VectorClock.scala

import scala.language.postfixOps

class VectorClock(private var name: String, private var countMap: Map[String, Int] = Map()) {
    if (!countMap.isDefinedAt(name)) countMap = countMap ++ Map(name -> 1)

    // increment
    def ++ = VectorClock(
        name,
        countMap ++ Map(name -> (countMap.getOrElse(name, 0) + 1))
    )

    // merge
    def +(other: VectorClock) = VectorClock(
        name,
        (countMap.keySet ++ other.countMap.keySet).map(key =>
            (key, math.max(countMap.getOrElse(key, 0), other.countMap.getOrElse(key, 0)))
        ).toMap ++ Map(name -> (math.max(countMap.getOrElse(name, 0), other.countMap.getOrElse(name, 0)) + 1))
    )

    override def toString: String = s"VectorClock($name, $countMap)"
}

object VectorClock {
    def apply(name: String, countMap: Map[String, Int] = Map()): VectorClock = new VectorClock(name, countMap)
}

val a = VectorClock("A", Map("B" -> 2))
val b = VectorClock("B")

println(a)
println(b)
println(a++)
println(a + b)
println((a + b)++)

a: VectorClock = VectorClock(A, Map(B -> 2, A -> 1))
b: VectorClock = VectorClock(B, Map(B -> 1))

VectorClock(A, Map(B -> 2, A -> 1))
VectorClock(B, Map(B -> 1))
VectorClock(A, Map(B -> 2, A -> 2))
VectorClock(A, Map(B -> 2, A -> 2))
VectorClock(A, Map(B -> 2, A -> 3))