Factorial in Kotlin

factorial.kt

/** If high-performance is needed consider using Guava BigIntegerMath.factorial instead **/
fun Long.factorial(): BigInteger {
    fun factorial(start: Long, n: Long): BigInteger {
        var i: Long
        if (n <= 16) {
            var r = BigInteger.valueOf(start)
            i = start + 1
            while (i < start + n) {
                r = r.multiply(BigInteger.valueOf(i))
                i++
            }
            return r
        }
        i = n / 2
        return factorial(start, i) * factorial(start + i, n - i)
    }
    if (this <= 0) return BigInteger.ONE
    return factorial(1, this)
}