Performance around functional programming in scala - scala

Question

I'm working with below stuff as a way to learn functional programming and scala, I came from a python background.
case class Point(x: Int, y:Int)
object Operation extends Enumeration {
type Operation = Value
val TurnOn, TurnOff, Toggle = Value
}
object Status extends Enumeration {
type Status = Value
val On, Off = Value
}
val inputs: List[String]
def parseInputs(s: String): (Point, Point, Operation)
Idea is that we have a light matrix(Point), each Point can be either On or Off as describe in Status.
My inputs is a series of command, asking to either TurnOn, TurnOff or Toggle all the lights from one Point to another Point (The rectangular area defined using two points are bottom-left corner and upper-right corner).
My original solution is like this:
type LightStatus = mutable.Map[Point, Status]
val lightStatus = mutable.Map[Point, Status]()
def updateStatus(p1: Point, p2: Point, op: Operation): Unit = {
(p1, p2) match {
case (Point(x1, y1), Point(x2, y2)) =>
for (x <- x1 to x2)
for (y <- y1 to y2) {
val p = Point(x, y)
val currentStatus = lightStatus.getOrElse(p, Off)
(op, currentStatus) match {
case (TurnOn, _) => lightStatus.update(p, On)
case (TurnOff, _) => lightStatus.update(p, Off)
case (Toggle, On) => lightStatus.update(p, Off)
case (Toggle, Off) => lightStatus.update(p, On)
}
}
}
}
for ((p1, p2, op) <- inputs.map(parseInputs)) {
updateStatus(p1, p2, op)
}
Now I have lightStatus as a map to describe the end status of the entire matrix. This works, but seems less functional to me as I was using a mutable Map instead of an immutable object, so I tried to re-factor this into a more functional way, I ended up with this:
inputs.flatMap(s => parseInputs(s) match {
case (Point(x1, y1), Point(x2, y2), op) =>
for (x <- x1 to x2;
y <- y1 to y2)
yield (Point(x, y), op)
}).foldLeft(Map[Point, Status]())((m, item) => {
item match {
case (p, op) =>
val currentStatus = m.getOrElse(p, Off)
(op, currentStatus) match {
case (TurnOn, _) => m.updated(p, On)
case (TurnOff, _) => m.updated(p, Off)
case (Toggle, On) => m.updated(p, Off)
case (Toggle, Off) => m.updated(p, On)
}
}
})
I have couple questions regarding this process:
My second version doesn't seem as clean and straightforward as the first version to me, I'm not sure if this is because I'm not that familiar with functional programming or I just wrote bad functional code.
Is there someway to simplify the syntax on second piece? Especially the (m, item) => ??? function in the foldLeft part? Something like (m, (point, operation)) => ??? gives me syntax error
The second piece of code takes significantly longer to run, which surprise me a bit as these two code essentially is doing the same thing, As I don't have too much Java background, Any idea what might be causing the performance issue?
Many thanks!

Answer 1

From a Functional Programming perspective, your code suffers from the fact that...
The lightStatus Map "maintains state" and thus requires mutation.
A large "area" of status changes == a large number of data updates.
If you can accept each light status as a Boolean value, here's a design that requires no mutation and has fast status updates even over very large areas.
case class Point(x: Int, y:Int)
class LightGrid private (status: Point => Boolean) {
def apply(p: Point): Boolean = status(p)
private def isWithin(p:Point, ll:Point, ur:Point) =
ll.x <= p.x && ll.y <= p.y && p.x <= ur.x && p.y <= ur.y
//each light op returns a new LightGrid
def turnOn(lowerLeft: Point, upperRight: Point): LightGrid =
new LightGrid(point =>
isWithin(point, lowerLeft, upperRight) || status(point))
def turnOff(lowerLeft: Point, upperRight: Point): LightGrid =
new LightGrid(point =>
!isWithin(point, lowerLeft, upperRight) && status(point))
def toggle(lowerLeft: Point, upperRight: Point): LightGrid =
new LightGrid(point =>
isWithin(point, lowerLeft, upperRight) ^ status(point))
}
object LightGrid { //the public constructor
def apply(): LightGrid = new LightGrid(_ => false)
}
usage:
val ON = true
val OFF = false
val lg = LightGrid().turnOn(Point(2,2), Point(11,11)) //easy numbers
.turnOff(Point(8,8), Point(10,10))
.toggle(Point(1,1), Point(9,9))
lg(Point(1,1)) //ON
lg(Point(7,7)) //OFF
lg(Point(8,8)) //ON
lg(Point(9,9)) //ON
lg(Point(10,10)) //OFF
lg(Point(11,11)) //ON
lg(Point(12,12)) //OFF