[SPARK-5550] [SQL] Support the case insensitive for UDF

SQL in HiveContext, should be case insensitive, however, the following query will fail.

```scala
udf.register("random0", ()  => { Math.random()})
assert(sql("SELECT RANDOM0() FROM src LIMIT 1").head().getDouble(0) >= 0.0)
```

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

Closes #4326 from chenghao-intel/udf_case_sensitive and squashes the following commits:

485cf66 [Cheng Hao] Support the case insensitive for UDF

1 files changed, 32 insertions, 4 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
index 760c49fbca..9f334f6d42 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
@@ -27,23 +27,25 @@ trait FunctionRegistry {
   def registerFunction(name: String, builder: FunctionBuilder): Unit
 
   def lookupFunction(name: String, children: Seq[Expression]): Expression
+
+  def caseSensitive: Boolean
 }
 
 trait OverrideFunctionRegistry extends FunctionRegistry {
 
-  val functionBuilders = new mutable.HashMap[String, FunctionBuilder]()
+  val functionBuilders = StringKeyHashMap[FunctionBuilder](caseSensitive)
 
   def registerFunction(name: String, builder: FunctionBuilder) = {
     functionBuilders.put(name, builder)
   }
 
   abstract override def lookupFunction(name: String, children: Seq[Expression]): Expression = {
-    functionBuilders.get(name).map(_(children)).getOrElse(super.lookupFunction(name,children))
+    functionBuilders.get(name).map(_(children)).getOrElse(super.lookupFunction(name, children))
   }
 }
 
-class SimpleFunctionRegistry extends FunctionRegistry {
-  val functionBuilders = new mutable.HashMap[String, FunctionBuilder]()
+class SimpleFunctionRegistry(val caseSensitive: Boolean) extends FunctionRegistry {
+  val functionBuilders = StringKeyHashMap[FunctionBuilder](caseSensitive)
 
   def registerFunction(name: String, builder: FunctionBuilder) = {
     functionBuilders.put(name, builder)
@@ -64,4 +66,30 @@ object EmptyFunctionRegistry extends FunctionRegistry {
   def lookupFunction(name: String, children: Seq[Expression]): Expression = {
     throw new UnsupportedOperationException
   }
+
+  def caseSensitive: Boolean = ???
+}
+
+/**
+ * Build a map with String type of key, and it also supports either key case
+ * sensitive or insensitive.
+ * TODO move this into util folder?
+ */
+object StringKeyHashMap {
+  def apply[T](caseSensitive: Boolean) = caseSensitive match {
+    case false => new StringKeyHashMap[T](_.toLowerCase)
+    case true => new StringKeyHashMap[T](identity)
+  }
+}
+
+class StringKeyHashMap[T](normalizer: (String) => String) {
+  private val base = new collection.mutable.HashMap[String, T]()
+
+  def apply(key: String): T = base(normalizer(key))
+
+  def get(key: String): Option[T] = base.get(normalizer(key))
+  def put(key: String, value: T): Option[T] = base.put(normalizer(key), value)
+  def remove(key: String): Option[T] = base.remove(normalizer(key))
+  def iterator: Iterator[(String, T)] = base.toIterator
 }
+