SCALA: monix Obeservable method - scala

Question

I am trying to use Monix Observable to control the big memory of file into smaller chunks of bytes so that it won't use up too many RAM to load the file's bytes.
However, when I using Observable.frominputStreram, it doesn't provide the Array[Byte] that fills into update() function from MessageDigest.
Any suggestions on my codes?
def SHA256_5(file: File)= {
val sha256 = MessageDigest.getInstance("SHA-256")
val in: Observable[Array[Byte]] = {
Observable.fromInputStream(Task(new FileInputStream(file)))
}
in.map(byteArray=>sha256.update(byteArray)).completed
sha256.digest().map("%02x".format(_)).mkString
}
def main(args: Array[String]): Unit = {
val path = "C:\\Users\\ME\\IdeaProjects\\HELLO\\src\\main\\scala\\TRY.scala"
println(SHA256_5(new File(path)))
}

Answer 1

in.map(byteArray=>sha256.update(byteArray)).completed
returns Task - it means that you have to execute that Task and when it finishes you will be able to call
sha256.digest().map("%02x".format(_)).mkString
because Task is used for lazily building asynchronous operation.
Try this instead:
def calcuateSHA(file: File) = for {
sha256 <- Task(MessageDigest.getInstance("SHA-256"))
in = Observable.fromInputStream(Task(new FileInputStream(file)))
_ <- in.map(byteArray=>sha256.update(byteArray)).completed
} yield sha256.digest().map("%02x".format(_)).mkString
def main(args: Array[String]): Unit = {
val path = "C:\\Users\\ME\\IdeaProjects\\HELLO\\src\\main\\scala\\TRY.scala"
import monix.execution.Implicits.global
Await.result(calcuateSHA(new File(path)).runToFuture, Duration.Inf)
}
for starters, or if you want to do it using build in Monix TaskApp instead of hacks for running asynchronous computation in a synchronous main:
object Test extends TaskApp {
def calcuateSHA(file: File) = for {
sha256 <- Task(MessageDigest.getInstance("SHA-256"))
in = Observable.fromInputStream(Task(new FileInputStream(file)))
_ <- in.map(byteArray=>sha256.update(byteArray)).completed
} yield sha256.digest().map("%02x".format(_)).mkString
def run(args: List[String]) = {
val path = "C:\\Users\\ME\\IdeaProjects\\HELLO\\src\\main\\scala\\TRY.scala"
for {
sha <- calcuateSHA(new File(path)
_ = println(sha)
} yield ExitCode.Success
}
}