Multiplies config files using a range of lines

kotgombinator.kt

// Usage:  java kotgombinate.jar Dir1 Dir2 Dir3 startOfRange endOfRange
// Output: Dir1(A, B) * Dir2(C, D) = Dir3 (A_C, A_D, B_C, B_D)
// Range is line-numbers so files in Dir1 and Dir2 need to correspond 1-to-1
import java.io.File
import kotlin.system.exitProcess

data class Butt(var fileContents: MutableList<List<String>>, var fileName: MutableList<String>)
var dir1 = Butt(ArrayList(), ArrayList())
var dir2 = Butt(ArrayList(), ArrayList())
var dir3 = Butt(ArrayList(), ArrayList())
var a: MutableList<Int> = ArrayList()
var range = Array(0, {0})
var out = ""
const val notInRange: String = "Input range in format a b e.g. 10 123"

fun main(args: Array<String>) {
//    for (e in args) {println(e)}
    if (args.isNotEmpty() ) {
        dir1 = readDir(args[0])
        dir2 = readDir(args[1])
        dir3 = readDir(args[2])
        a.add(args[3].toInt())
        a.add(args[4].toInt())
    }
    else{
        println("Null argument")
        exitProcess(0)
    }
    if (a[0] > a[1]) {
        println(notInRange)
        exitProcess(0)
    }

    range = Array(a[1] - a[0]) {i -> (i + a[0])}
    println("Range length: ${range.size}")


    var i = 0
    var o = 0
    dir2.fileName.forEach{
        val it2 = it
        var j = 0
        var g = 0
        var k:Int
        dir1.fileName.forEach() {
            dir3.fileName.add(it2 + " " + it)
            while (j < a[0]) {
                //println("Dir2: $j")
                out += dir2.fileContents[i][j] + "\n"
                j++
            }
            k = a[1] + 1
            for (e in range) {
                //println("Dir1: $e")
                out += dir1.fileContents[g][e] + "\n"
            }
            while (k < dir2.fileContents[i].size) {
                //println("Dir2: $k")
                out += dir2.fileContents[i][k] + "\n"
                k++
            }
            j = 0
            k = 0
            println("Dir3:  ${dir3.fileName}")
            println("No. Dir1 files $g")
            File("./Dir3/${dir3.fileName[o]}.bc").writeText(out)
            o++
            g++
            out = ""
        }
        println("No. Dir2 files $i")
        i++
    }

}
fun readDir(dir: String): Butt {
    val data = Butt(ArrayList(), ArrayList())
    File(dir).walkTopDown().forEach {
        if (!it.isDirectory)
            data.fileName.add(it.toString().substringAfterLast("/").substringBeforeLast("."))
        if (File(it.toString()).isFile)
            data.fileContents.add(File(it.toString()).readLines()) //add list of lines to list of lists
    }
    return data
}