[GitHub] spark pull request: [SPARK-12706] [SQL] grouping() and grouping_id...

[hvanhovell <gi...@git.apache.org>]

Github user hvanhovell commented on a diff in the pull request:

    https://github.com/apache/spark/pull/10677#discussion_r51643536
  
    --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/bitwiseExpressions.scala ---
    @@ -124,3 +124,47 @@ case class BitwiseNot(child: Expression) extends UnaryExpression with ExpectsInp
     
       protected override def nullSafeEval(input: Any): Any = not(input)
     }
    +
    +/**
    +  * A function that reverse the lowest N bits of a integer.
    +  *
    +  * Note: this is only used for grouping_id()
    +  */
    +case class BitwiseReverse(child: Expression, width: Int)
    +  extends UnaryExpression with ExpectsInputTypes {
    +
    +  override def inputTypes: Seq[AbstractDataType] = Seq(IntegerType)
    +
    +  override def dataType: DataType = IntegerType
    +
    +  override def toString: String = s"^$child"
    +
    +  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String = {
    +    nullSafeCodeGen(ctx, ev, c => {
    +      val v = ctx.freshName("v")
    +      val i = ctx.freshName("i")
    +      s"""
    +         | int $v = $c;
    +         | ${ev.value} = 0;
    +         | for (int $i = 0; $i < $width; $i ++) {
    +         |   ${ev.value} <<= 1;
    +         |   ${ev.value} |= $v & 1;
    +         |   $v >>>= 1;
    +         | }
    +       """.stripMargin
    +    })
    +  }
    +
    +  protected override def nullSafeEval(input: Any): Any = {
    +    var v = input.asInstanceOf[Int]
    --- End diff --
    
    Nit: We could also use ```Integer.reverse``` and a mask here. It saves some code and a branch.


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