fun String.unpack(): String {
    val builder = StringBuilder()
    var i = 0
    while (i < length) {
        if (this[i].isDigit()) {

            val repeatCount = this[i].digitToInt()
            i += 2
            val subStringStartIndex = i
            var openedBraces = 1
            while (openedBraces > 0) {
                when (this[i]) {
                    '[' -> ++openedBraces
                    ']' -> --openedBraces
                }
                ++i
            }
            val stringToRepeat = substring(subStringStartIndex until i - 1).unpack()

            builder.append(stringToRepeat.repeat(repeatCount))
        } else {
            builder.append(this[i])
            ++i
        }
    }
    return builder.toString()
}