1 | use super::exp; |
2 | |
3 | /* k is such that k*ln2 has minimal relative error and x - kln2 > log(FLT_MIN) */ |
4 | const K: i32 = 2043; |
5 | |
6 | /* expf(x)/2 for x >= log(FLT_MAX), slightly better than 0.5f*expf(x/2)*expf(x/2) */ |
7 | #[cfg_attr (all(test, assert_no_panic), no_panic::no_panic)] |
8 | pub(crate) fn k_expo2(x: f64) -> f64 { |
9 | let k_ln2: f64 = f64::from_bits(0x40962066151add8b); |
10 | /* note that k is odd and scale*scale overflows */ |
11 | let scale: f64 = f64::from_bits(((((0x3ff + K / 2) as u32) << 20) as u64) << 32); |
12 | /* exp(x - k ln2) * 2**(k-1) */ |
13 | exp(x - k_ln2) * scale * scale |
14 | } |
15 | |