lance_core/utils/
bit.rs

1// SPDX-License-Identifier: Apache-2.0
2// SPDX-FileCopyrightText: Copyright The Lance Authors
3
4pub fn is_pwr_two(n: u64) -> bool {
5    n & (n - 1) == 0
6}
7
8pub fn pad_bytes<const ALIGN: usize>(n: usize) -> usize {
9    debug_assert!(is_pwr_two(ALIGN as u64));
10    (ALIGN - (n & (ALIGN - 1))) & (ALIGN - 1)
11}
12
13pub fn pad_bytes_to(n: usize, align: usize) -> usize {
14    debug_assert!(is_pwr_two(align as u64));
15    (align - (n & (align - 1))) & (align - 1)
16}
17
18pub fn pad_bytes_u64<const ALIGN: u64>(n: u64) -> u64 {
19    debug_assert!(is_pwr_two(ALIGN));
20    (ALIGN - (n & (ALIGN - 1))) & (ALIGN - 1)
21}
22
23// This is a lookup table for the log2 of the first 256 numbers
24const LOG_TABLE_256: [u8; 256] = [
25    0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
26    6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
27    7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
28    7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
29    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
30    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
31    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
32    8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
33];
34
35/// Returns the number of bits needed to represent the given number
36///
37/// Inspired by https://graphics.stanford.edu/~seander/bithacks.html
38pub fn log_2_ceil(val: u32) -> u32 {
39    assert!(val > 0);
40    let upper_half = val >> 16;
41    if upper_half == 0 {
42        let third_quarter = val >> 8;
43        if third_quarter == 0 {
44            // Use lowest 8 bits (upper 24 are 0)
45            LOG_TABLE_256[val as usize] as u32
46        } else {
47            // Use bits 16..24 (0..16 are 0)
48            LOG_TABLE_256[third_quarter as usize] as u32 + 8
49        }
50    } else {
51        let first_quarter = upper_half >> 8;
52        if first_quarter == 0 {
53            // Use bits 8..16 (0..8 are 0)
54            16 + LOG_TABLE_256[upper_half as usize] as u32
55        } else {
56            // Use most significant bits (it's a big number!)
57            24 + LOG_TABLE_256[first_quarter as usize] as u32
58        }
59    }
60}
61
62#[cfg(test)]
63pub mod tests {
64    use crate::utils::bit::log_2_ceil;
65
66    #[test]
67    fn test_log_2_ceil() {
68        fn classic_approach(mut val: u32) -> u32 {
69            let mut counter = 0;
70            while val > 0 {
71                val >>= 1;
72                counter += 1;
73            }
74            counter
75        }
76
77        for i in 1..(16 * 1024) {
78            assert_eq!(log_2_ceil(i), classic_approach(i));
79        }
80        assert_eq!(log_2_ceil(50 * 1024), classic_approach(50 * 1024));
81        assert_eq!(
82            log_2_ceil(1024 * 1024 * 1024),
83            classic_approach(1024 * 1024 * 1024)
84        );
85    }
86}
lance_core/utils/bit.rs

lance_core/utils/
bit.rs
