arrow_buffer/util/
bit_chunk_iterator.rs

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9//   http://www.apache.org/licenses/LICENSE-2.0
10//
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15// specific language governing permissions and limitations
16// under the License.
17
18//! Types for iterating over bitmasks in 64-bit chunks
19
20use crate::util::bit_util::ceil;
21use std::fmt::Debug;
22
23/// Iterates over an arbitrarily aligned byte buffer
24///
25/// Yields an iterator of aligned u64, along with the leading and trailing
26/// u64 necessary to align the buffer to a 8-byte boundary
27///
28/// This is unlike [`BitChunkIterator`] which only exposes a trailing u64,
29/// and consequently has to perform more work for each read
30#[derive(Debug)]
31pub struct UnalignedBitChunk<'a> {
32    lead_padding: usize,
33    trailing_padding: usize,
34
35    prefix: Option<u64>,
36    chunks: &'a [u64],
37    suffix: Option<u64>,
38}
39
40impl<'a> UnalignedBitChunk<'a> {
41    /// Create a from a byte array, and and an offset and length in bits
42    pub fn new(buffer: &'a [u8], offset: usize, len: usize) -> Self {
43        if len == 0 {
44            return Self {
45                lead_padding: 0,
46                trailing_padding: 0,
47                prefix: None,
48                chunks: &[],
49                suffix: None,
50            };
51        }
52
53        let byte_offset = offset / 8;
54        let offset_padding = offset % 8;
55
56        let bytes_len = (len + offset_padding).div_ceil(8);
57        let buffer = &buffer[byte_offset..byte_offset + bytes_len];
58
59        let prefix_mask = compute_prefix_mask(offset_padding);
60
61        // If less than 8 bytes, read into prefix
62        if buffer.len() <= 8 {
63            let (suffix_mask, trailing_padding) = compute_suffix_mask(len, offset_padding);
64            let prefix = read_u64(buffer) & suffix_mask & prefix_mask;
65
66            return Self {
67                lead_padding: offset_padding,
68                trailing_padding,
69                prefix: Some(prefix),
70                chunks: &[],
71                suffix: None,
72            };
73        }
74
75        // If less than 16 bytes, read into prefix and suffix
76        if buffer.len() <= 16 {
77            let (suffix_mask, trailing_padding) = compute_suffix_mask(len, offset_padding);
78            let prefix = read_u64(&buffer[..8]) & prefix_mask;
79            let suffix = read_u64(&buffer[8..]) & suffix_mask;
80
81            return Self {
82                lead_padding: offset_padding,
83                trailing_padding,
84                prefix: Some(prefix),
85                chunks: &[],
86                suffix: Some(suffix),
87            };
88        }
89
90        // Read into prefix and suffix as needed
91        let (prefix, mut chunks, suffix) = unsafe { buffer.align_to::<u64>() };
92        assert!(
93            prefix.len() < 8 && suffix.len() < 8,
94            "align_to did not return largest possible aligned slice"
95        );
96
97        let (alignment_padding, prefix) = match (offset_padding, prefix.is_empty()) {
98            (0, true) => (0, None),
99            (_, true) => {
100                let prefix = chunks[0] & prefix_mask;
101                chunks = &chunks[1..];
102                (0, Some(prefix))
103            }
104            (_, false) => {
105                let alignment_padding = (8 - prefix.len()) * 8;
106
107                let prefix = (read_u64(prefix) & prefix_mask) << alignment_padding;
108                (alignment_padding, Some(prefix))
109            }
110        };
111
112        let lead_padding = offset_padding + alignment_padding;
113        let (suffix_mask, trailing_padding) = compute_suffix_mask(len, lead_padding);
114
115        let suffix = match (trailing_padding, suffix.is_empty()) {
116            (0, _) => None,
117            (_, true) => {
118                let suffix = chunks[chunks.len() - 1] & suffix_mask;
119                chunks = &chunks[..chunks.len() - 1];
120                Some(suffix)
121            }
122            (_, false) => Some(read_u64(suffix) & suffix_mask),
123        };
124
125        Self {
126            lead_padding,
127            trailing_padding,
128            prefix,
129            chunks,
130            suffix,
131        }
132    }
133
134    /// Returns the number of leading padding bits
135    pub fn lead_padding(&self) -> usize {
136        self.lead_padding
137    }
138
139    /// Returns the number of trailing padding bits
140    pub fn trailing_padding(&self) -> usize {
141        self.trailing_padding
142    }
143
144    /// Returns the prefix, if any
145    pub fn prefix(&self) -> Option<u64> {
146        self.prefix
147    }
148
149    /// Returns the suffix, if any
150    pub fn suffix(&self) -> Option<u64> {
151        self.suffix
152    }
153
154    /// Returns reference to the chunks
155    pub fn chunks(&self) -> &'a [u64] {
156        self.chunks
157    }
158
159    /// Returns an iterator over the chunks
160    pub fn iter(&self) -> UnalignedBitChunkIterator<'a> {
161        self.prefix
162            .into_iter()
163            .chain(self.chunks.iter().cloned())
164            .chain(self.suffix)
165    }
166
167    /// Counts the number of ones
168    pub fn count_ones(&self) -> usize {
169        self.iter().map(|x| x.count_ones() as usize).sum()
170    }
171}
172
173/// Iterator over an [`UnalignedBitChunk`]
174pub type UnalignedBitChunkIterator<'a> = std::iter::Chain<
175    std::iter::Chain<std::option::IntoIter<u64>, std::iter::Cloned<std::slice::Iter<'a, u64>>>,
176    std::option::IntoIter<u64>,
177>;
178
179#[inline]
180fn read_u64(input: &[u8]) -> u64 {
181    let len = input.len().min(8);
182    let mut buf = [0_u8; 8];
183    buf[..len].copy_from_slice(input);
184    u64::from_le_bytes(buf)
185}
186
187#[inline]
188fn compute_prefix_mask(lead_padding: usize) -> u64 {
189    !((1 << lead_padding) - 1)
190}
191
192#[inline]
193fn compute_suffix_mask(len: usize, lead_padding: usize) -> (u64, usize) {
194    let trailing_bits = (len + lead_padding) % 64;
195
196    if trailing_bits == 0 {
197        return (u64::MAX, 0);
198    }
199
200    let trailing_padding = 64 - trailing_bits;
201    let suffix_mask = (1 << trailing_bits) - 1;
202    (suffix_mask, trailing_padding)
203}
204
205/// Iterates over an arbitrarily aligned byte buffer 64 bits at a time
206///
207/// [`Self::iter`] yields iterator of `u64`, and a remainder. The first byte in the buffer
208/// will be the least significant byte in output u64
209#[derive(Debug)]
210pub struct BitChunks<'a> {
211    buffer: &'a [u8],
212    /// offset inside a byte, guaranteed to be between 0 and 7 (inclusive)
213    bit_offset: usize,
214    /// number of complete u64 chunks
215    chunk_len: usize,
216    /// number of remaining bits, guaranteed to be between 0 and 63 (inclusive)
217    remainder_len: usize,
218}
219
220impl<'a> BitChunks<'a> {
221    /// Create a new [`BitChunks`] from a byte array, and an offset and length in bits
222    pub fn new(buffer: &'a [u8], offset: usize, len: usize) -> Self {
223        assert!(
224            ceil(offset + len, 8) <= buffer.len(),
225            "offset + len out of bounds"
226        );
227
228        let byte_offset = offset / 8;
229        let bit_offset = offset % 8;
230
231        // number of complete u64 chunks
232        let chunk_len = len / 64;
233        // number of remaining bits
234        let remainder_len = len % 64;
235
236        BitChunks::<'a> {
237            buffer: &buffer[byte_offset..],
238            bit_offset,
239            chunk_len,
240            remainder_len,
241        }
242    }
243}
244
245/// Iterator over chunks of 64 bits represented as an u64
246#[derive(Debug)]
247pub struct BitChunkIterator<'a> {
248    buffer: &'a [u8],
249    bit_offset: usize,
250    chunk_len: usize,
251    index: usize,
252}
253
254impl<'a> BitChunks<'a> {
255    /// Returns the number of remaining bits, guaranteed to be between 0 and 63 (inclusive)
256    #[inline]
257    pub const fn remainder_len(&self) -> usize {
258        self.remainder_len
259    }
260
261    /// Returns the number of `u64` chunks
262    #[inline]
263    pub const fn chunk_len(&self) -> usize {
264        self.chunk_len
265    }
266
267    /// Returns the bitmask of remaining bits
268    #[inline]
269    pub fn remainder_bits(&self) -> u64 {
270        let bit_len = self.remainder_len;
271        if bit_len == 0 {
272            0
273        } else {
274            let bit_offset = self.bit_offset;
275            // number of bytes to read
276            // might be one more than sizeof(u64) if the offset is in the middle of a byte
277            let byte_len = ceil(bit_len + bit_offset, 8);
278            // pointer to remainder bytes after all complete chunks
279            let base = unsafe {
280                self.buffer
281                    .as_ptr()
282                    .add(self.chunk_len * std::mem::size_of::<u64>())
283            };
284
285            let mut bits = unsafe { std::ptr::read(base) } as u64 >> bit_offset;
286            for i in 1..byte_len {
287                let byte = unsafe { std::ptr::read(base.add(i)) };
288                bits |= (byte as u64) << (i * 8 - bit_offset);
289            }
290
291            bits & ((1 << bit_len) - 1)
292        }
293    }
294
295    /// Return the number of `u64` that are needed to represent all bits
296    /// (including remainder).
297    ///
298    /// This is equal to `chunk_len + 1` if there is a remainder,
299    /// otherwise it is equal to `chunk_len`.
300    #[inline]
301    pub fn num_u64s(&self) -> usize {
302        if self.remainder_len == 0 {
303            self.chunk_len
304        } else {
305            self.chunk_len + 1
306        }
307    }
308
309    /// Return the number of *bytes* that are needed to represent all bits
310    /// (including remainder).
311    #[inline]
312    pub fn num_bytes(&self) -> usize {
313        ceil(self.chunk_len * 64 + self.remainder_len, 8)
314    }
315
316    /// Returns an iterator over chunks of 64 bits represented as an `u64`
317    #[inline]
318    pub const fn iter(&self) -> BitChunkIterator<'a> {
319        BitChunkIterator::<'a> {
320            buffer: self.buffer,
321            bit_offset: self.bit_offset,
322            chunk_len: self.chunk_len,
323            index: 0,
324        }
325    }
326
327    /// Returns an iterator over chunks of 64 bits, with the remaining bits zero padded to 64-bits
328    #[inline]
329    pub fn iter_padded(&self) -> impl Iterator<Item = u64> + 'a {
330        self.iter().chain(std::iter::once(self.remainder_bits()))
331    }
332}
333
334impl<'a> IntoIterator for BitChunks<'a> {
335    type Item = u64;
336    type IntoIter = BitChunkIterator<'a>;
337
338    fn into_iter(self) -> Self::IntoIter {
339        self.iter()
340    }
341}
342
343impl Iterator for BitChunkIterator<'_> {
344    type Item = u64;
345
346    #[inline]
347    fn next(&mut self) -> Option<u64> {
348        let index = self.index;
349        if index >= self.chunk_len {
350            return None;
351        }
352
353        // cast to *const u64 should be fine since we are using read_unaligned below
354        #[allow(clippy::cast_ptr_alignment)]
355        let raw_data = self.buffer.as_ptr() as *const u64;
356
357        // bit-packed buffers are stored starting with the least-significant byte first
358        // so when reading as u64 on a big-endian machine, the bytes need to be swapped
359        let current = unsafe { std::ptr::read_unaligned(raw_data.add(index)).to_le() };
360
361        let bit_offset = self.bit_offset;
362
363        let combined = if bit_offset == 0 {
364            current
365        } else {
366            // the constructor ensures that bit_offset is in 0..8
367            // that means we need to read at most one additional byte to fill in the high bits
368            let next =
369                unsafe { std::ptr::read_unaligned(raw_data.add(index + 1) as *const u8) as u64 };
370
371            (current >> bit_offset) | (next << (64 - bit_offset))
372        };
373
374        self.index = index + 1;
375
376        Some(combined)
377    }
378
379    #[inline]
380    fn size_hint(&self) -> (usize, Option<usize>) {
381        (
382            self.chunk_len - self.index,
383            Some(self.chunk_len - self.index),
384        )
385    }
386}
387
388impl ExactSizeIterator for BitChunkIterator<'_> {
389    #[inline]
390    fn len(&self) -> usize {
391        self.chunk_len - self.index
392    }
393}
394
395#[cfg(test)]
396mod tests {
397    use rand::distr::uniform::UniformSampler;
398    use rand::distr::uniform::UniformUsize;
399    use rand::prelude::*;
400    use rand::rng;
401
402    use crate::buffer::Buffer;
403    use crate::util::bit_chunk_iterator::UnalignedBitChunk;
404
405    #[test]
406    fn test_iter_aligned() {
407        let input: &[u8] = &[0, 1, 2, 3, 4, 5, 6, 7];
408        let buffer: Buffer = Buffer::from(input);
409
410        let bitchunks = buffer.bit_chunks(0, 64);
411        let result = bitchunks.into_iter().collect::<Vec<_>>();
412
413        assert_eq!(vec![0x0706050403020100], result);
414    }
415
416    #[test]
417    fn test_iter_unaligned() {
418        let input: &[u8] = &[
419            0b00000000, 0b00000001, 0b00000010, 0b00000100, 0b00001000, 0b00010000, 0b00100000,
420            0b01000000, 0b11111111,
421        ];
422        let buffer: Buffer = Buffer::from(input);
423
424        let bitchunks = buffer.bit_chunks(4, 64);
425
426        assert_eq!(0, bitchunks.remainder_len());
427        assert_eq!(0, bitchunks.remainder_bits());
428
429        let result = bitchunks.into_iter().collect::<Vec<_>>();
430
431        assert_eq!(
432            vec![0b1111010000000010000000010000000010000000010000000010000000010000],
433            result
434        );
435    }
436
437    #[test]
438    fn test_iter_unaligned_remainder_1_byte() {
439        let input: &[u8] = &[
440            0b00000000, 0b00000001, 0b00000010, 0b00000100, 0b00001000, 0b00010000, 0b00100000,
441            0b01000000, 0b11111111,
442        ];
443        let buffer: Buffer = Buffer::from(input);
444
445        let bitchunks = buffer.bit_chunks(4, 66);
446
447        assert_eq!(2, bitchunks.remainder_len());
448        assert_eq!(0b00000011, bitchunks.remainder_bits());
449
450        let result = bitchunks.into_iter().collect::<Vec<_>>();
451
452        assert_eq!(
453            vec![0b1111010000000010000000010000000010000000010000000010000000010000],
454            result
455        );
456    }
457
458    #[test]
459    fn test_iter_unaligned_remainder_bits_across_bytes() {
460        let input: &[u8] = &[0b00111111, 0b11111100];
461        let buffer: Buffer = Buffer::from(input);
462
463        // remainder contains bits from both bytes
464        // result should be the highest 2 bits from first byte followed by lowest 5 bits of second bytes
465        let bitchunks = buffer.bit_chunks(6, 7);
466
467        assert_eq!(7, bitchunks.remainder_len());
468        assert_eq!(0b1110000, bitchunks.remainder_bits());
469    }
470
471    #[test]
472    fn test_iter_unaligned_remainder_bits_large() {
473        let input: &[u8] = &[
474            0b11111111, 0b00000000, 0b11111111, 0b00000000, 0b11111111, 0b00000000, 0b11111111,
475            0b00000000, 0b11111111,
476        ];
477        let buffer: Buffer = Buffer::from(input);
478
479        let bitchunks = buffer.bit_chunks(2, 63);
480
481        assert_eq!(63, bitchunks.remainder_len());
482        assert_eq!(
483            0b100_0000_0011_1111_1100_0000_0011_1111_1100_0000_0011_1111_1100_0000_0011_1111,
484            bitchunks.remainder_bits()
485        );
486    }
487
488    #[test]
489    fn test_iter_remainder_out_of_bounds() {
490        // allocating a full page should trigger a fault when reading out of bounds
491        const ALLOC_SIZE: usize = 4 * 1024;
492        let input = vec![0xFF_u8; ALLOC_SIZE];
493
494        let buffer: Buffer = Buffer::from_vec(input);
495
496        let bitchunks = buffer.bit_chunks(57, ALLOC_SIZE * 8 - 57);
497
498        assert_eq!(u64::MAX, bitchunks.iter().last().unwrap());
499        assert_eq!(0x7F, bitchunks.remainder_bits());
500    }
501
502    #[test]
503    #[should_panic(expected = "offset + len out of bounds")]
504    fn test_out_of_bound_should_panic_length_is_more_than_buffer_length() {
505        const ALLOC_SIZE: usize = 4 * 1024;
506        let input = vec![0xFF_u8; ALLOC_SIZE];
507
508        let buffer: Buffer = Buffer::from_vec(input);
509
510        // We are reading more than exists in the buffer
511        buffer.bit_chunks(0, (ALLOC_SIZE + 1) * 8);
512    }
513
514    #[test]
515    #[should_panic(expected = "offset + len out of bounds")]
516    fn test_out_of_bound_should_panic_length_is_more_than_buffer_length_but_not_when_not_using_ceil()
517     {
518        const ALLOC_SIZE: usize = 4 * 1024;
519        let input = vec![0xFF_u8; ALLOC_SIZE];
520
521        let buffer: Buffer = Buffer::from_vec(input);
522
523        // We are reading more than exists in the buffer
524        buffer.bit_chunks(0, (ALLOC_SIZE * 8) + 1);
525    }
526
527    #[test]
528    #[should_panic(expected = "offset + len out of bounds")]
529    fn test_out_of_bound_should_panic_when_offset_is_not_zero_and_length_is_the_entire_buffer_length()
530     {
531        const ALLOC_SIZE: usize = 4 * 1024;
532        let input = vec![0xFF_u8; ALLOC_SIZE];
533
534        let buffer: Buffer = Buffer::from_vec(input);
535
536        // We are reading more than exists in the buffer
537        buffer.bit_chunks(8, ALLOC_SIZE * 8);
538    }
539
540    #[test]
541    #[should_panic(expected = "offset + len out of bounds")]
542    fn test_out_of_bound_should_panic_when_offset_is_not_zero_and_length_is_the_entire_buffer_length_with_ceil()
543     {
544        const ALLOC_SIZE: usize = 4 * 1024;
545        let input = vec![0xFF_u8; ALLOC_SIZE];
546
547        let buffer: Buffer = Buffer::from_vec(input);
548
549        // We are reading more than exists in the buffer
550        buffer.bit_chunks(1, ALLOC_SIZE * 8);
551    }
552
553    #[test]
554    #[allow(clippy::assertions_on_constants)]
555    fn test_unaligned_bit_chunk_iterator() {
556        let buffer = Buffer::from(&[0xFF; 5]);
557        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 40);
558
559        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
560        assert_eq!(unaligned.lead_padding(), 0);
561        assert_eq!(unaligned.trailing_padding(), 24);
562        // 24x 1 bit then 40x 0 bits
563        assert_eq!(
564            unaligned.prefix(),
565            Some(0b0000000000000000000000001111111111111111111111111111111111111111)
566        );
567        assert_eq!(unaligned.suffix(), None);
568
569        let buffer = buffer.slice(1);
570        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 32);
571
572        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
573        assert_eq!(unaligned.lead_padding(), 0);
574        assert_eq!(unaligned.trailing_padding(), 32);
575        // 32x 1 bit then 32x 0 bits
576        assert_eq!(
577            unaligned.prefix(),
578            Some(0b0000000000000000000000000000000011111111111111111111111111111111)
579        );
580        assert_eq!(unaligned.suffix(), None);
581
582        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 5, 27);
583
584        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
585        assert_eq!(unaligned.lead_padding(), 5); // 5 % 8 == 5
586        assert_eq!(unaligned.trailing_padding(), 32);
587        // 5x 0 bit, 27x 1 bit then 32x 0 bits
588        assert_eq!(
589            unaligned.prefix(),
590            Some(0b0000000000000000000000000000000011111111111111111111111111100000)
591        );
592        assert_eq!(unaligned.suffix(), None);
593
594        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 12, 20);
595
596        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
597        assert_eq!(unaligned.lead_padding(), 4); // 12 % 8 == 4
598        assert_eq!(unaligned.trailing_padding(), 40);
599        // 4x 0 bit, 20x 1 bit then 40x 0 bits
600        assert_eq!(
601            unaligned.prefix(),
602            Some(0b0000000000000000000000000000000000000000111111111111111111110000)
603        );
604        assert_eq!(unaligned.suffix(), None);
605
606        let buffer = Buffer::from(&[0xFF; 14]);
607
608        // Verify buffer alignment
609        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
610        assert_eq!(prefix.len(), 0);
611        assert_eq!(aligned.len(), 1);
612        assert_eq!(suffix.len(), 6);
613
614        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 112);
615
616        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
617        assert_eq!(unaligned.lead_padding(), 0); // No offset and buffer aligned on 64-bit boundary
618        assert_eq!(unaligned.trailing_padding(), 16);
619        assert_eq!(unaligned.prefix(), Some(u64::MAX));
620        assert_eq!(unaligned.suffix(), Some((1 << 48) - 1));
621
622        let buffer = Buffer::from(&[0xFF; 16]);
623
624        // Verify buffer alignment
625        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
626        assert_eq!(prefix.len(), 0);
627        assert_eq!(aligned.len(), 2);
628        assert_eq!(suffix.len(), 0);
629
630        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 128);
631
632        assert_eq!(unaligned.prefix(), Some(u64::MAX));
633        assert_eq!(unaligned.suffix(), Some(u64::MAX));
634        assert!(unaligned.chunks().is_empty()); // Exactly 128 elements
635
636        let buffer = Buffer::from(&[0xFF; 64]);
637
638        // Verify buffer alignment
639        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
640        assert_eq!(prefix.len(), 0);
641        assert_eq!(aligned.len(), 8);
642        assert_eq!(suffix.len(), 0);
643
644        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 512);
645
646        // Buffer is completely aligned and larger than 128 elements -> all in chunks array
647        assert_eq!(unaligned.suffix(), None);
648        assert_eq!(unaligned.prefix(), None);
649        assert_eq!(unaligned.chunks(), [u64::MAX; 8].as_slice());
650        assert_eq!(unaligned.lead_padding(), 0);
651        assert_eq!(unaligned.trailing_padding(), 0);
652
653        let buffer = buffer.slice(1); // Offset buffer 1 byte off 64-bit alignment
654
655        // Verify buffer alignment
656        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
657        assert_eq!(prefix.len(), 7);
658        assert_eq!(aligned.len(), 7);
659        assert_eq!(suffix.len(), 0);
660
661        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 504);
662
663        // Need a prefix with 1 byte of lead padding to bring the buffer into alignment
664        assert_eq!(unaligned.prefix(), Some(u64::MAX - 0xFF));
665        assert_eq!(unaligned.suffix(), None);
666        assert_eq!(unaligned.chunks(), [u64::MAX; 7].as_slice());
667        assert_eq!(unaligned.lead_padding(), 8);
668        assert_eq!(unaligned.trailing_padding(), 0);
669
670        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 17, 300);
671
672        // Out of 64-bit alignment by 8 bits from buffer, and 17 bits from provided offset
673        //   => need 8 + 17 = 25 bits of lead padding + 39 bits in prefix
674        //
675        // This leaves 300 - 17 = 261 bits remaining
676        //   => 4x 64-bit aligned 64-bit chunks + 5 remaining bits
677        //   => trailing padding of 59 bits
678        assert_eq!(unaligned.lead_padding(), 25);
679        assert_eq!(unaligned.trailing_padding(), 59);
680        assert_eq!(unaligned.prefix(), Some(u64::MAX - (1 << 25) + 1));
681        assert_eq!(unaligned.suffix(), Some(0b11111));
682        assert_eq!(unaligned.chunks(), [u64::MAX; 4].as_slice());
683
684        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 17, 0);
685
686        assert_eq!(unaligned.prefix(), None);
687        assert_eq!(unaligned.suffix(), None);
688        assert!(unaligned.chunks().is_empty());
689        assert_eq!(unaligned.lead_padding(), 0);
690        assert_eq!(unaligned.trailing_padding(), 0);
691
692        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 17, 1);
693
694        assert_eq!(unaligned.prefix(), Some(2));
695        assert_eq!(unaligned.suffix(), None);
696        assert!(unaligned.chunks().is_empty());
697        assert_eq!(unaligned.lead_padding(), 1);
698        assert_eq!(unaligned.trailing_padding(), 62);
699    }
700
701    #[test]
702    #[cfg_attr(miri, ignore)]
703    fn fuzz_unaligned_bit_chunk_iterator() {
704        let mut rng = rng();
705
706        let uusize = UniformUsize::new(usize::MIN, usize::MAX).unwrap();
707        for _ in 0..100 {
708            let mask_len = rng.random_range(0..1024);
709            let bools: Vec<_> = std::iter::from_fn(|| Some(rng.random()))
710                .take(mask_len)
711                .collect();
712
713            let buffer = Buffer::from_iter(bools.iter().cloned());
714
715            let max_offset = 64.min(mask_len);
716            let offset = uusize.sample(&mut rng).checked_rem(max_offset).unwrap_or(0);
717
718            let max_truncate = 128.min(mask_len - offset);
719            let truncate = uusize
720                .sample(&mut rng)
721                .checked_rem(max_truncate)
722                .unwrap_or(0);
723
724            let unaligned =
725                UnalignedBitChunk::new(buffer.as_slice(), offset, mask_len - offset - truncate);
726
727            let bool_slice = &bools[offset..mask_len - truncate];
728
729            let count = unaligned.count_ones();
730            let expected_count = bool_slice.iter().filter(|x| **x).count();
731
732            assert_eq!(count, expected_count);
733
734            let collected: Vec<u64> = unaligned.iter().collect();
735
736            let get_bit = |idx: usize| -> bool {
737                let padded_index = idx + unaligned.lead_padding();
738                let byte_idx = padded_index / 64;
739                let bit_idx = padded_index % 64;
740                (collected[byte_idx] & (1 << bit_idx)) != 0
741            };
742
743            for (idx, b) in bool_slice.iter().enumerate() {
744                assert_eq!(*b, get_bit(idx))
745            }
746        }
747    }
748}
arrow_buffer/util/bit_chunk_iterator.rs

arrow_buffer/util/
bit_chunk_iterator.rs
