Merge pull request #31 from avibryant/aggregator

Aggregator trait provides a generic mapPlusMap-like abstraction

src/main/scala/com/twitter/algebird/Aggregator.scala

@@ -0,0 +1,22 @@
+package com.twitter.algebird
+
+trait Aggregator[A,B,C] extends Function1[TraversableOnce[A], C] with java.io.Serializable {
+  def prepare(input : A) : B
+  def reduce(l : B, r : B) : B
+  def present(reduction : B) : C
+
+  def reduce(items : TraversableOnce[B]) : B = items.reduce{reduce(_,_)}
+  def apply(inputs : TraversableOnce[A]) : C = present(reduce(inputs.map{prepare(_)}))
+}
+
+trait MonoidAggregator[A,B,C] extends Aggregator[A,B,C] {
+  def monoid : Monoid[B]
+  def reduce(l : B, r : B) : B = monoid.plus(l, r)
+  override def reduce(items : TraversableOnce[B]) : B = items.foldLeft(monoid.zero){reduce(_,_)}
+}
+
+object Averager extends MonoidAggregator[Double, AveragedValue, Double] {
+  val monoid = AveragedGroup
+  def prepare(value : Double) = AveragedValue(value)
+  def present(average : AveragedValue) = average.value
+}