pxfm/gamma/
lgamma_r.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::common::{is_integer, is_odd_integer};
30use crate::gamma::lgamma::lgamma_core;
31
32/// Computes log(gamma(x))
33///
34/// Returns gamma value + sign.
35pub fn f_lgamma_r(x: f64) -> (f64, i32) {
36    // filter out exceptional cases
37    let xb = x.to_bits().wrapping_shl(1);
38    if xb >= 0x7ffu64 << 53 || xb == 0 {
39        if x.is_nan() {
40            return (f64::NAN, 1);
41        }
42        if xb == 0 {
43            return (f64::INFINITY, 1 - 2 * ((x.to_bits() >> 63) as i32));
44        }
45        if xb.wrapping_shl(11) == 0 {
46            return (f64::INFINITY, 1);
47        }
48    }
49
50    if is_integer(x) {
51        if x == 2. || x == 1. {
52            return (0., 1);
53        }
54        if x.is_sign_negative() {
55            let is_odd = (!is_odd_integer(x)) as i32;
56            return (f64::INFINITY, 1 - (is_odd << 1));
57        }
58    }
59
60    let (r, sign_gam) = lgamma_core(x);
61    (r.to_f64(), sign_gam)
62}
63
64#[cfg(test)]
65mod tests {
66    use crate::f_lgamma_r;
67
68    #[test]
69    fn test_lgamma_rf() {
70        assert_eq!(f_lgamma_r(-0.), (f64::INFINITY, -1));
71        assert_eq!(f_lgamma_r(f64::NEG_INFINITY), (f64::INFINITY, 1));
72        assert_eq!(f_lgamma_r(f64::INFINITY), (f64::INFINITY, 1));
73        assert_eq!(f_lgamma_r(0.), (f64::INFINITY, 1));
74        assert_eq!(f_lgamma_r(5.), (3.1780538303479458, 1));
75        assert_eq!(f_lgamma_r(-4.5), (-2.813084081769316, -1));
76        assert_eq!(f_lgamma_r(-2.0015738), (5.759666328573806, -1));
77    }
78}
pxfm/gamma/lgamma_r.rs

pxfm/gamma/
lgamma_r.rs
