Method count

datafusion/functions-aggregate/src/hyperloglog.rs:147–157

Guess the number of unique elements seen by the HyperLogLog.

(&self)

    /// Guess the number of unique elements seen by the HyperLogLog.
    pub fn count(&self) -> usize {
        let histogram = self.get_histogram();
        let m = NUM_REGISTERS as f64;
        let mut z = m * hll_tau((m - histogram[HLL_Q + 1] as f64) / m);
        for i in histogram[1..=HLL_Q].iter().rev() {
            z += *i as f64;
            z *= 0.5;
        }
        z += m * hll_sigma(histogram[0] as f64 / m);
        (0.5 / 2_f64.ln() * m * m / z).round() as usize
    }
}

/// Helper function sigma as defined in