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Combine - Combine PerKey
------------------------

After creating a keyed PCollection (for example, by using a GroupByKey transform), a common pattern 
is to combine the collection of values associated with each key into a single, merged value. This 
pattern of a GroupByKey followed by merging the collection of values is equivalent to Combine 
PerKey transform. The combine function you supply to Combine PerKey must be an associative 
reduction function or a subclass of CombineFn.

**Kata:** Implement the sum of scores per player using 
[Combine.perKey](https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/CombineFnBase.GlobalCombineFn.html).

<div class="hint">
  Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/CombineFnBase.GlobalCombineFn.html">
  Combine.perKey(GlobalCombineFn)</a>.
</div>

<div class="hint">
  Extend the
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Combine.BinaryCombineFn.html">
    Combine.BinaryCombineFn</a> class that counts the sum of the number.
</div>

<div class="hint">
  Refer to the Beam Programming Guide
  <a href="https://beam.apache.org/documentation/programming-guide/#combining-values-in-a-keyed-pcollection">
    "Combining values in a keyed PCollection"</a> section for more information.
</div>
