How do I combine fastutil maps in scala?

Question

What's the quickest way to combine two Object2IntOpenHashMap[String] in scala? Looking to combine these two maps:

  val foo = new Object2IntOpenHashMap[String]
  foo.put("foo", 1)
  val bar = new Object2IntOpenHashMap[String]
  bar.put("foo", 1)
  bar.put("bar", 1)

And produce an output of {"foo" : 2, "bar" : 1}.

Answer 1

Below is the imperative way to combine the 2 Object2IntOpenHashMap values.

    val foo = new Object2IntOpenHashMap[String]
    foo.put("foo", 1)
    val bar = new Object2IntOpenHashMap[String]
    bar.put("foo", 1)
    bar.put("bar", 1)

    bar.keySet().forEach(x => {
        val barValue = bar.getInt(x)
        foo.computeInt(x ,  (_, v) => if(v == null) barValue else barValue + v)
    })
   println(foo)

The above println(foo) will print {bar=>1, foo=>2}.

But if you want more functional way you should use more functional ibraries like cats or scalaz. I did this using cats -

            import cats.Semigroup
    import cats.implicits._
    import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap

    implicit val Object2IntOpenHashMapSemiGroup = new Semigroup[Object2IntOpenHashMap[String]] {

        override def combine(x: Object2IntOpenHashMap[String], y: Object2IntOpenHashMap[String]): Object2IntOpenHashMap[String] = {
        val result: Object2IntOpenHashMap[String] = y.clone()


        x.keySet().forEach(x => {
            val barValue = y.getInt(x)
            result.computeInt(x ,  (_, v) => if(v == null) barValue else barValue +v)
        })
        result
        }
    }
    println(foo combine bar)
    println(Object2IntOpenHashMapSemiGroup.combine(foo, bar))

You will get the same result as before. You can see the documentation here for semigroup here.

Answer 2

Found an alternative using the fast entry set:

  val foo = new Object2IntOpenHashMap[String]
  foo.put("foo", 1)
  val bar = new Object2IntOpenHashMap[String]
  bar.put("foo", 1)
  bar.put("bar", 1)

  val mapIter = bar.object2IntEntrySet().fastIterator()
  while(mapIter.hasNext()) {
    val x = mapIter.next()
    foo.put(x.getKey(), x.getIntValue() + foo.getOrDefault(x.getKey(), 0))
  }
  println(foo)