1use crate::intrinsics::unlikely;
2use crate::iter::adapters::SourceIter;
3use crate::iter::adapters::zip::try_get_unchecked;
4use crate::iter::{
5 FusedIterator, InPlaceIterable, TrustedFused, TrustedLen, TrustedRandomAccess,
6 TrustedRandomAccessNoCoerce,
7};
8use crate::num::NonZero;
9use crate::ops::{ControlFlow, Try};
10
11/// An iterator that skips over `n` elements of `iter`.
12///
13/// This `struct` is created by the [`skip`] method on [`Iterator`]. See its
14/// documentation for more.
15///
16/// [`skip`]: Iterator::skip
17/// [`Iterator`]: trait.Iterator.html
18#[derive(Clone, Debug)]
19#[must_use = "iterators are lazy and do nothing unless consumed"]
20#[stable(feature = "rust1", since = "1.0.0")]
21pub struct Skip<I> {
22 iter: I,
23 n: usize,
24}
25
26impl<I> Skip<I> {
27 pub(in crate::iter) fn new(iter: I, n: usize) -> Skip<I> {
28 Skip { iter, n }
29 }
30}
31
32#[stable(feature = "rust1", since = "1.0.0")]
33impl<I> Iterator for Skip<I>
34where
35 I: Iterator,
36{
37 type Item = <I as Iterator>::Item;
38
39 #[inline]
40 fn next(&mut self) -> Option<I::Item> {
41 if unlikely(self.n > 0) {
42 self.iter.nth(crate::mem::take(&mut self.n))
43 } else {
44 self.iter.next()
45 }
46 }
47
48 #[inline]
49 fn nth(&mut self, n: usize) -> Option<I::Item> {
50 if self.n > 0 {
51 let skip: usize = crate::mem::take(&mut self.n);
52 // Checked add to handle overflow case.
53 let n = match skip.checked_add(n) {
54 Some(nth) => nth,
55 None => {
56 // In case of overflow, load skip value, before loading `n`.
57 // Because the amount of elements to iterate is beyond `usize::MAX`, this
58 // is split into two `nth` calls where the `skip` `nth` call is discarded.
59 self.iter.nth(skip - 1)?;
60 n
61 }
62 };
63 // Load nth element including skip.
64 self.iter.nth(n)
65 } else {
66 self.iter.nth(n)
67 }
68 }
69
70 #[inline]
71 fn count(mut self) -> usize {
72 if self.n > 0 {
73 // nth(n) skips n+1
74 if self.iter.nth(self.n - 1).is_none() {
75 return 0;
76 }
77 }
78 self.iter.count()
79 }
80
81 #[inline]
82 fn last(mut self) -> Option<I::Item> {
83 if self.n > 0 {
84 // nth(n) skips n+1
85 self.iter.nth(self.n - 1)?;
86 }
87 self.iter.last()
88 }
89
90 #[inline]
91 fn size_hint(&self) -> (usize, Option<usize>) {
92 let (lower, upper) = self.iter.size_hint();
93
94 let lower = lower.saturating_sub(self.n);
95 let upper = match upper {
96 Some(x) => Some(x.saturating_sub(self.n)),
97 None => None,
98 };
99
100 (lower, upper)
101 }
102
103 #[inline]
104 fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
105 where
106 Self: Sized,
107 Fold: FnMut(Acc, Self::Item) -> R,
108 R: Try<Output = Acc>,
109 {
110 let n = self.n;
111 self.n = 0;
112 if n > 0 {
113 // nth(n) skips n+1
114 if self.iter.nth(n - 1).is_none() {
115 return try { init };
116 }
117 }
118 self.iter.try_fold(init, fold)
119 }
120
121 #[inline]
122 fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
123 where
124 Fold: FnMut(Acc, Self::Item) -> Acc,
125 {
126 if self.n > 0 {
127 // nth(n) skips n+1
128 if self.iter.nth(self.n - 1).is_none() {
129 return init;
130 }
131 }
132 self.iter.fold(init, fold)
133 }
134
135 #[inline]
136 #[rustc_inherit_overflow_checks]
137 fn advance_by(&mut self, mut n: usize) -> Result<(), NonZero<usize>> {
138 let skip_inner = self.n;
139 let skip_and_advance = skip_inner.saturating_add(n);
140
141 let remainder = match self.iter.advance_by(skip_and_advance) {
142 Ok(()) => 0,
143 Err(n) => n.get(),
144 };
145 let advanced_inner = skip_and_advance - remainder;
146 n -= advanced_inner.saturating_sub(skip_inner);
147 self.n = self.n.saturating_sub(advanced_inner);
148
149 // skip_and_advance may have saturated
150 if unlikely(remainder == 0 && n > 0) {
151 n = match self.iter.advance_by(n) {
152 Ok(()) => 0,
153 Err(n) => n.get(),
154 }
155 }
156
157 NonZero::new(n).map_or(Ok(()), Err)
158 }
159
160 #[doc(hidden)]
161 unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
162 where
163 Self: TrustedRandomAccessNoCoerce,
164 {
165 // SAFETY: the caller must uphold the contract for
166 // `Iterator::__iterator_get_unchecked`.
167 //
168 // Dropping the skipped prefix when index 0 is passed is safe
169 // since
170 // * the caller passing index 0 means that the inner iterator has more items than `self.n`
171 // * TRA contract requires that get_unchecked will only be called once
172 // (unless elements are copyable)
173 // * it does not conflict with in-place iteration since index 0 must be accessed
174 // before something is written into the storage used by the prefix
175 unsafe {
176 if Self::MAY_HAVE_SIDE_EFFECT && idx == 0 {
177 for skipped_idx in 0..self.n {
178 drop(try_get_unchecked(&mut self.iter, skipped_idx));
179 }
180 }
181
182 try_get_unchecked(&mut self.iter, idx + self.n)
183 }
184 }
185}
186
187#[stable(feature = "rust1", since = "1.0.0")]
188impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}
189
190#[stable(feature = "double_ended_skip_iterator", since = "1.9.0")]
191impl<I> DoubleEndedIterator for Skip<I>
192where
193 I: DoubleEndedIterator + ExactSizeIterator,
194{
195 fn next_back(&mut self) -> Option<Self::Item> {
196 if self.len() > 0 { self.iter.next_back() } else { None }
197 }
198
199 #[inline]
200 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
201 let len = self.len();
202 if n < len {
203 self.iter.nth_back(n)
204 } else {
205 if len > 0 {
206 // consume the original iterator
207 self.iter.nth_back(len - 1);
208 }
209 None
210 }
211 }
212
213 fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
214 where
215 Self: Sized,
216 Fold: FnMut(Acc, Self::Item) -> R,
217 R: Try<Output = Acc>,
218 {
219 fn check<T, Acc, R: Try<Output = Acc>>(
220 mut n: usize,
221 mut fold: impl FnMut(Acc, T) -> R,
222 ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> {
223 move |acc, x| {
224 n -= 1;
225 let r = fold(acc, x);
226 if n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) }
227 }
228 }
229
230 let n = self.len();
231 if n == 0 { try { init } } else { self.iter.try_rfold(init, check(n, fold)).into_try() }
232 }
233
234 impl_fold_via_try_fold! { rfold -> try_rfold }
235
236 #[inline]
237 fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
238 let min = crate::cmp::min(self.len(), n);
239 let rem = self.iter.advance_back_by(min);
240 assert!(rem.is_ok(), "ExactSizeIterator contract violation");
241 NonZero::new(n - min).map_or(Ok(()), Err)
242 }
243}
244
245#[stable(feature = "fused", since = "1.26.0")]
246impl<I> FusedIterator for Skip<I> where I: FusedIterator {}
247
248#[unstable(issue = "none", feature = "trusted_fused")]
249unsafe impl<I: TrustedFused> TrustedFused for Skip<I> {}
250
251#[unstable(issue = "none", feature = "inplace_iteration")]
252unsafe impl<I> SourceIter for Skip<I>
253where
254 I: SourceIter,
255{
256 type Source = I::Source;
257
258 #[inline]
259 unsafe fn as_inner(&mut self) -> &mut I::Source {
260 // SAFETY: unsafe function forwarding to unsafe function with the same requirements
261 unsafe { SourceIter::as_inner(&mut self.iter) }
262 }
263}
264
265#[unstable(issue = "none", feature = "inplace_iteration")]
266unsafe impl<I: InPlaceIterable> InPlaceIterable for Skip<I> {
267 const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
268 const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
269}
270
271#[doc(hidden)]
272#[unstable(feature = "trusted_random_access", issue = "none")]
273unsafe impl<I> TrustedRandomAccess for Skip<I> where I: TrustedRandomAccess {}
274
275#[doc(hidden)]
276#[unstable(feature = "trusted_random_access", issue = "none")]
277unsafe impl<I> TrustedRandomAccessNoCoerce for Skip<I>
278where
279 I: TrustedRandomAccessNoCoerce,
280{
281 const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
282}
283
284// SAFETY: This adapter is shortening. TrustedLen requires the upper bound to be calculated correctly.
285// These requirements can only be satisfied when the upper bound of the inner iterator's upper
286// bound is never `None`. I: TrustedRandomAccess happens to provide this guarantee while
287// I: TrustedLen would not.
288#[unstable(feature = "trusted_len", issue = "37572")]
289unsafe impl<I> TrustedLen for Skip<I> where I: Iterator + TrustedRandomAccess {}
290