Window function support in Scala/Java DataFrame DSL

class Column {
  ...
  def over(window: Window): Column
  ...
}

object Window {
  def partitionBy(...): Window
  def orderBy(...): Window

  object Frame {
    def unbounded: Frame
    def preceding(n: Long): Frame
    def following(n: Long): Frame
  }

  class Frame
}

class Window {
  def orderBy(...): Window
  def rowsBetween(Frame, Frame): Window
  def rangeBetween(Frame, Frame): Window  // maybe add this later
}

df.select(
  avg(“age”).over(Window.partitionBy(“..”, “..”).orderBy(“..”, “..”)
    .rowsBetween(Frame.unbounded, Frame.currentRow))
)

df.select(
  avg(“age”).over(Window.partitionBy(“..”, “..”).orderBy(“..”, “..”)
    .rowsBetween(Frame.preceding(50), Frame.following(10)))
)