NumberExt.kt

fun Long.toUnits(units: Array<String>): String {
    var num = this
    var rem = 0L
    var index = -1
    while ("$num".length > 4 && index < units.lastIndex) {
        rem = num % 10000
        num /= 10000
        index += 1
    }
    
    val unit = if (index > -1) units[index] else ""
    val numString = if ("$num".length == 1) 
    	"${num}.${rem / 1000}" else 
    	"%,d".format(num)

    return numString + unit
}

fun main() {
    val units = arrayOf("万", "億")
    println(7777L.toUnits(units)) // 7,777
    println(770000L.toUnits(units)) // 77万
    println(77000L.toUnits(units)) // 7.7万
    println(77770000L.toUnits(units)) // 7,777万
    println(770000000L.toUnits(units)) // 7.7億
    println(7777770000000L.toUnits(units)) // 77,777億
}