libm/math/
modf.rs

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pub fn modf(x: f64) -> (f64, f64) {
    let rv2: f64;
    let mut u = x.to_bits();
    let mask: u64;
    let e = ((u >> 52 & 0x7ff) as i32) - 0x3ff;

    /* no fractional part */
    if e >= 52 {
        rv2 = x;
        if e == 0x400 && (u << 12) != 0 {
            /* nan */
            return (x, rv2);
        }
        u &= 1 << 63;
        return (f64::from_bits(u), rv2);
    }

    /* no integral part*/
    if e < 0 {
        u &= 1 << 63;
        rv2 = f64::from_bits(u);
        return (x, rv2);
    }

    mask = ((!0) >> 12) >> e;
    if (u & mask) == 0 {
        rv2 = x;
        u &= 1 << 63;
        return (f64::from_bits(u), rv2);
    }
    u &= !mask;
    rv2 = f64::from_bits(u);
    return (x - rv2, rv2);
}