is there a nicer way of writing this? ```inputStream.apply { skip(from.toLong()) }.use { var by...

# codereview

[tipsy]

is there a nicer way of writing this?

inputStream.apply { skip(from.toLong()) }.use {
    var bytesLeft = bytesToWrite
    while (bytesLeft != 0) {
        val read = it.read(buffer, 0, Math.min(buffer.size, bytesLeft))
        outputStream.write(buffer, 0, read)
        bytesLeft -= read
    }
}

[Matteo Mirk]

since Java 9, InputStream has the method transferTo(OutputStream)
https://docs.oracle.com/javase/9/docs/api/java/io/InputStream.html#transferTo-java.io.OutputStream-
So you can write:

inputStream.apply { 
    skip(from.toLong()) 
}.transferTo(outputStream)

This works well if both are simple In/Out streams, otherwise if you’re dealing with files (FileInputStream/FileOutputStream) you should prefer Files.copy as it will be faster.

[trevjones]

i'd probably get rid of the .apply and move the skip into the use block

[tipsy]

thanks @Matteo Mirk, unfortunately i'm targeting java8

[Matteo Mirk]

oh… that’s really unfortunate! 😞

[Matteo Mirk]

Anyway it’s still worth try copying the code from OpenJDK 9 and create an extension function

[tipsy]

🤔

[tipsy]

sure, why not

[Matteo Mirk]

Or use your implementation, as you please, but I would encapsulate it into an ext fun for readability

[Matteo Mirk]

🆒

[tipsy]

i trust openjdk more

[trevjones]

kotlin has a copyTo ext function already as well

[trevjones]

message has been deleted

[trevjones]

inputStream.use {
  it.skip(from.toLong())
  it.copyTo(outputStream)
}

[tipsy]

can you limit the bytes to write in it?

[tipsy]

i don't want to write the whole inputstream

[trevjones]

ah not with this one. but maybe there is something like a .subset(drop, truncate) to wrap the input stream. that way you can compose it rather than handwriting

[Matteo Mirk]

kotlin has a copyTo ext function already as well

Ohh cool didn’t know it! Of course it has one 😉

[trevjones]

big detail I missed was the "bytesToWrite" arg 😕
maybe a nice raw Sequence<Byte> so you could do something like .takeWhile (if that is a thing, or maybe I just am stuck in reactivex thinking too far?)

[trevjones]

I dunno, overthinking, probably just wrap up what you have in an ext fun , drop the apply so if skip pukes you don't leak the stream?

[tipsy]

private fun InputStream.copyTo(outputStream: OutputStream, from: Int, to: Int, buffer: ByteArray = ByteArray(1024)) = this.use {
    it.skip(from.toLong())
    var bytesLeft = to
    while (bytesLeft != 0) {
        val read = it.read(buffer, 0, Math.min(buffer.size, bytesLeft))
        outputStream.write(buffer, 0, read)
        bytesLeft -= read
    }
}

[tipsy]

reads a lot nicer, thank you

[tipsy]

wish i could get rid of that while, but i'll live

[trevjones]

one other nit, which I would expect intellij to be screaming about. Math.min looks from java but kotlin has a top level min I thought?

[tipsy]

it suggests buffer.size.coerceAtMost(bytesLeft))

[tipsy]

but i found that much harder to read..

[trevjones]

I agree, feels like prioritizing an extension function just to use an extension function. plus min(x,y) etc are so common everywhere 🤔