()) => match b.next_back() {
266 maybe!(self.a.nth_back(n))
270 fn rfind<P>(&mut self, mut predicate: P) -> Option<Self::Item>
272 P: FnMut(&Self::Item) -> bool,
274 match fuse!(self.b.rfind(&mut predicate)) {
275 None => maybe!(self.a.rfind(predicate)),
280 fn try_rfold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
283 F: FnMut(Acc, Self::Item) -> R,
286 if let Some(ref mut b) = self.b {
287 acc = b.try_rfold(acc, &mut f)?;
290 if let Some(ref mut a) = self.a {
291 acc = a.try_rfold(acc, f)?;
292 // we don't fuse the second iterator
297 fn rfold<Acc, F>(self, mut acc: Acc, mut f: F) -> Acc
299 F: FnMut(Acc, Self::Item) -> Acc,
301 if let Some(b) = self.b {
302 acc = b.rfold(acc, &mut f);
304 if let Some(a) = self.a {
305 acc = a.rfold(acc, f);
311 // Note: *both* must be fused to handle double-ended iterators.
312 #[stable(feature = "fused", since = "1.26.0")]
313 impl<A, B> FusedIterator for Chain<A, B>
316 B: FusedIterator<Item = A::Item>,
320 #[unstable(feature = "trusted_len", issue = "37572")]
321 unsafe impl<A, B> TrustedLen for Chain<A, B>
324 B: TrustedLen<Item = A::Item>,