My purely functional Scala solution to the equilibrium index problem by codility

equilibrium.scala

// My scala solution (purely functional with no mutable variables)
// to the sample codility problem for finding
// any equilibrium index in an array of ints with up to
// 10 million entries. Description of problem is here:
// http://blog.codility.com/2011/03/solutions-for-task-equi.html
// this is O(N) and uses a scala List (instead of an Array) to maximize the
// speed of .head and .tail operations

import scala.annotation.tailrec

object Solution {
  def solution(A: Array[Int]): Int = {
    // convert to Long to avoid overflows, then to List[Long]
    val B: List[Long] = A.map(_.toLong).toList
    testEquil(B, 0L, B.sum, 0)
  }

  @tailrec
  def testEquil(A: List[Long], leftSum: Long, rightSum: Long, P: Int): Int = {
    if (A.isEmpty) return -1
    if (leftSum == rightSum - A.head) return P
    testEquil(A.tail, leftSum + A.head, rightSum - A.head, P+1)
  }
}

// example call with 10 million entries
val result = Solution.solution(Array.fill(5000000)(1) ++ Array(100) ++ Array.fill(5000000)(1))
println(result)