arrow_buffer/util/
bit_chunk_iterator.rs

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9//   http://www.apache.org/licenses/LICENSE-2.0
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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        let end = offset.checked_add(len).expect("offset + len out of bounds");
224        assert!(ceil(end, 8) <= buffer.len(), "offset + len out of bounds");
225
226        let byte_offset = offset / 8;
227        let bit_offset = offset % 8;
228
229        // number of complete u64 chunks
230        let chunk_len = len / 64;
231        // number of remaining bits
232        let remainder_len = len % 64;
233
234        BitChunks::<'a> {
235            buffer: &buffer[byte_offset..],
236            bit_offset,
237            chunk_len,
238            remainder_len,
239        }
240    }
241}
242
243/// Iterator over chunks of 64 bits represented as an u64
244#[derive(Debug)]
245pub struct BitChunkIterator<'a> {
246    buffer: &'a [u8],
247    bit_offset: usize,
248    chunk_len: usize,
249    index: usize,
250}
251
252impl<'a> BitChunks<'a> {
253    /// Returns the number of remaining bits, guaranteed to be between 0 and 63 (inclusive)
254    #[inline]
255    pub const fn remainder_len(&self) -> usize {
256        self.remainder_len
257    }
258
259    /// Returns the number of `u64` chunks
260    #[inline]
261    pub const fn chunk_len(&self) -> usize {
262        self.chunk_len
263    }
264
265    /// Returns the bitmask of remaining bits
266    #[inline]
267    pub fn remainder_bits(&self) -> u64 {
268        let bit_len = self.remainder_len;
269        if bit_len == 0 {
270            0
271        } else {
272            let bit_offset = self.bit_offset;
273            // number of bytes to read
274            // might be one more than sizeof(u64) if the offset is in the middle of a byte
275            let byte_len = ceil(bit_len + bit_offset, 8);
276            // pointer to remainder bytes after all complete chunks
277            let base = unsafe {
278                self.buffer
279                    .as_ptr()
280                    .add(self.chunk_len * std::mem::size_of::<u64>())
281            };
282
283            let mut bits = unsafe { std::ptr::read(base) } as u64 >> bit_offset;
284            for i in 1..byte_len {
285                let byte = unsafe { std::ptr::read(base.add(i)) };
286                bits |= (byte as u64) << (i * 8 - bit_offset);
287            }
288
289            bits & ((1 << bit_len) - 1)
290        }
291    }
292
293    /// Return the number of `u64` that are needed to represent all bits
294    /// (including remainder).
295    ///
296    /// This is equal to `chunk_len + 1` if there is a remainder,
297    /// otherwise it is equal to `chunk_len`.
298    #[inline]
299    pub fn num_u64s(&self) -> usize {
300        if self.remainder_len == 0 {
301            self.chunk_len
302        } else {
303            self.chunk_len + 1
304        }
305    }
306
307    /// Return the number of *bytes* that are needed to represent all bits
308    /// (including remainder).
309    #[inline]
310    pub fn num_bytes(&self) -> usize {
311        ceil(self.chunk_len * 64 + self.remainder_len, 8)
312    }
313
314    /// Returns an iterator over chunks of 64 bits represented as an `u64`
315    #[inline]
316    pub const fn iter(&self) -> BitChunkIterator<'a> {
317        BitChunkIterator::<'a> {
318            buffer: self.buffer,
319            bit_offset: self.bit_offset,
320            chunk_len: self.chunk_len,
321            index: 0,
322        }
323    }
324
325    /// Returns an iterator over chunks of 64 bits, with the remaining bits zero padded to 64-bits
326    #[inline]
327    pub fn iter_padded(&self) -> impl Iterator<Item = u64> + 'a {
328        self.iter().chain(std::iter::once(self.remainder_bits()))
329    }
330}
331
332impl<'a> IntoIterator for BitChunks<'a> {
333    type Item = u64;
334    type IntoIter = BitChunkIterator<'a>;
335
336    fn into_iter(self) -> Self::IntoIter {
337        self.iter()
338    }
339}
340
341impl Iterator for BitChunkIterator<'_> {
342    type Item = u64;
343
344    #[inline]
345    fn next(&mut self) -> Option<u64> {
346        let index = self.index;
347        if index >= self.chunk_len {
348            return None;
349        }
350
351        // cast to *const u64 should be fine since we are using read_unaligned below
352        #[allow(clippy::cast_ptr_alignment)]
353        let raw_data = self.buffer.as_ptr() as *const u64;
354
355        // bit-packed buffers are stored starting with the least-significant byte first
356        // so when reading as u64 on a big-endian machine, the bytes need to be swapped
357        let current = unsafe { std::ptr::read_unaligned(raw_data.add(index)).to_le() };
358
359        let bit_offset = self.bit_offset;
360
361        let combined = if bit_offset == 0 {
362            current
363        } else {
364            // the constructor ensures that bit_offset is in 0..8
365            // that means we need to read at most one additional byte to fill in the high bits
366            let next =
367                unsafe { std::ptr::read_unaligned(raw_data.add(index + 1) as *const u8) as u64 };
368
369            (current >> bit_offset) | (next << (64 - bit_offset))
370        };
371
372        self.index = index + 1;
373
374        Some(combined)
375    }
376
377    #[inline]
378    fn size_hint(&self) -> (usize, Option<usize>) {
379        (
380            self.chunk_len - self.index,
381            Some(self.chunk_len - self.index),
382        )
383    }
384}
385
386impl ExactSizeIterator for BitChunkIterator<'_> {
387    #[inline]
388    fn len(&self) -> usize {
389        self.chunk_len - self.index
390    }
391}
392
393#[cfg(test)]
394mod tests {
395    use rand::distr::uniform::UniformSampler;
396    use rand::distr::uniform::UniformUsize;
397    use rand::prelude::*;
398    use rand::rng;
399
400    use crate::buffer::Buffer;
401    use crate::util::bit_chunk_iterator::UnalignedBitChunk;
402
403    #[test]
404    fn test_iter_aligned() {
405        let input: &[u8] = &[0, 1, 2, 3, 4, 5, 6, 7];
406        let buffer: Buffer = Buffer::from(input);
407
408        let bitchunks = buffer.bit_chunks(0, 64);
409        let result = bitchunks.into_iter().collect::<Vec<_>>();
410
411        assert_eq!(vec![0x0706050403020100], result);
412    }
413
414    #[test]
415    fn test_iter_unaligned() {
416        let input: &[u8] = &[
417            0b00000000, 0b00000001, 0b00000010, 0b00000100, 0b00001000, 0b00010000, 0b00100000,
418            0b01000000, 0b11111111,
419        ];
420        let buffer: Buffer = Buffer::from(input);
421
422        let bitchunks = buffer.bit_chunks(4, 64);
423
424        assert_eq!(0, bitchunks.remainder_len());
425        assert_eq!(0, bitchunks.remainder_bits());
426
427        let result = bitchunks.into_iter().collect::<Vec<_>>();
428
429        assert_eq!(
430            vec![0b1111010000000010000000010000000010000000010000000010000000010000],
431            result
432        );
433    }
434
435    #[test]
436    fn test_iter_unaligned_remainder_1_byte() {
437        let input: &[u8] = &[
438            0b00000000, 0b00000001, 0b00000010, 0b00000100, 0b00001000, 0b00010000, 0b00100000,
439            0b01000000, 0b11111111,
440        ];
441        let buffer: Buffer = Buffer::from(input);
442
443        let bitchunks = buffer.bit_chunks(4, 66);
444
445        assert_eq!(2, bitchunks.remainder_len());
446        assert_eq!(0b00000011, bitchunks.remainder_bits());
447
448        let result = bitchunks.into_iter().collect::<Vec<_>>();
449
450        assert_eq!(
451            vec![0b1111010000000010000000010000000010000000010000000010000000010000],
452            result
453        );
454    }
455
456    #[test]
457    fn test_iter_unaligned_remainder_bits_across_bytes() {
458        let input: &[u8] = &[0b00111111, 0b11111100];
459        let buffer: Buffer = Buffer::from(input);
460
461        // remainder contains bits from both bytes
462        // result should be the highest 2 bits from first byte followed by lowest 5 bits of second bytes
463        let bitchunks = buffer.bit_chunks(6, 7);
464
465        assert_eq!(7, bitchunks.remainder_len());
466        assert_eq!(0b1110000, bitchunks.remainder_bits());
467    }
468
469    #[test]
470    fn test_iter_unaligned_remainder_bits_large() {
471        let input: &[u8] = &[
472            0b11111111, 0b00000000, 0b11111111, 0b00000000, 0b11111111, 0b00000000, 0b11111111,
473            0b00000000, 0b11111111,
474        ];
475        let buffer: Buffer = Buffer::from(input);
476
477        let bitchunks = buffer.bit_chunks(2, 63);
478
479        assert_eq!(63, bitchunks.remainder_len());
480        assert_eq!(
481            0b100_0000_0011_1111_1100_0000_0011_1111_1100_0000_0011_1111_1100_0000_0011_1111,
482            bitchunks.remainder_bits()
483        );
484    }
485
486    #[test]
487    fn test_iter_remainder_out_of_bounds() {
488        // allocating a full page should trigger a fault when reading out of bounds
489        const ALLOC_SIZE: usize = 4 * 1024;
490        let input = vec![0xFF_u8; ALLOC_SIZE];
491
492        let buffer: Buffer = Buffer::from_vec(input);
493
494        let bitchunks = buffer.bit_chunks(57, ALLOC_SIZE * 8 - 57);
495
496        assert_eq!(u64::MAX, bitchunks.iter().last().unwrap());
497        assert_eq!(0x7F, bitchunks.remainder_bits());
498    }
499
500    #[test]
501    #[should_panic(expected = "offset + len out of bounds")]
502    fn test_out_of_bound_should_panic_length_is_more_than_buffer_length() {
503        const ALLOC_SIZE: usize = 4 * 1024;
504        let input = vec![0xFF_u8; ALLOC_SIZE];
505
506        let buffer: Buffer = Buffer::from_vec(input);
507
508        // We are reading more than exists in the buffer
509        buffer.bit_chunks(0, (ALLOC_SIZE + 1) * 8);
510    }
511
512    #[test]
513    #[should_panic(expected = "offset + len out of bounds")]
514    fn test_out_of_bound_should_panic_length_is_more_than_buffer_length_but_not_when_not_using_ceil()
515     {
516        const ALLOC_SIZE: usize = 4 * 1024;
517        let input = vec![0xFF_u8; ALLOC_SIZE];
518
519        let buffer: Buffer = Buffer::from_vec(input);
520
521        // We are reading more than exists in the buffer
522        buffer.bit_chunks(0, (ALLOC_SIZE * 8) + 1);
523    }
524
525    #[test]
526    #[should_panic(expected = "offset + len out of bounds")]
527    fn test_out_of_bound_should_panic_when_offset_is_not_zero_and_length_is_the_entire_buffer_length()
528     {
529        const ALLOC_SIZE: usize = 4 * 1024;
530        let input = vec![0xFF_u8; ALLOC_SIZE];
531
532        let buffer: Buffer = Buffer::from_vec(input);
533
534        // We are reading more than exists in the buffer
535        buffer.bit_chunks(8, ALLOC_SIZE * 8);
536    }
537
538    #[test]
539    #[should_panic(expected = "offset + len out of bounds")]
540    fn test_out_of_bound_should_panic_when_offset_is_not_zero_and_length_is_the_entire_buffer_length_with_ceil()
541     {
542        const ALLOC_SIZE: usize = 4 * 1024;
543        let input = vec![0xFF_u8; ALLOC_SIZE];
544
545        let buffer: Buffer = Buffer::from_vec(input);
546
547        // We are reading more than exists in the buffer
548        buffer.bit_chunks(1, ALLOC_SIZE * 8);
549    }
550
551    #[test]
552    #[should_panic(expected = "offset + len out of bounds")]
553    fn test_out_of_bound_should_panic_when_offset_and_length_overflow() {
554        let buffer = Buffer::from(vec![0xFF_u8; 8]);
555        buffer.bit_chunks(1, usize::MAX);
556    }
557
558    #[test]
559    #[allow(clippy::assertions_on_constants)]
560    fn test_unaligned_bit_chunk_iterator() {
561        let buffer = Buffer::from(&[0xFF; 5]);
562        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 40);
563
564        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
565        assert_eq!(unaligned.lead_padding(), 0);
566        assert_eq!(unaligned.trailing_padding(), 24);
567        // 24x 1 bit then 40x 0 bits
568        assert_eq!(
569            unaligned.prefix(),
570            Some(0b0000000000000000000000001111111111111111111111111111111111111111)
571        );
572        assert_eq!(unaligned.suffix(), None);
573
574        let buffer = buffer.slice(1);
575        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 32);
576
577        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
578        assert_eq!(unaligned.lead_padding(), 0);
579        assert_eq!(unaligned.trailing_padding(), 32);
580        // 32x 1 bit then 32x 0 bits
581        assert_eq!(
582            unaligned.prefix(),
583            Some(0b0000000000000000000000000000000011111111111111111111111111111111)
584        );
585        assert_eq!(unaligned.suffix(), None);
586
587        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 5, 27);
588
589        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
590        assert_eq!(unaligned.lead_padding(), 5); // 5 % 8 == 5
591        assert_eq!(unaligned.trailing_padding(), 32);
592        // 5x 0 bit, 27x 1 bit then 32x 0 bits
593        assert_eq!(
594            unaligned.prefix(),
595            Some(0b0000000000000000000000000000000011111111111111111111111111100000)
596        );
597        assert_eq!(unaligned.suffix(), None);
598
599        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 12, 20);
600
601        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
602        assert_eq!(unaligned.lead_padding(), 4); // 12 % 8 == 4
603        assert_eq!(unaligned.trailing_padding(), 40);
604        // 4x 0 bit, 20x 1 bit then 40x 0 bits
605        assert_eq!(
606            unaligned.prefix(),
607            Some(0b0000000000000000000000000000000000000000111111111111111111110000)
608        );
609        assert_eq!(unaligned.suffix(), None);
610
611        let buffer = Buffer::from(&[0xFF; 14]);
612
613        // Verify buffer alignment
614        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
615        assert_eq!(prefix.len(), 0);
616        assert_eq!(aligned.len(), 1);
617        assert_eq!(suffix.len(), 6);
618
619        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 112);
620
621        assert!(unaligned.chunks().is_empty()); // Less than 128 elements
622        assert_eq!(unaligned.lead_padding(), 0); // No offset and buffer aligned on 64-bit boundary
623        assert_eq!(unaligned.trailing_padding(), 16);
624        assert_eq!(unaligned.prefix(), Some(u64::MAX));
625        assert_eq!(unaligned.suffix(), Some((1 << 48) - 1));
626
627        let buffer = Buffer::from(&[0xFF; 16]);
628
629        // Verify buffer alignment
630        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
631        assert_eq!(prefix.len(), 0);
632        assert_eq!(aligned.len(), 2);
633        assert_eq!(suffix.len(), 0);
634
635        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 128);
636
637        assert_eq!(unaligned.prefix(), Some(u64::MAX));
638        assert_eq!(unaligned.suffix(), Some(u64::MAX));
639        assert!(unaligned.chunks().is_empty()); // Exactly 128 elements
640
641        let buffer = Buffer::from(&[0xFF; 64]);
642
643        // Verify buffer alignment
644        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
645        assert_eq!(prefix.len(), 0);
646        assert_eq!(aligned.len(), 8);
647        assert_eq!(suffix.len(), 0);
648
649        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 512);
650
651        // Buffer is completely aligned and larger than 128 elements -> all in chunks array
652        assert_eq!(unaligned.suffix(), None);
653        assert_eq!(unaligned.prefix(), None);
654        assert_eq!(unaligned.chunks(), [u64::MAX; 8].as_slice());
655        assert_eq!(unaligned.lead_padding(), 0);
656        assert_eq!(unaligned.trailing_padding(), 0);
657
658        let buffer = buffer.slice(1); // Offset buffer 1 byte off 64-bit alignment
659
660        // Verify buffer alignment
661        let (prefix, aligned, suffix) = unsafe { buffer.as_slice().align_to::<u64>() };
662        assert_eq!(prefix.len(), 7);
663        assert_eq!(aligned.len(), 7);
664        assert_eq!(suffix.len(), 0);
665
666        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 0, 504);
667
668        // Need a prefix with 1 byte of lead padding to bring the buffer into alignment
669        assert_eq!(unaligned.prefix(), Some(u64::MAX - 0xFF));
670        assert_eq!(unaligned.suffix(), None);
671        assert_eq!(unaligned.chunks(), [u64::MAX; 7].as_slice());
672        assert_eq!(unaligned.lead_padding(), 8);
673        assert_eq!(unaligned.trailing_padding(), 0);
674
675        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 17, 300);
676
677        // Out of 64-bit alignment by 8 bits from buffer, and 17 bits from provided offset
678        //   => need 8 + 17 = 25 bits of lead padding + 39 bits in prefix
679        //
680        // This leaves 300 - 17 = 261 bits remaining
681        //   => 4x 64-bit aligned 64-bit chunks + 5 remaining bits
682        //   => trailing padding of 59 bits
683        assert_eq!(unaligned.lead_padding(), 25);
684        assert_eq!(unaligned.trailing_padding(), 59);
685        assert_eq!(unaligned.prefix(), Some(u64::MAX - (1 << 25) + 1));
686        assert_eq!(unaligned.suffix(), Some(0b11111));
687        assert_eq!(unaligned.chunks(), [u64::MAX; 4].as_slice());
688
689        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 17, 0);
690
691        assert_eq!(unaligned.prefix(), None);
692        assert_eq!(unaligned.suffix(), None);
693        assert!(unaligned.chunks().is_empty());
694        assert_eq!(unaligned.lead_padding(), 0);
695        assert_eq!(unaligned.trailing_padding(), 0);
696
697        let unaligned = UnalignedBitChunk::new(buffer.as_slice(), 17, 1);
698
699        assert_eq!(unaligned.prefix(), Some(2));
700        assert_eq!(unaligned.suffix(), None);
701        assert!(unaligned.chunks().is_empty());
702        assert_eq!(unaligned.lead_padding(), 1);
703        assert_eq!(unaligned.trailing_padding(), 62);
704    }
705
706    #[test]
707    #[cfg_attr(miri, ignore)]
708    fn fuzz_unaligned_bit_chunk_iterator() {
709        let mut rng = rng();
710
711        let uusize = UniformUsize::new(usize::MIN, usize::MAX).unwrap();
712        for _ in 0..100 {
713            let mask_len = rng.random_range(0..1024);
714            let bools: Vec<_> = std::iter::from_fn(|| Some(rng.random()))
715                .take(mask_len)
716                .collect();
717
718            let buffer = Buffer::from_iter(bools.iter().cloned());
719
720            let max_offset = 64.min(mask_len);
721            let offset = uusize.sample(&mut rng).checked_rem(max_offset).unwrap_or(0);
722
723            let max_truncate = 128.min(mask_len - offset);
724            let truncate = uusize
725                .sample(&mut rng)
726                .checked_rem(max_truncate)
727                .unwrap_or(0);
728
729            let unaligned =
730                UnalignedBitChunk::new(buffer.as_slice(), offset, mask_len - offset - truncate);
731
732            let bool_slice = &bools[offset..mask_len - truncate];
733
734            let count = unaligned.count_ones();
735            let expected_count = bool_slice.iter().filter(|x| **x).count();
736
737            assert_eq!(count, expected_count);
738
739            let collected: Vec<u64> = unaligned.iter().collect();
740
741            let get_bit = |idx: usize| -> bool {
742                let padded_index = idx + unaligned.lead_padding();
743                let byte_idx = padded_index / 64;
744                let bit_idx = padded_index % 64;
745                (collected[byte_idx] & (1 << bit_idx)) != 0
746            };
747
748            for (idx, b) in bool_slice.iter().enumerate() {
749                assert_eq!(*b, get_bit(idx))
750            }
751        }
752    }
753}
arrow_buffer/util/bit_chunk_iterator.rs

arrow_buffer/util/
bit_chunk_iterator.rs
