pxfm/square_root/
rsqrtf.rs

1/*
2 * // Copyright (c) Radzivon Bartoshyk 8/2025. All rights reserved.
3 * //
4 * // Redistribution and use in source and binary forms, with or without modification,
5 * // are permitted provided that the following conditions are met:
6 * //
7 * // 1.  Redistributions of source code must retain the above copyright notice, this
8 * // list of conditions and the following disclaimer.
9 * //
10 * // 2.  Redistributions in binary form must reproduce the above copyright notice,
11 * // this list of conditions and the following disclaimer in the documentation
12 * // and/or other materials provided with the distribution.
13 * //
14 * // 3.  Neither the name of the copyright holder nor the names of its
15 * // contributors may be used to endorse or promote products derived from
16 * // this software without specific prior written permission.
17 * //
18 * // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
19 * // AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
20 * // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
21 * // DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
22 * // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
23 * // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
24 * // SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
25 * // CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
26 * // OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
27 * // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
28 */
29
30/// Computes 1/sqrt(x)
31///
32/// Max ULP 0.5
33#[inline]
34pub fn f_rsqrtf(x: f32) -> f32 {
35    let ix = x.to_bits();
36
37    // filter out exceptional cases
38    if ix >= 0xffu32 << 23 || ix == 0 {
39        if ix.wrapping_shl(1) == 0 {
40            return 1.0 / x; // +/-0
41        }
42        if (ix >> 31) != 0 {
43            // x < 0
44            return f32::NAN;
45        }
46        if ix.wrapping_shl(9) == 0 {
47            // |x| == Inf
48            return 0.;
49        }
50        return x + x; // nan
51    }
52
53    let dx = x as f64;
54    ((1. / dx) * dx.sqrt()) as f32
55}
56
57#[cfg(test)]
58mod tests {
59    use super::*;
60
61    #[test]
62    fn test_rsqrtf() {
63        assert_eq!(f_rsqrtf(0.0), f32::INFINITY);
64        assert_eq!(f_rsqrtf(4.0), 0.5);
65        assert_eq!(f_rsqrtf(9.0), 1. / 3.);
66        assert_eq!(f_rsqrtf(-0.0), f32::NEG_INFINITY);
67        assert!(f_rsqrtf(f32::NAN).is_nan());
68    }
69}
pxfm/square_root/rsqrtf.rs

pxfm/square_root/
rsqrtf.rs
