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statrs 0.3.0

Statistical computing library for Rust
Documentation 
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use std::f64;
use function::internal;

/// Calculates a numerically stable `exp(x) - 1`
///
/// # Remarks
///
/// Returns `f64::NAN` if `pow` is `f64::NAN`
pub fn exp_minus_one(pow: f64) -> f64 {
    if pow.is_nan() {
        return f64::NAN;
    }

    let x = pow.abs();
    if x > 0.1 {
        pow.exp() - 1.0
    } else {
        let mut k = 0;
        let mut term = 1.0;
        internal::series(|| {
            k += 1;
            term *= pow;
            term /= k as f64;
            term
        })
    }
}

#[cfg_attr(rustfmt, rustfmt_skip)]
#[cfg(test)]
mod test {
    use std::f64;
    
    #[test]
    fn test_exp_minus_one() {
        assert!(super::exp_minus_one(f64::NAN).is_nan());
        assert_eq!(super::exp_minus_one(f64::INFINITY), f64::INFINITY);
        assert_eq!(super::exp_minus_one(f64::NEG_INFINITY), -1.0);
        assert_eq!(super::exp_minus_one(-50.0), -0.99999999999999999999980712501520360822169826571834729874);
        assert_eq!(super::exp_minus_one(-2.5), -0.91791500137610120483047132553284019216219587898456);
        assert_almost_eq!(super::exp_minus_one(-0.1), -0.09516258196404042683575094055356337880529463901959, 1e-8);
        assert_eq!(super::exp_minus_one(0.0), 0.0);
        assert_almost_eq!(super::exp_minus_one(0.1), 0.1051709180756476248117078264902466682245471947375187, 1e-8);
        assert_eq!(super::exp_minus_one(2.5), 11.182493960703473438070175951167966183182767790063161);
        assert_eq!(super::exp_minus_one(50.0), 5.1847055285870724640864533229334853848274691005838464e21);
    }
}