How do I implement the following method in Scala.js?
import scala.scalajs.js
def toScalaArray(input: js.typedarray.Uint8Array): Array[Byte] =
// code in question
3 Answers
All the current answers require iterating over the array in user space.
Scala.js has optimizer supported conversions for typed arrays (in fact, Array[Byte] are typed arrays in modern configs). You'll likely get better performance by doing this:
import scala.scalajs.js.typedarray._
def toScalaArray(input: Uint8Array): Array[Byte] = {
// Create a view as Int8 on the same underlying data.
new Int8Array(input.buffer, input.byteOffset, input.length).toArray
}
The additional new Int8Array is necessary to re-interpret the underlying buffer as signed (the Byte type is signed). Only then, Scala.js will provide the built in conversion to Array[Byte].
When looking at the generated code, you'll see no user space loop is necessary: The built-in slice method is used to copy the TypedArray. This will almost certainly not be beatable in terms of performance by any user-space loop.
$c_Lhelloworld_HelloWorld$.prototype.toScalaArray__sjs_js_typedarray_Uint8Array__AB = (function(input) {
var array = new Int8Array(input.buffer, $uI(input.byteOffset), $uI(input.length));
return new $ac_B(array.slice())
});
If we compare this with the currently accepted answer (input.view.map(_.toByte).toArray) we see quite a difference (comments mine):
$c_Lhelloworld_HelloWorld$.prototype.toScalaArray__sjs_js_typedarray_Uint8Array__AB = (function(input) {
var this$2 = new $c_sjs_js_IterableOps(input);
var this$5 = new $c_sc_IterableLike$$anon$1(this$2);
// We need a function
var f = new $c_sjsr_AnonFunction1(((x$1$2) => {
var x$1 = $uS(x$1$2);
return ((x$1 << 24) >> 24)
}));
new $c_sc_IterableView$$anon$1();
// Here's the view: So indeed no intermediate allocations.
var this$8 = new $c_sc_IterableViewLike$$anon$6(this$5, f);
var len = $f_sc_TraversableOnce__size__I(this$8);
var result = new $ac_B(len);
// This function actually will traverse.
$f_sc_TraversableOnce__copyToArray__O__I__V(this$8, result, 0);
return result
});
3 Comments
gzm0
Yes :-/ The byte array view had the wrong parameters. It's fixed.
R A
I've done some tests and found no performance difference with my other proposed solution. Both are ~1.8 faster than
view variant thoughedited per request: tl;dr
input.view.map(_.toByte).toArray
Original answer
I'm not intimately familiar with Scala-js, but I can elaborate on some of the questions that came up in the comments, and improve upon your self-answer.
Also I don't quite get why I need toByte calls
class Uint8Array extends Object with TypedArray[Short, Uint8Array]
Scala treats a Uint8Array as a collection of Short, whereas you are expecting it to be a collection of Byte
Uint8Array's toArray method notes:
This member is added by an implicit conversion from
Uint8ArraytoIterableOps[Short]performed by methoditerableOpsinscala.scalajs.js.LowestPrioAnyImplicits.
So the method is returning an Array[Short] which you then .map to convert the Shorts to Bytes.
In your answer you posted
input.toArray.map(_.toByte)
which is technically correct, but it has the downside of allocating an intermediate array of the Shorts. To avoid this allocation, you can perform the .map operation on a .view of the array, then call .toArray on the view.
Views in Scala (and by extension Scala.js) are lightweight objects that reference an original collection plus some kind of transformation/filtering function, which can be iterated like any other collection. You can compose many transformation/filters on a view without having to allocate intermediate collections to represent the results. See the docs page (linked) for more.
input.view.map(_.toByte).toArray
Depending on how you intend to pass the resulting value around, you may not even need to call .toArray. For example if all you need to do is iterate the elements later on, you could just pass the view around as an Iterable[Byte] without ever having to allocate a separate array.
import scala.scalajs.js
def toScalaArray(input: js.typedarray.Uint8Array): Array[Byte] =
input.toArray.map(_.toByte)
3 Comments
Andrey Tyukin
This was just a guess, I didn't actually attempt to test it. Does everything actually work out with signs etc.? Short can represent +255, signed byte cannot (aren't the sign issues the reason why they made it an
Iterable[Short] in the first place?)R A
@AndreyTyukin I only tested two-way conversion with -127 number and it worked fine
R A
Uint8Array possible values are within 0 and 255. numbers 128-255 get converted to negative Byte numbers with no data loss (e.g. 255 maps to -1)
toArray.map(_.toByte), because that would at the very least allocate two arrays, and one of them would be twice as large as what's actually stored in the memory. I clicked around through the API, but found no obvious way to do the same without creating any intermediate data structures. I feel that the question is clear enough even as it is.