double_ended.rs.html -- source
Description: Source to the Rust file `src/libcore/iter/traits/double_ended.rs`.
Keywords: rust, rustlang, rust-lang

      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

    use crate::iter::LoopState;
    use crate::ops::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.
    ///
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// # 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]: trait.DoubleEndedIterator.html
        ///
        /// # 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>;

        /// Returns the `n`th element from the end of the iterator.
        ///
        /// This is essentially the reversed version of [`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.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        /// [`nth`]: ../../std/iter/trait.Iterator.html#method.nth
        ///
        /// # 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, mut n: usize) -> Option<Self::Item> {
            for x in self.rev() {
                if n == 0 {
                    return Some(x);
                }
                n -= 1;
            }
            None
        }

        /// This is the reverse version of [`try_fold()`]: it takes elements
        /// starting from the back of the iterator.
        ///
        /// [`try_fold()`]: trait.Iterator.html#method.try_fold
        ///
        /// # 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 [`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.
        ///
        /// [`fold()`]: trait.Iterator.html#method.fold
        ///
        /// # 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)`]: ../../std/option/enum.Option.html#variant.Some
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # 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) -> LoopState<(), T> {
                move |(), x| {
                    if predicate(&x) { LoopState::Break(x) } else { LoopState::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 nth_back(&mut self, n: usize) -> Option<I::Item> {
            (**self).nth_back(n)
        }
    }