/
bit_util.rs
94 lines (81 loc) · 3.25 KB
/
bit_util.rs
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// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.
use arrow::util::bit_chunk_iterator::UnalignedBitChunk;
use std::ops::Range;
/// Counts the number of set bits in the provided range
pub fn count_set_bits(bytes: &[u8], range: Range<usize>) -> usize {
let unaligned = UnalignedBitChunk::new(bytes, range.start, range.end - range.start);
unaligned.count_ones()
}
/// Iterates through the set bit positions in `bytes` in reverse order
pub fn iter_set_bits_rev(bytes: &[u8]) -> impl Iterator<Item = usize> + '_ {
let bit_length = bytes.len() * 8;
let unaligned = UnalignedBitChunk::new(bytes, 0, bit_length);
let mut chunk_end_idx =
bit_length + unaligned.lead_padding() + unaligned.trailing_padding();
let iter = unaligned
.prefix()
.into_iter()
.chain(unaligned.chunks().iter().cloned())
.chain(unaligned.suffix().into_iter());
iter.rev().flat_map(move |mut chunk| {
let chunk_idx = chunk_end_idx - 64;
chunk_end_idx = chunk_idx;
std::iter::from_fn(move || {
if chunk != 0 {
let bit_pos = 63 - chunk.leading_zeros();
chunk ^= 1 << bit_pos;
return Some(chunk_idx + (bit_pos as usize));
}
None
})
})
}
#[cfg(test)]
mod tests {
use super::*;
use arrow::array::BooleanBufferBuilder;
use rand::prelude::*;
#[test]
fn test_bit_fns() {
let mut rng = thread_rng();
let mask_length = rng.gen_range(1..1024);
let bools: Vec<_> = std::iter::from_fn(|| Some(rng.next_u32() & 1 == 0))
.take(mask_length)
.collect();
let mut nulls = BooleanBufferBuilder::new(mask_length);
bools.iter().for_each(|b| nulls.append(*b));
let actual: Vec<_> = iter_set_bits_rev(nulls.as_slice()).collect();
let expected: Vec<_> = bools
.iter()
.enumerate()
.rev()
.filter_map(|(x, y)| y.then(|| x))
.collect();
assert_eq!(actual, expected);
assert_eq!(iter_set_bits_rev(&[]).count(), 0);
assert_eq!(count_set_bits(&[], 0..0), 0);
assert_eq!(count_set_bits(&[0xFF], 1..1), 0);
for _ in 0..20 {
let start = rng.gen_range(0..bools.len());
let end = rng.gen_range(start..bools.len());
let actual = count_set_bits(nulls.as_slice(), start..end);
let expected = bools[start..end].iter().filter(|x| **x).count();
assert_eq!(actual, expected);
}
}
}