0a33394d17
This also adds expo2 for the __expo2 function, and combine_words() to replace the INSERT_WORDS macro. Closes #35
14 lines
516 B
Rust
14 lines
516 B
Rust
use super::{combine_words, exp};
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/* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */
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pub(crate) fn expo2(x: f64) -> f64 {
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/* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN) */
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const K: i32 = 2043;
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let kln2 = f64::from_bits(0x40962066151add8b);
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/* note that k is odd and scale*scale overflows */
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let scale = combine_words(((0x3ff + K / 2) as u32) << 20, 0);
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/* exp(x - k ln2) * 2**(k-1) */
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return exp(x - kln2) * scale * scale;
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}
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