1use super::expm1;
2
3/* tanh(x) = (exp(x) - exp(-x))/(exp(x) + exp(-x))
4 * = (exp(2*x) - 1)/(exp(2*x) - 1 + 2)
5 * = (1 - exp(-2*x))/(exp(-2*x) - 1 + 2)
6 */
7#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
8pub fn tanh(mut x: f64) -> f64 {
9 let mut uf: f64 = x;
10 let mut ui: u64 = f64::to_bits(uf);
11
12 let w: u32;
13 let sign: bool;
14 let mut t: f64;
15
16 /* x = |x| */
17 sign = ui >> 63 != 0;
18 ui &= !1 / 2;
19 uf = f64::from_bits(ui);
20 x = uf;
21 w = (ui >> 32) as u32;
22
23 if w > 0x3fe193ea {
24 /* |x| > log(3)/2 ~= 0.5493 or nan */
25 if w > 0x40340000 {
26 /* |x| > 20 or nan */
27 /* note: this branch avoids raising overflow */
28 t = 1.0 - 0.0 / x;
29 } else {
30 t = expm1(2.0 * x);
31 t = 1.0 - 2.0 / (t + 2.0);
32 }
33 } else if w > 0x3fd058ae {
34 /* |x| > log(5/3)/2 ~= 0.2554 */
35 t = expm1(2.0 * x);
36 t = t / (t + 2.0);
37 } else if w >= 0x00100000 {
38 /* |x| >= 0x1p-1022, up to 2ulp error in [0.1,0.2554] */
39 t = expm1(-2.0 * x);
40 t = -t / (t + 2.0);
41 } else {
42 /* |x| is subnormal */
43 /* note: the branch above would not raise underflow in [0x1p-1023,0x1p-1022) */
44 force_eval!(x as f32);
45 t = x;
46 }
47
48 if sign {
49 -t
50 } else {
51 t
52 }
53}
54