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    //! The exponent estimator.

    /// Finds `k_0` such that `10^(k_0-1) < mant * 2^exp <= 10^(k_0+1)`.
    ///
    /// This is used to approximate `k = ceil(log_10 (mant * 2^exp))`;
    /// the true `k` is either `k_0` or `k_0+1`.
    #[doc(hidden)]
    pub fn estimate_scaling_factor(mant: u64, exp: i16) -> i16 {
        // 2^(nbits-1) < mant <= 2^nbits if mant > 0
        let nbits = 64 - (mant - 1).leading_zeros() as i64;
        // 1292913986 = floor(2^32 * log_10 2)
        // therefore this always underestimates (or is exact), but not much.
        (((nbits + exp as i64) * 1292913986) >> 32) as i16
    }