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
use crate::iter::{adapters::SourceIter, FusedIterator, TrustedLen};
use crate::ops::{ControlFlow, Try};

/// An iterator with a `peek()` that returns an optional reference to the next
/// element.
///
/// This `struct` is created by the [`peekable`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`peekable`]: Iterator::peekable
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Peekable<I: Iterator> {
    iter: I,
    /// Remember a peeked value, even if it was None.
    peeked: Option<Option<I::Item>>,
}

impl<I: Iterator> Peekable<I> {
    pub(in crate::iter) fn new(iter: I) -> Peekable<I> {
        Peekable { iter, peeked: None }
    }
}

// Peekable must remember if a None has been seen in the `.peek()` method.
// It ensures that `.peek(); .peek();` or `.peek(); .next();` only advances the
// underlying iterator at most once. This does not by itself make the iterator
// fused.
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator> Iterator for Peekable<I> {
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        match self.peeked.take() {
            Some(v) => v,
            None => self.iter.next(),
        }
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn count(mut self) -> usize {
        match self.peeked.take() {
            Some(None) => 0,
            Some(Some(_)) => 1 + self.iter.count(),
            None => self.iter.count(),
        }
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        match self.peeked.take() {
            Some(None) => None,
            Some(v @ Some(_)) if n == 0 => v,
            Some(Some(_)) => self.iter.nth(n - 1),
            None => self.iter.nth(n),
        }
    }

    #[inline]
    fn last(mut self) -> Option<I::Item> {
        let peek_opt = match self.peeked.take() {
            Some(None) => return None,
            Some(v) => v,
            None => None,
        };
        self.iter.last().or(peek_opt)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let peek_len = match self.peeked {
            Some(None) => return (0, Some(0)),
            Some(Some(_)) => 1,
            None => 0,
        };
        let (lo, hi) = self.iter.size_hint();
        let lo = lo.saturating_add(peek_len);
        let hi = match hi {
            Some(x) => x.checked_add(peek_len),
            None => None,
        };
        (lo, hi)
    }

    #[inline]
    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        let acc = match self.peeked.take() {
            Some(None) => return try { init },
            Some(Some(v)) => f(init, v)?,
            None => init,
        };
        self.iter.try_fold(acc, f)
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let acc = match self.peeked {
            Some(None) => return init,
            Some(Some(v)) => fold(init, v),
            None => init,
        };
        self.iter.fold(acc, fold)
    }
}

#[stable(feature = "double_ended_peek_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for Peekable<I>
where
    I: DoubleEndedIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        match self.peeked.as_mut() {
            Some(v @ Some(_)) => self.iter.next_back().or_else(|| v.take()),
            Some(None) => None,
            None => self.iter.next_back(),
        }
    }

    #[inline]
    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<Output = B>,
    {
        match self.peeked.take() {
            Some(None) => try { init },
            Some(Some(v)) => match self.iter.try_rfold(init, &mut f).branch() {
                ControlFlow::Continue(acc) => f(acc, v),
                ControlFlow::Break(r) => {
                    self.peeked = Some(Some(v));
                    R::from_residual(r)
                }
            },
            None => self.iter.try_rfold(init, f),
        }
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        match self.peeked {
            Some(None) => init,
            Some(Some(v)) => {
                let acc = self.iter.rfold(init, &mut fold);
                fold(acc, v)
            }
            None => self.iter.rfold(init, fold),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator> FusedIterator for Peekable<I> {}

impl<I: Iterator> Peekable<I> {
    /// Returns a reference to the next() value without advancing the iterator.
    ///
    /// Like [`next`], if there is a value, it is wrapped in a `Some(T)`.
    /// But if the iteration is over, `None` is returned.
    ///
    /// [`next`]: Iterator::next
    ///
    /// Because `peek()` returns a reference, and many iterators iterate over
    /// references, there can be a possibly confusing situation where the
    /// return value is a double reference. You can see this effect in the
    /// examples below.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let xs = [1, 2, 3];
    ///
    /// let mut iter = xs.iter().peekable();
    ///
    /// // peek() lets us see into the future
    /// assert_eq!(iter.peek(), Some(&&1));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// assert_eq!(iter.next(), Some(&2));
    ///
    /// // The iterator does not advance even if we `peek` multiple times
    /// assert_eq!(iter.peek(), Some(&&3));
    /// assert_eq!(iter.peek(), Some(&&3));
    ///
    /// assert_eq!(iter.next(), Some(&3));
    ///
    /// // After the iterator is finished, so is `peek()`
    /// assert_eq!(iter.peek(), None);
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn peek(&mut self) -> Option<&I::Item> {
        let iter = &mut self.iter;
        self.peeked.get_or_insert_with(|| iter.next()).as_ref()
    }

    /// Returns a mutable reference to the next() value without advancing the iterator.
    ///
    /// Like [`next`], if there is a value, it is wrapped in a `Some(T)`.
    /// But if the iteration is over, `None` is returned.
    ///
    /// Because `peek_mut()` returns a reference, and many iterators iterate over
    /// references, there can be a possibly confusing situation where the
    /// return value is a double reference. You can see this effect in the examples
    /// below.
    ///
    /// [`next`]: Iterator::next
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut iter = [1, 2, 3].iter().peekable();
    ///
    /// // Like with `peek()`, we can see into the future without advancing the iterator.
    /// assert_eq!(iter.peek_mut(), Some(&mut &1));
    /// assert_eq!(iter.peek_mut(), Some(&mut &1));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// // Peek into the iterator and set the value behind the mutable reference.
    /// if let Some(p) = iter.peek_mut() {
    ///     assert_eq!(*p, &2);
    ///     *p = &5;
    /// }
    ///
    /// // The value we put in reappears as the iterator continues.
    /// assert_eq!(iter.collect::<Vec<_>>(), vec![&5, &3]);
    /// ```
    #[inline]
    #[stable(feature = "peekable_peek_mut", since = "1.53.0")]
    pub fn peek_mut(&mut self) -> Option<&mut I::Item> {
        let iter = &mut self.iter;
        self.peeked.get_or_insert_with(|| iter.next()).as_mut()
    }

    /// Consume and return the next value of this iterator if a condition is true.
    ///
    /// If `func` returns `true` for the next value of this iterator, consume and return it.
    /// Otherwise, return `None`.
    ///
    /// # Examples
    /// Consume a number if it's equal to 0.
    /// ```
    /// let mut iter = (0..5).peekable();
    /// // The first item of the iterator is 0; consume it.
    /// assert_eq!(iter.next_if(|&x| x == 0), Some(0));
    /// // The next item returned is now 1, so `consume` will return `false`.
    /// assert_eq!(iter.next_if(|&x| x == 0), None);
    /// // `next_if` saves the value of the next item if it was not equal to `expected`.
    /// assert_eq!(iter.next(), Some(1));
    /// ```
    ///
    /// Consume any number less than 10.
    /// ```
    /// let mut iter = (1..20).peekable();
    /// // Consume all numbers less than 10
    /// while iter.next_if(|&x| x < 10).is_some() {}
    /// // The next value returned will be 10
    /// assert_eq!(iter.next(), Some(10));
    /// ```
    #[stable(feature = "peekable_next_if", since = "1.51.0")]
    pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {
        match self.next() {
            Some(matched) if func(&matched) => Some(matched),
            other => {
                // Since we called `self.next()`, we consumed `self.peeked`.
                assert!(self.peeked.is_none());
                self.peeked = Some(other);
                None
            }
        }
    }

    /// Consume and return the next item if it is equal to `expected`.
    ///
    /// # Example
    /// Consume a number if it's equal to 0.
    /// ```
    /// let mut iter = (0..5).peekable();
    /// // The first item of the iterator is 0; consume it.
    /// assert_eq!(iter.next_if_eq(&0), Some(0));
    /// // The next item returned is now 1, so `consume` will return `false`.
    /// assert_eq!(iter.next_if_eq(&0), None);
    /// // `next_if_eq` saves the value of the next item if it was not equal to `expected`.
    /// assert_eq!(iter.next(), Some(1));
    /// ```
    #[stable(feature = "peekable_next_if", since = "1.51.0")]
    pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>
    where
        T: ?Sized,
        I::Item: PartialEq<T>,
    {
        self.next_if(|next| next == expected)
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Peekable<I> where I: TrustedLen {}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<S: Iterator, I: Iterator> SourceIter for Peekable<I>
where
    I: SourceIter<Source = S>,
{
    type Source = S;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut S {
        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}