pxfm/gamma/
betaf.rs

1/*
2 * // Copyright (c) Radzivon Bartoshyk 9/2025. 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 */
29use crate::exponents::core_expdf;
30use crate::gamma::lgamma_rf::lgamma_coref;
31
32/// Computes beta function
33pub fn f_betaf(a: f32, b: f32) -> f32 {
34    let ax = a.to_bits();
35    let bx = b.to_bits();
36
37    if ax >= 0xffu32 << 23 || ax == 0 || bx >= 0xffu32 << 23 || bx == 0 {
38        if (ax >> 31) != 0 || (bx >> 31) != 0 {
39            // |a| < 0 or |b| < 0
40            return f32::NAN;
41        }
42        if ax.wrapping_shl(1) == 0 || bx.wrapping_shl(1) == 0 {
43            // |a| == 0 || |b| == 0
44            if ax.wrapping_shl(1) != 0 || bx.wrapping_shl(1) != 0 {
45                return f32::INFINITY;
46            }
47            return f32::NAN;
48        }
49        if a.is_infinite() || b.is_infinite() {
50            // |a| == inf or |b| == inf
51            return 0.;
52        }
53        return a + f32::NAN; // nan
54    }
55
56    let mut sign = 1i32;
57    let (mut y, sgngamf) = lgamma_coref(a + b);
58    sign *= sgngamf; /* keep track of the sign */
59    let (y1, sgngamf) = lgamma_coref(b);
60    y = y1 - y;
61    sign *= sgngamf;
62    let (y1, sgngamf) = lgamma_coref(a);
63    y += y1;
64    sign *= sgngamf;
65    if y <= -104. {
66        return 0.;
67    }
68    if y >= 89. {
69        // x > 89
70        return f32::INFINITY;
71    }
72    (core_expdf(y) * (sign as f64)) as f32
73}
74
75#[cfg(test)]
76mod tests {
77    use crate::f_betaf;
78
79    #[test]
80    fn test_betaf() {
81        assert_eq!(f_betaf(f32::INFINITY, 1.), 0.);
82        assert_eq!(f_betaf(f32::INFINITY, 0.), f32::INFINITY);
83        assert!(f_betaf(0., 0.).is_nan());
84        assert_eq!(f_betaf(0., f32::INFINITY), f32::INFINITY);
85        assert!(f_betaf(-5., 15.).is_nan());
86        assert!(f_betaf(5., -15.).is_nan());
87        assert!(f_betaf(f32::NAN, 15.).is_nan());
88        assert!(f_betaf(15., f32::NAN).is_nan());
89        assert_eq!(f_betaf(f32::INFINITY, 1.), 0.);
90        assert_eq!(f_betaf(1., f32::INFINITY), 0.);
91        assert_eq!(f_betaf(5., 3.), 0.00952381);
92        assert_eq!(f_betaf(3., 5.), 0.00952381);
93        assert_eq!(f_betaf(12., 23.), 1.5196995e-10);
94    }
95}
pxfm/gamma/betaf.rs

pxfm/gamma/
betaf.rs
