Avoid duplicated computation in guards - scala

Question

Here is the code:
import java.util.{Calendar, Date, GregorianCalendar}
import com.mongodb.casbah.Imports._
import com.mongodb.casbah.commons.conversions.scala._
case class Quota(date: Date, used: Int)
object MongoDateDemo extends App {
val client = InsertUsers.getClient
val db = client("github")
val quota = db("quota")
val rand = scala.util.Random
// quota.drop()
// (1 to 100).foreach { _ =>
// quota += DBObject("date" -> new Date(), "used" -> rand.nextInt(10))
// Thread.sleep(1000)
// }
val minuteInMilliseconds = 60 * 1000
def thresholdDate(minute: Int) = new Date(new Date() .getTime - minuteInMilliseconds * minute) // since a minute ago
val fields = DBObject("_id" -> 0, "used" -> 1)
val x = quota.find("date" $gte thresholdDate(28), fields).collect {
case x if x.getAs[Int]("used").isDefined => x.getAs[Int]("used").get
}
println(x.toList.sum)
// val y = x.map {
// case dbo: DBObject => Quota(dbo.getAs[Date]("date").getOrElse(new Date(0)), dbo.getAs[Int]("used").getOrElse(0))
// }
}
It's reading documents from a collection and filter out those that don't have "used" defined, then summing up the numbers.
The x.getAs[Int]("used") part is duplicated computation, how can I avoid it?

Answer 1

Not much of a Scala programmer, but isn't that what flatMap is for?
quota
.find("date" $gte thresholdDate(38), fields)
.flatMap(_.getAs[Int]("used").toList)

Answer 2

Since this is not possible to avoid, I had to do it in two steps, map into Options then collect. I used view method so that the collection is not traversed twice:
import java.util.{Calendar, Date, GregorianCalendar}
import com.mongodb.casbah.Imports._
import com.mongodb.casbah.commons.conversions.scala._
case class Quota(date: Date, used: Int)
object MongoDateDemo extends App {
val client = InsertUsers.getClient
val db = client("github")
val quota = db("quota")
val rand = scala.util.Random
// quota.drop()
// (1 to 100).foreach { _ =>
// quota += DBObject("date" -> new Date(), "used" -> rand.nextInt(10))
// Thread.sleep(1000)
// }
val minuteInMilliseconds = 60 * 1000
def thresholdDate(minute: Int) = new Date(new Date() .getTime - minuteInMilliseconds * minute) // since a minute ago
val fields = DBObject("_id" -> 0, "used" -> 1)
val usedNumbers = quota.find("date" $gte thresholdDate(38), fields).toList.view.map {
_.getAs[Int]("used")
}.collect {
case Some(i) => i
}.force
println(usedNumbers.sum)
// val y = x.map {
// case dbo: DBObject => Quota(dbo.getAs[Date]("date").getOrElse(new Date(0)), dbo.getAs[Int]("used").getOrElse(0))
// }
}

Answer 3

Assuming getAs returns an Option, this should do what you want:
val x = quota.find("date" $gte thresholdDate(28), fields).flatMap { _.getAs[Int]("used") }
This is similar to doing this:
scala> List(Some(1), Some(2), None, Some(4)).flatMap(x => x)
res: List[Int] = List(1, 2, 4)
Or this:
scala> (1 to 20).flatMap(x => if(x%2 == 0) Some(x) else None)
res: scala.collection.immutable.IndexedSeq[Int] = Vector(2, 4, 6, 8, 10, 12, 14, 16, 18, 20)