Sieve of Eratosthenes

Sieve.kt

class Sieve(val max: Int) {
    private val isPrimeArray: BooleanArray

    init {
        val array = BooleanArray(max + 1) {
            true
        }
        array[0] = false
        array[1] = false
        val root = sqrt(max.toDouble()).toInt()
        for (i in 0..root) {
            if (array[i]) {
                for (j in i * i until array.size step i) {
                    array[j] = false
                }
            }
        }
        isPrimeArray = array
    }

    fun isPrime(x: Int): Boolean {
        if (x > max) {
            throw IllegalArgumentException("x should same or smaller than $max")
        }
        return isPrimeArray[x]
    }
}