Calculate all happy numbers from 0 to 100

HappyNumbers.kt

/**
 * What's a happy number? https://en.wikipedia.org/wiki/Happy_number
 * 
 * Simple brute force solution that just evaluates if the happy number sequence reaches one within [MAX_DEPTH] iterations.
 */
object HappyNumbers {

    const val MAX_DEPTH = 10_000

    @JvmStatic
    fun main(args: Array<String>) {
        val numbers = (0..100).filter { checkNumber(it) }
        println(numbers.mapIndexed { index, i -> "$index: $i" }.joinToString("\n"))
    }

    fun checkNumber(number: Int, depth: Int = 0): Boolean =
        number == 1 || (depth < MAX_DEPTH && checkNumber(squareSumOfDigits(number), depth + 1))

    private fun squareSumOfDigits(number: Int): Int {
        return number.toString().map { it.digitToInt() * it.digitToInt() }.sum()
    }
}