GitHub user techaddict opened a pull request:
https://github.com/apache/spark/pull/13347
[SPARK-15598] Change Aggregator.zero to Aggregator.init
## What changes were proposed in this pull request?
org.apache.spark.sql.expressions.Aggregator currently requires defining the
zero value for an aggregator. This is actually a limitation making it difficult
to implement APIs such as reduce. In reduce (or reduceByKey), a single
associative and commutative reduce function is specified by the user, and there
is no definition of zero value.
A small tweak to the API is to change zero to init, taking an input,
similar to the following:
```scala
abstract class Aggregator[-IN, BUF, OUT] extends Serializable {
def init(a: IN): BUF
def reduce(b: BUF, a: IN): BUF
def merge(b1: BUF, b2: BUF): BUF
def finish(reduction: BUF): OUT
}
```
Then reduce can be implemented using:
`f: (T, T) => T`
```scala
new Aggregator[T, T, T] {
override def init(a: T): T = identify
override def reduce(b: T, a: T): T = f(b, a)
override def merge(b1: T, b2: T): T = f(b1, b2)
override def finish(reduction: T): T = identify
}
```
## How was this patch tested?
Existing tests
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/techaddict/spark SPARK-15598
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/13347.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #13347
----
commit c278f445b62b10f82d73739d883fa8a35dc81f7a
Author: Sandeep Singh <[email protected]>
Date: 2016-05-27T02:42:15Z
[SPARK-15598] Change Aggregator.zero to Aggregator.init
----
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