package activeactive

class Calculator {
  def main(args:Array[String]): Double = eval(args.toList)

  def eval(args:List[String]): Double = args match {
    case null =>
      throw new IllegalArgumentException(
        "args cannot be null-valued")
    case Nil =>
      throw new IllegalArgumentException(
        "You must provide a function name as the first argument")
    case "sum" :: rest => sum(convertList(rest))
    case "prod" :: rest => product(convertList(rest))
    case "mean" :: rest => mean(convertList(rest))
    case "sqrt" :: rest => sqrt(convertList(rest))
    case _ => throw new IllegalArgumentException(
      "invalid function name. Use 'sum', 'prod', 'mean', or 'sqrt'.")
  }

  def convertList(list: List[String]): List[Double] = list match {
    case Nil => Nil
    case x :: subList => x.toDouble :: convertList(subList)
  }

  def sum(list: List[Double]) =
    (0D :: list) reduceLeft ((x, y) => x+y)

  def product(list: List[Double]) =
    (1D :: list) reduceLeft ((x, y) => x*y)

  def mean(list: List[Double]) = list match {
    case Nil => throw new IllegalArgumentException(
      "The mean function requires at least one operand")
    case _ => sum(list) / list.size
  }

  def sqrt(list: List[Double]) =
    if (sum(list) <= 0)
      throw new IllegalArgumentException(
        "the input values '" + list.toString +
          "' resulted in a sum <= zero.")
    else if (sum(list) == 1D) 1D
    else Math.sqrt(sum(list))
}