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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