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    use super::{log, log1p, sqrt};

    const LN2: f64 = 0.693147180559945309417232121458176568; /* 0x3fe62e42,  0xfefa39ef*/

    /// Inverse hyperbolic cosine (f64)
    ///
    /// Calculates the inverse hyperbolic cosine of `x`.
    /// Is defined as `log(x + sqrt(x*x-1))`.
    /// `x` must be a number greater than or equal to 1.
    pub fn acosh(x: f64) -> f64 {
        let u = x.to_bits();
        let e = ((u >> 52) as usize) & 0x7ff;

        /* x < 1 domain error is handled in the called functions */

        if e < 0x3ff + 1 {
            /* |x| < 2, up to 2ulp error in [1,1.125] */
            return log1p(x - 1.0 + sqrt((x - 1.0) * (x - 1.0) + 2.0 * (x - 1.0)));
        }
        if e < 0x3ff + 26 {
            /* |x| < 0x1p26 */
            return log(2.0 * x - 1.0 / (x + sqrt(x * x - 1.0)));
        }
        /* |x| >= 0x1p26 or nan */
        return log(x) + LN2;
    }

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