Scala implementation for Exponential Moving Average indicator

Ema.scala

object Ema {
  def apply(period: Int)(data: Vector[Double]): Vector[Double] = {
    if (period < 1) throw new IllegalArgumentException(s"Period ($period) is smaller than 1")
    if (period > data.size) throw new IllegalArgumentException(s"Period ($period) is bigger than data size (${data.size})")

    val factor = 2d / (period + 1)

    data.iterator
      .drop(1)
      .scanLeft(data.head) { (prevEma, value) => value * factor + (1d - factor) * prevEma }
      .toVector
  }
}

EmaSpec.scala

import org.scalatest.matchers.should.Matchers
import org.scalatest.wordspec.AnyWordSpec

class EmaSpec extends AnyWordSpec with Matchers {
  "validates input" in {
    an[IllegalArgumentException] shouldBe thrownBy { Ema(period = 0)(Vector.fill(0)(0d)) }

    an[IllegalArgumentException] shouldBe thrownBy { Ema(period = 0)(Vector.fill(1)(0d)) }

    an[IllegalArgumentException] shouldBe thrownBy { Ema(period = 2)(Vector.fill(1)(0d)) }
  }

  "returns same size" in {
    Ema(period = 1)(Vector.fill(1)(0d)).size shouldBe 1

    Ema(period = 3)(Vector.fill(16)(0d)).size shouldBe 16
  }

  "calculates correctly for still data" in {
    val data = Vector.fill(16)(1d)

    Ema(period = 16)(data) should contain theSameElementsInOrderAs data
  }

  "calculates correctly for moving data" in {
    val data = Vector(1d, 2d, 3d, 4d)

    Ema(period = 3)(data) should contain theSameElementsInOrderAs Vector(1d, 1.5d, 2.25d, 3.125d)
  }
}