core/iter/traits/double_ended.rs
1use crate::num::NonZero;
2use crate::ops::{ControlFlow, Try};
3
4/// An iterator able to yield elements from both ends.
5///
6/// Something that implements `DoubleEndedIterator` has one extra capability
7/// over something that implements [`Iterator`]: the ability to also take
8/// `Item`s from the back, as well as the front.
9///
10/// It is important to note that both back and forth work on the same range,
11/// and do not cross: iteration is over when they meet in the middle.
12///
13/// In a similar fashion to the [`Iterator`] protocol, once a
14/// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it
15/// again may or may not ever return [`Some`] again. [`next()`] and
16/// [`next_back()`] are interchangeable for this purpose.
17///
18/// [`next_back()`]: DoubleEndedIterator::next_back
19/// [`next()`]: Iterator::next
20///
21/// # Examples
22///
23/// Basic usage:
24///
25/// ```
26/// let numbers = vec![1, 2, 3, 4, 5, 6];
27///
28/// let mut iter = numbers.iter();
29///
30/// assert_eq!(Some(&1), iter.next());
31/// assert_eq!(Some(&6), iter.next_back());
32/// assert_eq!(Some(&5), iter.next_back());
33/// assert_eq!(Some(&2), iter.next());
34/// assert_eq!(Some(&3), iter.next());
35/// assert_eq!(Some(&4), iter.next());
36/// assert_eq!(None, iter.next());
37/// assert_eq!(None, iter.next_back());
38/// ```
39#[stable(feature = "rust1", since = "1.0.0")]
40#[rustc_diagnostic_item = "DoubleEndedIterator"]
41pub trait DoubleEndedIterator: Iterator {
42 /// Removes and returns an element from the end of the iterator.
43 ///
44 /// Returns `None` when there are no more elements.
45 ///
46 /// The [trait-level] docs contain more details.
47 ///
48 /// [trait-level]: DoubleEndedIterator
49 ///
50 /// # Examples
51 ///
52 /// Basic usage:
53 ///
54 /// ```
55 /// let numbers = vec![1, 2, 3, 4, 5, 6];
56 ///
57 /// let mut iter = numbers.iter();
58 ///
59 /// assert_eq!(Some(&1), iter.next());
60 /// assert_eq!(Some(&6), iter.next_back());
61 /// assert_eq!(Some(&5), iter.next_back());
62 /// assert_eq!(Some(&2), iter.next());
63 /// assert_eq!(Some(&3), iter.next());
64 /// assert_eq!(Some(&4), iter.next());
65 /// assert_eq!(None, iter.next());
66 /// assert_eq!(None, iter.next_back());
67 /// ```
68 ///
69 /// # Remarks
70 ///
71 /// The elements yielded by `DoubleEndedIterator`'s methods may differ from
72 /// the ones yielded by [`Iterator`]'s methods:
73 ///
74 /// ```
75 /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
76 /// let uniq_by_fst_comp = || {
77 /// let mut seen = std::collections::HashSet::new();
78 /// vec.iter().copied().filter(move |x| seen.insert(x.0))
79 /// };
80 ///
81 /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
82 /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
83 ///
84 /// assert_eq!(
85 /// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
86 /// vec![(1, 'a'), (2, 'a')]
87 /// );
88 /// assert_eq!(
89 /// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
90 /// vec![(2, 'b'), (1, 'c')]
91 /// );
92 /// ```
93 #[stable(feature = "rust1", since = "1.0.0")]
94 fn next_back(&mut self) -> Option<Self::Item>;
95
96 /// Advances the iterator from the back by `n` elements.
97 ///
98 /// `advance_back_by` is the reverse version of [`advance_by`]. This method will
99 /// eagerly skip `n` elements starting from the back by calling [`next_back`] up
100 /// to `n` times until [`None`] is encountered.
101 ///
102 /// `advance_back_by(n)` will return `Ok(())` if the iterator successfully advances by
103 /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered, where `k`
104 /// is remaining number of steps that could not be advanced because the iterator ran out.
105 /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
106 /// Otherwise, `k` is always less than `n`.
107 ///
108 /// Calling `advance_back_by(0)` can do meaningful work, for example [`Flatten`] can advance its
109 /// outer iterator until it finds an inner iterator that is not empty, which then often
110 /// allows it to return a more accurate `size_hint()` than in its initial state.
111 ///
112 /// [`advance_by`]: Iterator::advance_by
113 /// [`Flatten`]: crate::iter::Flatten
114 /// [`next_back`]: DoubleEndedIterator::next_back
115 ///
116 /// # Examples
117 ///
118 /// Basic usage:
119 ///
120 /// ```
121 /// #![feature(iter_advance_by)]
122 ///
123 /// use std::num::NonZero;
124 ///
125 /// let a = [3, 4, 5, 6];
126 /// let mut iter = a.iter();
127 ///
128 /// assert_eq!(iter.advance_back_by(2), Ok(()));
129 /// assert_eq!(iter.next_back(), Some(&4));
130 /// assert_eq!(iter.advance_back_by(0), Ok(()));
131 /// assert_eq!(iter.advance_back_by(100), Err(NonZero::new(99).unwrap())); // only `&3` was skipped
132 /// ```
133 ///
134 /// [`Ok(())`]: Ok
135 /// [`Err(k)`]: Err
136 #[inline]
137 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
138 fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
139 for i in 0..n {
140 if self.next_back().is_none() {
141 // SAFETY: `i` is always less than `n`.
142 return Err(unsafe { NonZero::new_unchecked(n - i) });
143 }
144 }
145 Ok(())
146 }
147
148 /// Returns the `n`th element from the end of the iterator.
149 ///
150 /// This is essentially the reversed version of [`Iterator::nth()`].
151 /// Although like most indexing operations, the count starts from zero, so
152 /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the
153 /// second, and so on.
154 ///
155 /// Note that all elements between the end and the returned element will be
156 /// consumed, including the returned element. This also means that calling
157 /// `nth_back(0)` multiple times on the same iterator will return different
158 /// elements.
159 ///
160 /// `nth_back()` will return [`None`] if `n` is greater than or equal to the
161 /// length of the iterator.
162 ///
163 /// # Examples
164 ///
165 /// Basic usage:
166 ///
167 /// ```
168 /// let a = [1, 2, 3];
169 /// assert_eq!(a.iter().nth_back(2), Some(&1));
170 /// ```
171 ///
172 /// Calling `nth_back()` multiple times doesn't rewind the iterator:
173 ///
174 /// ```
175 /// let a = [1, 2, 3];
176 ///
177 /// let mut iter = a.iter();
178 ///
179 /// assert_eq!(iter.nth_back(1), Some(&2));
180 /// assert_eq!(iter.nth_back(1), None);
181 /// ```
182 ///
183 /// Returning `None` if there are less than `n + 1` elements:
184 ///
185 /// ```
186 /// let a = [1, 2, 3];
187 /// assert_eq!(a.iter().nth_back(10), None);
188 /// ```
189 #[inline]
190 #[stable(feature = "iter_nth_back", since = "1.37.0")]
191 fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
192 if self.advance_back_by(n).is_err() {
193 return None;
194 }
195 self.next_back()
196 }
197
198 /// This is the reverse version of [`Iterator::try_fold()`]: it takes
199 /// elements starting from the back of the iterator.
200 ///
201 /// # Examples
202 ///
203 /// Basic usage:
204 ///
205 /// ```
206 /// let a = ["1", "2", "3"];
207 /// let sum = a.iter()
208 /// .map(|&s| s.parse::<i32>())
209 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
210 /// assert_eq!(sum, Ok(6));
211 /// ```
212 ///
213 /// Short-circuiting:
214 ///
215 /// ```
216 /// let a = ["1", "rust", "3"];
217 /// let mut it = a.iter();
218 /// let sum = it
219 /// .by_ref()
220 /// .map(|&s| s.parse::<i32>())
221 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
222 /// assert!(sum.is_err());
223 ///
224 /// // Because it short-circuited, the remaining elements are still
225 /// // available through the iterator.
226 /// assert_eq!(it.next_back(), Some(&"1"));
227 /// ```
228 #[inline]
229 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
230 fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
231 where
232 Self: Sized,
233 F: FnMut(B, Self::Item) -> R,
234 R: Try<Output = B>,
235 {
236 let mut accum = init;
237 while let Some(x) = self.next_back() {
238 accum = f(accum, x)?;
239 }
240 try { accum }
241 }
242
243 /// An iterator method that reduces the iterator's elements to a single,
244 /// final value, starting from the back.
245 ///
246 /// This is the reverse version of [`Iterator::fold()`]: it takes elements
247 /// starting from the back of the iterator.
248 ///
249 /// `rfold()` takes two arguments: an initial value, and a closure with two
250 /// arguments: an 'accumulator', and an element. The closure returns the value that
251 /// the accumulator should have for the next iteration.
252 ///
253 /// The initial value is the value the accumulator will have on the first
254 /// call.
255 ///
256 /// After applying this closure to every element of the iterator, `rfold()`
257 /// returns the accumulator.
258 ///
259 /// This operation is sometimes called 'reduce' or 'inject'.
260 ///
261 /// Folding is useful whenever you have a collection of something, and want
262 /// to produce a single value from it.
263 ///
264 /// Note: `rfold()` combines elements in a *right-associative* fashion. For associative
265 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
266 /// operators like `-` the order will affect the final result.
267 /// For a *left-associative* version of `rfold()`, see [`Iterator::fold()`].
268 ///
269 /// # Examples
270 ///
271 /// Basic usage:
272 ///
273 /// ```
274 /// let a = [1, 2, 3];
275 ///
276 /// // the sum of all of the elements of a
277 /// let sum = a.iter()
278 /// .rfold(0, |acc, &x| acc + x);
279 ///
280 /// assert_eq!(sum, 6);
281 /// ```
282 ///
283 /// This example demonstrates the right-associative nature of `rfold()`:
284 /// it builds a string, starting with an initial value
285 /// and continuing with each element from the back until the front:
286 ///
287 /// ```
288 /// let numbers = [1, 2, 3, 4, 5];
289 ///
290 /// let zero = "0".to_string();
291 ///
292 /// let result = numbers.iter().rfold(zero, |acc, &x| {
293 /// format!("({x} + {acc})")
294 /// });
295 ///
296 /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
297 /// ```
298 #[doc(alias = "foldr")]
299 #[inline]
300 #[stable(feature = "iter_rfold", since = "1.27.0")]
301 fn rfold<B, F>(mut self, init: B, mut f: F) -> B
302 where
303 Self: Sized,
304 F: FnMut(B, Self::Item) -> B,
305 {
306 let mut accum = init;
307 while let Some(x) = self.next_back() {
308 accum = f(accum, x);
309 }
310 accum
311 }
312
313 /// Searches for an element of an iterator from the back that satisfies a predicate.
314 ///
315 /// `rfind()` takes a closure that returns `true` or `false`. It applies
316 /// this closure to each element of the iterator, starting at the end, and if any
317 /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
318 /// `false`, it returns [`None`].
319 ///
320 /// `rfind()` is short-circuiting; in other words, it will stop processing
321 /// as soon as the closure returns `true`.
322 ///
323 /// Because `rfind()` takes a reference, and many iterators iterate over
324 /// references, this leads to a possibly confusing situation where the
325 /// argument is a double reference. You can see this effect in the
326 /// examples below, with `&&x`.
327 ///
328 /// [`Some(element)`]: Some
329 ///
330 /// # Examples
331 ///
332 /// Basic usage:
333 ///
334 /// ```
335 /// let a = [1, 2, 3];
336 ///
337 /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
338 ///
339 /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
340 /// ```
341 ///
342 /// Stopping at the first `true`:
343 ///
344 /// ```
345 /// let a = [1, 2, 3];
346 ///
347 /// let mut iter = a.iter();
348 ///
349 /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
350 ///
351 /// // we can still use `iter`, as there are more elements.
352 /// assert_eq!(iter.next_back(), Some(&1));
353 /// ```
354 #[inline]
355 #[stable(feature = "iter_rfind", since = "1.27.0")]
356 fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
357 where
358 Self: Sized,
359 P: FnMut(&Self::Item) -> bool,
360 {
361 #[inline]
362 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
363 move |(), x| {
364 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
365 }
366 }
367
368 self.try_rfold((), check(predicate)).break_value()
369 }
370}
371
372#[stable(feature = "rust1", since = "1.0.0")]
373impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
374 fn next_back(&mut self) -> Option<I::Item> {
375 (**self).next_back()
376 }
377 fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
378 (**self).advance_back_by(n)
379 }
380 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
381 (**self).nth_back(n)
382 }
383 fn rfold<B, F>(self, init: B, f: F) -> B
384 where
385 F: FnMut(B, Self::Item) -> B,
386 {
387 self.spec_rfold(init, f)
388 }
389 fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
390 where
391 F: FnMut(B, Self::Item) -> R,
392 R: Try<Output = B>,
393 {
394 self.spec_try_rfold(init, f)
395 }
396}
397
398/// Helper trait to specialize `rfold` and `rtry_fold` for `&mut I where I: Sized`
399trait DoubleEndedIteratorRefSpec: DoubleEndedIterator {
400 fn spec_rfold<B, F>(self, init: B, f: F) -> B
401 where
402 F: FnMut(B, Self::Item) -> B;
403
404 fn spec_try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
405 where
406 F: FnMut(B, Self::Item) -> R,
407 R: Try<Output = B>;
408}
409
410impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIteratorRefSpec for &mut I {
411 default fn spec_rfold<B, F>(self, init: B, mut f: F) -> B
412 where
413 F: FnMut(B, Self::Item) -> B,
414 {
415 let mut accum = init;
416 while let Some(x) = self.next_back() {
417 accum = f(accum, x);
418 }
419 accum
420 }
421
422 default fn spec_try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
423 where
424 F: FnMut(B, Self::Item) -> R,
425 R: Try<Output = B>,
426 {
427 let mut accum = init;
428 while let Some(x) = self.next_back() {
429 accum = f(accum, x)?;
430 }
431 try { accum }
432 }
433}
434
435impl<I: DoubleEndedIterator> DoubleEndedIteratorRefSpec for &mut I {
436 impl_fold_via_try_fold! { spec_rfold -> spec_try_rfold }
437
438 fn spec_try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
439 where
440 F: FnMut(B, Self::Item) -> R,
441 R: Try<Output = B>,
442 {
443 (**self).try_rfold(init, f)
444 }
445}