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
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
use crate::ops::{ControlFlow, Try};

/// An iterator able to yield elements from both ends.
///
/// Something that implements `DoubleEndedIterator` has one extra capability
/// over something that implements [`Iterator`]: the ability to also take
/// `Item`s from the back, as well as the front.
///
/// It is important to note that both back and forth work on the same range,
/// and do not cross: iteration is over when they meet in the middle.
///
/// In a similar fashion to the [`Iterator`] protocol, once a
/// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it
/// again may or may not ever return [`Some`] again. [`next()`] and
/// [`next_back()`] are interchangeable for this purpose.
///
/// [`next_back()`]: DoubleEndedIterator::next_back
/// [`next()`]: Iterator::next
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let numbers = vec![1, 2, 3, 4, 5, 6];
///
/// let mut iter = numbers.iter();
///
/// assert_eq!(Some(&1), iter.next());
/// assert_eq!(Some(&6), iter.next_back());
/// assert_eq!(Some(&5), iter.next_back());
/// assert_eq!(Some(&2), iter.next());
/// assert_eq!(Some(&3), iter.next());
/// assert_eq!(Some(&4), iter.next());
/// assert_eq!(None, iter.next());
/// assert_eq!(None, iter.next_back());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub trait DoubleEndedIterator: Iterator {
    /// Removes and returns an element from the end of the iterator.
    ///
    /// Returns `None` when there are no more elements.
    ///
    /// The [trait-level] docs contain more details.
    ///
    /// [trait-level]: DoubleEndedIterator
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let numbers = vec![1, 2, 3, 4, 5, 6];
    ///
    /// let mut iter = numbers.iter();
    ///
    /// assert_eq!(Some(&1), iter.next());
    /// assert_eq!(Some(&6), iter.next_back());
    /// assert_eq!(Some(&5), iter.next_back());
    /// assert_eq!(Some(&2), iter.next());
    /// assert_eq!(Some(&3), iter.next());
    /// assert_eq!(Some(&4), iter.next());
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next_back());
    /// ```
    ///
    /// # Remarks
    ///
    /// The elements yielded by `DoubleEndedIterator`'s methods may differ from
    /// the ones yielded by [`Iterator`]'s methods:
    ///
    /// ```
    /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
    /// let uniq_by_fst_comp = || {
    ///     let mut seen = std::collections::HashSet::new();
    ///     vec.iter().copied().filter(move |x| seen.insert(x.0))
    /// };
    ///
    /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
    /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
    ///
    /// assert_eq!(
    ///     uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
    ///     vec![(1, 'a'), (2, 'a')]
    /// );
    /// assert_eq!(
    ///     uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
    ///     vec![(2, 'b'), (1, 'c')]
    /// );
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn next_back(&mut self) -> Option<Self::Item>;

    /// Advances the iterator from the back by `n` elements.
    ///
    /// `advance_back_by` is the reverse version of [`advance_by`]. This method will
    /// eagerly skip `n` elements starting from the back by calling [`next_back`] up
    /// to `n` times until [`None`] is encountered.
    ///
    /// `advance_back_by(n)` will return [`Ok(())`] if the iterator successfully advances by
    /// `n` elements, or [`Err(k)`] if [`None`] is encountered, where `k` is the number of
    /// elements the iterator is advanced by before running out of elements (i.e. the length
    /// of the iterator). Note that `k` is always less than `n`.
    ///
    /// Calling `advance_back_by(0)` does not consume any elements and always returns [`Ok(())`].
    ///
    /// [`advance_by`]: Iterator::advance_by
    /// [`next_back`]: DoubleEndedIterator::next_back
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_advance_by)]
    ///
    /// let a = [3, 4, 5, 6];
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.advance_back_by(2), Ok(()));
    /// assert_eq!(iter.next_back(), Some(&4));
    /// assert_eq!(iter.advance_back_by(0), Ok(()));
    /// assert_eq!(iter.advance_back_by(100), Err(1)); // only `&3` was skipped
    /// ```
    #[inline]
    #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
        for i in 0..n {
            self.next_back().ok_or(i)?;
        }
        Ok(())
    }

    /// Returns the `n`th element from the end of the iterator.
    ///
    /// This is essentially the reversed version of [`Iterator::nth()`].
    /// Although like most indexing operations, the count starts from zero, so
    /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the
    /// second, and so on.
    ///
    /// Note that all elements between the end and the returned element will be
    /// consumed, including the returned element. This also means that calling
    /// `nth_back(0)` multiple times on the same iterator will return different
    /// elements.
    ///
    /// `nth_back()` will return [`None`] if `n` is greater than or equal to the
    /// length of the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().nth_back(2), Some(&1));
    /// ```
    ///
    /// Calling `nth_back()` multiple times doesn't rewind the iterator:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.nth_back(1), Some(&2));
    /// assert_eq!(iter.nth_back(1), None);
    /// ```
    ///
    /// Returning `None` if there are less than `n + 1` elements:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().nth_back(10), None);
    /// ```
    #[inline]
    #[stable(feature = "iter_nth_back", since = "1.37.0")]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        self.advance_back_by(n).ok()?;
        self.next_back()
    }

    /// This is the reverse version of [`Iterator::try_fold()`]: it takes
    /// elements starting from the back of the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = ["1", "2", "3"];
    /// let sum = a.iter()
    ///     .map(|&s| s.parse::<i32>())
    ///     .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
    /// assert_eq!(sum, Ok(6));
    /// ```
    ///
    /// Short-circuiting:
    ///
    /// ```
    /// let a = ["1", "rust", "3"];
    /// let mut it = a.iter();
    /// let sum = it
    ///     .by_ref()
    ///     .map(|&s| s.parse::<i32>())
    ///     .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
    /// assert!(sum.is_err());
    ///
    /// // Because it short-circuited, the remaining elements are still
    /// // available through the iterator.
    /// assert_eq!(it.next_back(), Some(&"1"));
    /// ```
    #[inline]
    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        let mut accum = init;
        while let Some(x) = self.next_back() {
            accum = f(accum, x)?;
        }
        Try::from_ok(accum)
    }

    /// An iterator method that reduces the iterator's elements to a single,
    /// final value, starting from the back.
    ///
    /// This is the reverse version of [`Iterator::fold()`]: it takes elements
    /// starting from the back of the iterator.
    ///
    /// `rfold()` takes two arguments: an initial value, and a closure with two
    /// arguments: an 'accumulator', and an element. The closure returns the value that
    /// the accumulator should have for the next iteration.
    ///
    /// The initial value is the value the accumulator will have on the first
    /// call.
    ///
    /// After applying this closure to every element of the iterator, `rfold()`
    /// returns the accumulator.
    ///
    /// This operation is sometimes called 'reduce' or 'inject'.
    ///
    /// Folding is useful whenever you have a collection of something, and want
    /// to produce a single value from it.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// // the sum of all of the elements of a
    /// let sum = a.iter()
    ///            .rfold(0, |acc, &x| acc + x);
    ///
    /// assert_eq!(sum, 6);
    /// ```
    ///
    /// This example builds a string, starting with an initial value
    /// and continuing with each element from the back until the front:
    ///
    /// ```
    /// let numbers = [1, 2, 3, 4, 5];
    ///
    /// let zero = "0".to_string();
    ///
    /// let result = numbers.iter().rfold(zero, |acc, &x| {
    ///     format!("({} + {})", x, acc)
    /// });
    ///
    /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
    /// ```
    #[inline]
    #[stable(feature = "iter_rfold", since = "1.27.0")]
    fn rfold<B, F>(mut self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        let mut accum = init;
        while let Some(x) = self.next_back() {
            accum = f(accum, x);
        }
        accum
    }

    /// Searches for an element of an iterator from the back that satisfies a predicate.
    ///
    /// `rfind()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, starting at the end, and if any
    /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
    /// `false`, it returns [`None`].
    ///
    /// `rfind()` is short-circuiting; in other words, it will stop processing
    /// as soon as the closure returns `true`.
    ///
    /// Because `rfind()` takes a reference, and many iterators iterate over
    /// references, this leads to a possibly confusing situation where the
    /// argument is a double reference. You can see this effect in the
    /// examples below, with `&&x`.
    ///
    /// [`Some(element)`]: Some
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
    ///
    /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next_back(), Some(&1));
    /// ```
    #[inline]
    #[stable(feature = "iter_rfind", since = "1.27.0")]
    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        #[inline]
        fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
            move |(), x| {
                if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
            }
        }

        self.try_rfold((), check(predicate)).break_value()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
    fn next_back(&mut self) -> Option<I::Item> {
        (**self).next_back()
    }
    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
        (**self).advance_back_by(n)
    }
    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
        (**self).nth_back(n)
    }
}