[jira] [Created] (SPARK-10801) StatCounter uses mutability, is not thread-safe and hard to understand its implementation

["Gianmario Spacagna (JIRA)" <ji...@apache.org>]

Gianmario Spacagna created SPARK-10801:
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             Summary: StatCounter uses mutability, is not thread-safe and hard to understand its implementation
                 Key: SPARK-10801
                 URL: https://issues.apache.org/jira/browse/SPARK-10801
             Project: Spark
          Issue Type: Bug
          Components: Spark Core
            Reporter: Gianmario Spacagna


The current implementation of StatCounter is mutable and not thread-safe.
The API for creating it is also limiting since that it only expose the constructor using a TraversableOnce[Double].
More over the current implementation does not offer any equality.

My proposal is to use case classes to store the minimum amount of fields necessary to compute the statistics and make it so that it would be easy to apply the Monoid pattern to reduce an RDD or a Scala collection of StatCounter into a single StatCounter.

I have re-implemented and tested StatCounter at my work after I found a bug when trying to merge multiple stat counter in parallel using Scalaz Monoid. I would like to send a pull request of that functional, clean and concise re-implementation.

This would be the declaration of the class:

case class StatCounter(n: Long, sum: Double, sos: Double, min: Double, max: Double)

That would also change the implementation of variance into a single line:
def variance = (sos - n * mean * mean) / (n - 1)



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