 Solution

class MultiSetVarImp[T] extends MultiSetVar[T] {
  private var table = Map.empty[T, Int]

  def count(t: T): Int = table.getOrElse(t, 0)
  def present: Seq[T] = table.keys.toVector

  def copy(other: CompleteMultiSet[T]): Unit = {
    val saved = other.present.distinct.iterator
      .map(t => (t, other.count(t))).filter(_._2 != 0).toMap
    table = saved
  }

  private def snapshot(other: MultiSetVar[T]): Map[T, Int] =
    other.present.distinct.iterator
      .map(t => (t, other.count(t))).filter(_._2 != 0).toMap

  private def update(other: MultiSetVar[T], op: (Int, Int) => Int): Unit = {
    val left = table
    val right = snapshot(other)
    val keys = left.keySet ++ right.keySet
    val builder = Map.newBuilder[T, Int]
    keys.foreach { t =>
      val n = op(left.getOrElse(t, 0), right.getOrElse(t, 0))
      if (n != 0) builder += ((t, n))
    }
    val replacement = builder.result()
    table = replacement
  }

  def +(other: MultiSetVar[T]): Unit =
    update(other, (a, b) => java.lang.Math.addExact(a, b))

  def -(other: MultiSetVar[T]): Unit =
    update(other, (a, b) => java.lang.Math.subtractExact(a, b))

  def max(other: MultiSetVar[T]): Unit =
    update(other, (a, b) => math.max(a, b))

  def min(other: MultiSetVar[T]): Unit =
    update(other, (a, b) => math.min(a, b))

  def <=(other: CompleteMultiSet[T]): Boolean = {
    val left = table
    (left.keySet ++ other.present).forall { t =>
      left.getOrElse(t, 0) <= other.count(t)
    }
  }
}