Find prime factors for an integer

prime_factors.scala

import scala.math.{ceil,sqrt}

object Main {

    def isPrime(n: Int): Boolean = 
        if (n <= 2) (n == 2) else {
            val limit = ceil(sqrt(n)).toInt
            def _check(i: Int): Boolean =
                if (i > limit) true
                else if (n % i == 0) false
                     else _check(i + 1)
            _check(2)
        }

    def nextPrime(p: Int):Int = 
        if (isPrime(p + 1)) (p + 1) else nextPrime(p + 1)

    def divide(x: Int, f:Int):(Int, Int) = {
        def _divide(x: Int, c: Int):(Int, Int) =
            if (x % f != 0) (x, c) else _divide(x / f, c + 1)
        _divide(x, 0)
    }

    def factors(n: Int):List[(Int,Int)] = {
        def _factors(x: Int, p: Int):List[(Int, Int)] = {
            if (x <= 1) Nil else if (isPrime(x)) (x,1) :: Nil
            else {
                val (r, c) = divide(x, p)
                if (c != 0) (p, c) :: _factors(r, nextPrime(p))
                else _factors(r, nextPrime(p))
            }
        }
        _factors(n, 2)
    }

    def main(args: Array[String]) {
        println(factors(1999978910))
        // List((2,1), (5,1), (29,1), (6896479,1))
    }
}