[GitHub] [datasketches-java] jmalkin commented on a diff in pull request #430: HLL relative error

["jmalkin (via GitHub)" <gi...@apache.org>]

jmalkin commented on code in PR #430:
URL: https://github.com/apache/datasketches-java/pull/430#discussion_r1119284930


##########
src/main/java/org/apache/datasketches/hll/BaseHllSketch.java:
##########
@@ -118,8 +120,14 @@ public static final int getSerializationVersion(final Memory mem) {
    * <a href="{@docRoot}/resources/dictionary.html#numStdDev">Number of Standard Deviations</a>
    * @return the current (approximate) RelativeError
    */
-  public double getRelErr(final boolean upperBound, final boolean unioned,
+  public static double getRelErr(final boolean upperBound, final boolean unioned,
       final int lgConfigK, final int numStdDev) {
+    HllUtil.checkLgK(lgConfigK);
+    if (lgConfigK > 12) {
+      final double rseFactor = unioned ? HLL_NON_HIP_RSE_FACTOR : HLL_HIP_RSE_FACTOR;
+      final int configK = 1 << lgConfigK;
+      return (upperBound ? -1.0 : 1.0) * (numStdDev * rseFactor) / Math.sqrt(configK);

Review Comment:
   If we know that there's a guaranteed direction for an error, it's reasonable and I'd argue correct to reflect that.



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