Parallel sha256 checksum with kotlin

MultiSHA.kt

import java.io.File
import java.security.MessageDigest
import kotlinx.coroutines.*

val digester = MessageDigest.getInstance("SHA-256")

suspend fun check(file_path: String, checksum_in: String) = withContext(Dispatchers.Default){
    val target_file = File(file_path)

    if(target_file.isFile){
        val byte_array = target_file.readBytes()
        val checksum_out_raw = digester.digest(byte_array)
        val string_builder = StringBuilder();

        checksum_out_raw.asUByteArray().forEach{
            string_builder.append( it.toString(16).padStart(2,'0'))
        }
        val checksum_out = string_builder.toString()
        if(checksum_in==checksum_out) println("$file_path [OK]") else println("$file_path [FAILED]")
    }

}

fun main(args : Array<String>) {
    if(args.size<1){
        println("Missing the checksum file.")
        return
    }
    val checksum_file = File(args[0])
    if(!checksum_file.isFile){
        println("$checksum_file is not a file.")
        return
    }
    runBlocking {
        checksum_file.reader().readLines().forEach { line ->
            val split = line.split("  ")
            val checksum_in = split[0]
            val file_path = split[1]
            check(file_path, checksum_in)
        }
    }
}