[SPARK-9155][SQL] codegen StringSpace

Jira https://issues.apache.org/jira/browse/SPARK-9155

Author: Tarek Auel <tarek.auel@googlemail.com>

Closes #7531 from tarekauel/SPARK-9155 and squashes the following commits:

423c426 [Tarek Auel] [SPARK-9155] language typo fix
e34bd1b [Tarek Auel] [SPARK-9155] moved creation of blank string to UTF8String
4bc33e6 [Tarek Auel] [SPARK-9155] codegen StringSpace

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringOperations.scala

@@ -593,17 +593,19 @@ case class StringReverse(child: Expression) extends UnaryExpression with String2
  * Returns a n spaces string.
  */
 case class StringSpace(child: Expression)
-  extends UnaryExpression with ImplicitCastInputTypes with CodegenFallback {
+  extends UnaryExpression with ImplicitCastInputTypes {
 
   override def dataType: DataType = StringType
   override def inputTypes: Seq[DataType] = Seq(IntegerType)
 
   override def nullSafeEval(s: Any): Any = {
-    val length = s.asInstanceOf[Integer]
+    val length = s.asInstanceOf[Int]
+    UTF8String.blankString(if (length < 0) 0 else length)
+  }
 
-    val spaces = new Array[Byte](if (length < 0) 0 else length)
-    java.util.Arrays.fill(spaces, ' '.asInstanceOf[Byte])
-    UTF8String.fromBytes(spaces)
+  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String = {
+    nullSafeCodeGen(ctx, ev, (length) =>
+      s"""${ev.primitive} = UTF8String.blankString(($length < 0) ? 0 : $length);""")
   }
 
   override def prettyName: String = "space"

unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -20,6 +20,7 @@ package org.apache.spark.unsafe.types;
 import javax.annotation.Nonnull;
 import java.io.Serializable;
 import java.io.UnsupportedEncodingException;
+import java.util.Arrays;
 
 import org.apache.spark.unsafe.PlatformDependent;
 import org.apache.spark.unsafe.array.ByteArrayMethods;
@@ -77,6 +78,15 @@ public final class UTF8String implements Comparable<UTF8String>, Serializable {
     }
   }
 
+  /**
+   * Creates an UTF8String that contains `length` spaces.
+   */
+  public static UTF8String blankString(int length) {
+    byte[] spaces = new byte[length];
+    Arrays.fill(spaces, (byte) ' ');
+    return fromBytes(spaces);
+  }
+
   protected UTF8String(Object base, long offset, int size) {
     this.base = base;
     this.offset = offset;

unsafe/src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java

@@ -286,4 +286,12 @@ public class UTF8StringSuite {
     assertEquals(
         UTF8String.fromString("世界千世").levenshteinDistance(UTF8String.fromString("千a世b")),4);
   }
+
+  @Test
+  public void createBlankString() {
+    assertEquals(fromString(" "), blankString(1));
+    assertEquals(fromString("  "), blankString(2));
+    assertEquals(fromString("   "), blankString(3));
+    assertEquals(fromString(""), blankString(0));
+  }
 }