LinearFit2.scala

object LinearFit2 {
    case class Point(x: Double = 0.0, y: Double = 0.0) {
        def +(that: Point): Point = Point(this.x + that.x, this.y + that.y)
    }
    
    implicit def tupleToPoint(tuple: (Double, Double)): Point = Point(tuple._1, tuple._2)

    def through (points : Point*) : Point = through(points.toList)
    def through (points : List[Point]) : Point = {
        val total = points.foldLeft(Point())(_ + _)
        val (xAverage, yAverage) = (total.x / points.size , total.y / points.size)

        val Point(squareSumXX, squareSumXY) = points.foldLeft(Point()) {
            (squareSums: Point, point: Point) => Point(
                squareSums.x + pow(point.x - xAverage, 2),
                squareSums.y + (point.x - xAverage) * (point.y - yAverage)
            )
        }

        val b = (squareSumXY / squareSumXX)
        
        Point(yAverage - b * xAverage,  b)
    }
}

class LinearFit2Test extends Specification {
  import LinearFit2._

  "A LinearFit2" should {
    "fit a slope for points on a straight line" in {
      LinearFit2 through (Point(0.0, 1.0), Point(1.0, 1.0), Point(2.0, 1.0)) must beEqualTo(Point(1, 0))
      LinearFit2 through (Point(0.0, 0.0), Point(1.0, 1.0), Point(2.0, 2.0)) must beEqualTo(Point(0, 1))
    }
    "fit a slope for points not a straight line" in {
      LinearFit2 through (Point(0.0, 1.0), Point(1.0, 2.0), Point(2.0, 1.0)) must beEqualTo(Point(4.0/3, 0))
      LinearFit2 through (Point(0.0, 0.0), Point(1.0, 0.0), Point(1.0, 2.0), Point(2.0, 2.0)) must beEqualTo(Point(0, 1))
    }
    "accept a List as input type" in {
      LinearFit2 through (List(Point(0.0, 1.0), Point(1.0, 1.0), Point(2.0, 1.0))) must beEqualTo(Point(1, 0))
    }
    "convert input tuples to input Points" in {
      LinearFit2 through ((0.0, 1.0), (1.0, 1.0), (2.0, 1.0)) must beEqualTo(Point(1, 0))
    }
    "convert output points to output tuples" in {
        val output: (Double, Double) = LinearFit2 through ((0.0, 1.0), (1.0, 1.0), (2.0, 1.0))

        output must beEqualTo((1.0, 0.0))
    }
  }
}