code: private static final double getHllBitMapEstimate(
        final int lgConfigK, final int curMin, final int numAtCurMin) {
      final int configK = 1 << lgConfigK;
      final int numUnhitBuckets =  (curMin == 0) ? numAtCurMin : 0;
  
      //This will eventually go away.
      if (numUnhitBuckets == 0) {
        return configK * Math.log(configK / 0.5);
      }
  
      final int numHitBuckets = configK - numUnhitBuckets;
      return HarmonicNumbers.getBitMapEstimate(configK, numHitBuckets);
    }
docs: In C: again-two-registers.c hhb_get_improved_linear_counting_estimate L1274