pxfm/gamma/
lnbeta.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::double_double::DoubleDouble;
30use crate::gamma::lgamma::lgamma_core;
31
32/// Computes log(beta(x)) function
33pub fn f_lnbeta(a: f64, b: f64) -> f64 {
34    let ax = a.to_bits();
35    let bx = b.to_bits();
36
37    if ax >= 0x7ffu64 << 52 || ax == 0 || bx >= 0x7ffu64 << 52 || bx == 0 {
38        if (ax >> 63) != 0 || (bx >> 63) != 0 {
39            // |a| < 0 or |b| < 0
40            return f64::NAN;
41        }
42        if ax.wrapping_shl(1) == 0 || bx.wrapping_shl(1) == 0 {
43            // |a| == 0 || |b| == 0
44            if ax == 0 || bx == 0 {
45                // |a| == 0 || |b| == 0
46                if ax.wrapping_shl(1) != 0 || bx.wrapping_shl(1) != 0 {
47                    return f64::INFINITY;
48                }
49                return f64::NAN;
50            }
51            return f64::NAN;
52        }
53        if a.is_infinite() || b.is_infinite() {
54            // |a| == inf or |b| == inf
55            return f64::NEG_INFINITY;
56        }
57        return a + f64::NAN; // nan
58    }
59
60    let (mut y, _) = lgamma_core(DoubleDouble::from_full_exact_add(a, b).to_f64());
61    let (y1, _) = lgamma_core(b);
62    y = DoubleDouble::quick_dd_sub(y1, y);
63    let (y1, _) = lgamma_core(a);
64    y = DoubleDouble::quick_dd_add(y1, y);
65    y.to_f64()
66}
67
68pub(crate) fn lnbeta_core(a: f64, b: f64) -> DoubleDouble {
69    let (mut y, _) = lgamma_core(DoubleDouble::from_full_exact_add(a, b).to_f64());
70    let (y1, _) = lgamma_core(b);
71    y = DoubleDouble::quick_dd_sub(y1, y);
72    let (y1, _) = lgamma_core(a);
73    DoubleDouble::quick_dd_add(y1, y)
74}
75
76#[cfg(test)]
77mod tests {
78    use crate::f_lnbeta;
79
80    #[test]
81    fn test_beta() {
82        assert_eq!(f_lnbeta(f64::INFINITY, 0.), f64::INFINITY);
83        assert!(f_lnbeta(0., 0.).is_nan());
84        assert_eq!(f_lnbeta(0., f64::INFINITY), f64::INFINITY);
85        assert!(f_lnbeta(-5., 15.).is_nan());
86        assert!(f_lnbeta(5., -15.).is_nan());
87        assert!(f_lnbeta(f64::NAN, 15.).is_nan());
88        assert!(f_lnbeta(15., f64::NAN).is_nan());
89        assert_eq!(f_lnbeta(f64::INFINITY, 1.), f64::NEG_INFINITY);
90        assert_eq!(f_lnbeta(1., f64::INFINITY), f64::NEG_INFINITY);
91        assert_eq!(f_lnbeta(5., 3.), -4.653960350157523);
92        assert_eq!(f_lnbeta(3., 5.), -4.653960350157523);
93        assert_eq!(f_lnbeta(12., 23.), -22.607338344488568);
94    }
95}
pxfm/gamma/lnbeta.rs

pxfm/gamma/
lnbeta.rs
