[SPARK-4375] [SQL] Add 0 argument support for udf

Author: Cheng Hao <hao.cheng@intel.com>

Closes #3595 from chenghao-intel/udf0 and squashes the following commits:

a858973 [Cheng Hao] Add 0 arguments support for udf

sql/core/src/main/scala/org/apache/spark/sql/UdfRegistration.scala

@@ -72,14 +72,13 @@ private[sql] trait UDFRegistration {
     functionRegistry.registerFunction(name, builder)
   }
 
-  /** registerFunction 1-22 were generated by this script
+  /** registerFunction 0-22 were generated by this script
 
-    (1 to 22).map { x =>
-      val types = (1 to x).map(x => "_").reduce(_ + ", " + _)
+    (0 to 22).map { x =>
+      val types = (1 to x).foldRight("T")((_, s) => {s"_, $s"})
       s"""
-        def registerFunction[T: TypeTag](name: String, func: Function$x[$types, T]): Unit = {
-          def builder(e: Seq[Expression]) =
-            ScalaUdf(func, ScalaReflection.schemaFor[T].dataType, e)
+        def registerFunction[T: TypeTag](name: String, func: Function$x[$types]): Unit = {
+          def builder(e: Seq[Expression]) = ScalaUdf(func, ScalaReflection.schemaFor[T].dataType, e)
           functionRegistry.registerFunction(name, builder)
         }
       """
@@ -87,6 +86,11 @@ private[sql] trait UDFRegistration {
   */
 
   // scalastyle:off
+  def registerFunction[T: TypeTag](name: String, func: Function0[T]): Unit = {
+    def builder(e: Seq[Expression]) = ScalaUdf(func, ScalaReflection.schemaFor[T].dataType, e)
+    functionRegistry.registerFunction(name, builder)
+  }
+
   def registerFunction[T: TypeTag](name: String, func: Function1[_, T]): Unit = {
     def builder(e: Seq[Expression]) = ScalaUdf(func, ScalaReflection.schemaFor[T].dataType, e)
     functionRegistry.registerFunction(name, builder)

sql/core/src/test/scala/org/apache/spark/sql/UDFSuite.scala

@@ -31,6 +31,11 @@ class UDFSuite extends QueryTest {
     assert(sql("SELECT strLenScala('test')").first().getInt(0) === 4)
   }
 
+  test("ZeroArgument UDF") {
+    registerFunction("random0", () => { Math.random()})
+    assert(sql("SELECT random0()").first().getDouble(0) >= 0.0)
+  }
+
   test("TwoArgument UDF") {
     registerFunction("strLenScala", (_: String).length + (_:Int))
     assert(sql("SELECT strLenScala('test', 1)").first().getInt(0) === 5)