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
use crate::array;
use crate::iter::adapters::SourceIter;
use crate::iter::{
    ByRefSized, FusedIterator, InPlaceIterable, TrustedFused, TrustedRandomAccessNoCoerce,
};
use crate::num::NonZero;
use crate::ops::{ControlFlow, NeverShortCircuit, Try};

/// An iterator over `N` elements of the iterator at a time.
///
/// The chunks do not overlap. If `N` does not divide the length of the
/// iterator, then the last up to `N-1` elements will be omitted.
///
/// This `struct` is created by the [`array_chunks`][Iterator::array_chunks]
/// method on [`Iterator`]. See its documentation for more.
#[derive(Debug, Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
pub struct ArrayChunks<I: Iterator, const N: usize> {
    iter: I,
    remainder: Option<array::IntoIter<I::Item, N>>,
}

impl<I, const N: usize> ArrayChunks<I, N>
where
    I: Iterator,
{
    #[track_caller]
    pub(in crate::iter) fn new(iter: I) -> Self {
        assert!(N != 0, "chunk size must be non-zero");
        Self { iter, remainder: None }
    }

    /// Returns an iterator over the remaining elements of the original iterator
    /// that are not going to be returned by this iterator. The returned
    /// iterator will yield at most `N-1` elements.
    #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
    #[inline]
    pub fn into_remainder(self) -> Option<array::IntoIter<I::Item, N>> {
        self.remainder
    }
}

#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
impl<I, const N: usize> Iterator for ArrayChunks<I, N>
where
    I: Iterator,
{
    type Item = [I::Item; N];

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        self.try_for_each(ControlFlow::Break).break_value()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper) = self.iter.size_hint();

        (lower / N, upper.map(|n| n / N))
    }

    #[inline]
    fn count(self) -> usize {
        self.iter.count() / N
    }

    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 mut acc = init;
        loop {
            match self.iter.next_chunk() {
                Ok(chunk) => acc = f(acc, chunk)?,
                Err(remainder) => {
                    // Make sure to not override `self.remainder` with an empty array
                    // when `next` is called after `ArrayChunks` exhaustion.
                    self.remainder.get_or_insert(remainder);

                    break try { acc };
                }
            }
        }
    }

    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        <Self as SpecFold>::fold(self, init, f)
    }
}

#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
impl<I, const N: usize> DoubleEndedIterator for ArrayChunks<I, N>
where
    I: DoubleEndedIterator + ExactSizeIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        self.try_rfold((), |(), x| ControlFlow::Break(x)).break_value()
    }

    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>,
    {
        // We are iterating from the back we need to first handle the remainder.
        self.next_back_remainder();

        let mut acc = init;
        let mut iter = ByRefSized(&mut self.iter).rev();

        // NB remainder is handled by `next_back_remainder`, so
        // `next_chunk` can't return `Err` with non-empty remainder
        // (assuming correct `I as ExactSizeIterator` impl).
        while let Ok(mut chunk) = iter.next_chunk() {
            // FIXME: do not do double reverse
            //        (we could instead add `next_chunk_back` for example)
            chunk.reverse();
            acc = f(acc, chunk)?
        }

        try { acc }
    }

    impl_fold_via_try_fold! { rfold -> try_rfold }
}

impl<I, const N: usize> ArrayChunks<I, N>
where
    I: DoubleEndedIterator + ExactSizeIterator,
{
    /// Updates `self.remainder` such that `self.iter.len` is divisible by `N`.
    fn next_back_remainder(&mut self) {
        // Make sure to not override `self.remainder` with an empty array
        // when `next_back` is called after `ArrayChunks` exhaustion.
        if self.remainder.is_some() {
            return;
        }

        // We use the `ExactSizeIterator` implementation of the underlying
        // iterator to know how many remaining elements there are.
        let rem = self.iter.len() % N;

        // Take the last `rem` elements out of `self.iter`.
        let mut remainder =
            // SAFETY: `unwrap_err` always succeeds because x % N < N for all x.
            unsafe { self.iter.by_ref().rev().take(rem).next_chunk().unwrap_err_unchecked() };

        // We used `.rev()` above, so we need to re-reverse the reminder
        remainder.as_mut_slice().reverse();
        self.remainder = Some(remainder);
    }
}

#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
impl<I, const N: usize> FusedIterator for ArrayChunks<I, N> where I: FusedIterator {}

#[unstable(issue = "none", feature = "trusted_fused")]
unsafe impl<I, const N: usize> TrustedFused for ArrayChunks<I, N> where I: TrustedFused + Iterator {}

#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
impl<I, const N: usize> ExactSizeIterator for ArrayChunks<I, N>
where
    I: ExactSizeIterator,
{
    #[inline]
    fn len(&self) -> usize {
        self.iter.len() / N
    }

    #[inline]
    fn is_empty(&self) -> bool {
        self.iter.len() < N
    }
}

trait SpecFold: Iterator {
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B;
}

impl<I, const N: usize> SpecFold for ArrayChunks<I, N>
where
    I: Iterator,
{
    #[inline]
    default fn fold<B, F>(mut self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
    }
}

impl<I, const N: usize> SpecFold for ArrayChunks<I, N>
where
    I: Iterator + TrustedRandomAccessNoCoerce,
{
    #[inline]
    fn fold<B, F>(mut self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        let mut accum = init;
        let inner_len = self.iter.size();
        let mut i = 0;
        // Use a while loop because (0..len).step_by(N) doesn't optimize well.
        while inner_len - i >= N {
            let chunk = crate::array::from_fn(|local| {
                // SAFETY: The method consumes the iterator and the loop condition ensures that
                // all accesses are in bounds and only happen once.
                unsafe {
                    let idx = i + local;
                    self.iter.__iterator_get_unchecked(idx)
                }
            });
            accum = f(accum, chunk);
            i += N;
        }

        // unlike try_fold this method does not need to take care of the remainder
        // since `self` will be dropped

        accum
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I, const N: usize> SourceIter for ArrayChunks<I, N>
where
    I: SourceIter + Iterator,
{
    type Source = I::Source;

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

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I: InPlaceIterable + Iterator, const N: usize> InPlaceIterable for ArrayChunks<I, N> {
    const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
    const MERGE_BY: Option<NonZero<usize>> = const {
        match (I::MERGE_BY, NonZero::new(N)) {
            (Some(m), Some(n)) => m.checked_mul(n),
            _ => None,
        }
    };
}