ArrayRotation.scala

object ArrayRotation {
  def rotate(a: Array[Int], k: Int): Array[Int] = {
    val n = a.length
    lazy val kRn = k % n

    if (n == 0 || kRn == 0)
      a
    else
      Array.tabulate(n) { i =>
        val j = i - kRn

        if (j < 0) a(j + n)
        else a(j)
      }
  }
}

ArrayRotationSpec.scala

import org.scalatest.flatspec.AnyFlatSpec

class ArrayRotationSpec extends AnyFlatSpec {
  private val a = Array(1, 2, 3, 4)

  "shifting by length" should "no-effect" in {
    assert(ArrayRotation.rotate(a, 4) === a)
  }
  "n = 0" should "no-effect" in {
    assert(ArrayRotation.rotate(Array(), 5).isEmpty)
  }
  "k = 0" should "no-effect" in {
    assert(ArrayRotation.rotate(a, 0) === a)
  }
  "shifting once" should "work" in {
    assert(ArrayRotation.rotate(a, 1) === Array(4, 1, 2, 3))
  }
  "shifting twice" should "work" in {
    assert(ArrayRotation.rotate(a, 2) === Array(3, 4, 1, 2))
  }
  "fully rotating more than once" should "work" in {
    assert(ArrayRotation.rotate(a, 10) === Array(3, 4, 1, 2))
  }
}