[GitHub] cloud-fan commented on a change in pull request #23304: [SPARK-26353][SQL]Add typed aggregate functions(max/min) to the example module.

[GitBox <gi...@apache.org>]

cloud-fan commented on a change in pull request #23304: [SPARK-26353][SQL]Add typed aggregate functions(max/min) to the example module.
URL: https://github.com/apache/spark/pull/23304#discussion_r257271503
 
 

 ##########
 File path: examples/src/main/scala/org/apache/spark/examples/sql/SimpleTypedAggregator.scala
 ##########
 @@ -84,3 +91,71 @@ class TypedAverage[IN](val f: IN => Double) extends Aggregator[IN, (Double, Long
   }
   override def outputEncoder: Encoder[Double] = Encoders.scalaDouble
 }
+
+class TypedMin[IN](val f: IN => Double) extends Aggregator[IN, MutableDouble, Option[Double]] {
+  override def zero: MutableDouble = null
+  override def reduce(b: MutableDouble, a: IN): MutableDouble = {
+    if (b == null) {
+      new MutableDouble(f(a))
+    } else {
+      b.value = math.min(b.value, f(a))
+      b
+    }
+  }
+  override def merge(b1: MutableDouble, b2: MutableDouble): MutableDouble = {
+    if (b1 == null) {
+      b2
+    } else if (b2 == null) {
+      b1
+    } else {
+      b1.value = math.min(b1.value, b2.value)
+      b1
+    }
+  }
+  override def finish(reduction: MutableDouble): Option[Double] = {
+    if (reduction != null) {
+      Some(reduction.value)
+    } else {
+      None
+    }
+  }
+
+  override def bufferEncoder: Encoder[MutableDouble] = Encoders.kryo[MutableDouble]
+  override def outputEncoder: Encoder[Option[Double]] = ExpressionEncoder[Option[Double]]()
 
 Review comment:
   can we use `Encoders.product[Option[Double]]`? Or we can use `Encoders.DOUBLE` and use null to represent `None`.
   
   The thing is, `ExpressionEncoder` is an internal class and we should avoid exposing it in the example module.

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