[swift-users] Help! Slicing an array is very expensive

[Joe Groff]

> On May 8, 2017, at 4:47 PM, Rick Mann via swift-users <swift-users at swift.org> wrote:
> 
> I have this C library that interacts with some hardware over the network that produces a ton of data. It tells me up front the maximum size the data might be so I can allocate a buffer for it, then does a bunch of network requests downloading that data into the buffer, then tells me when it's done and what the final, smaller size is.
> 
> Thanks to previous discussions on the list, I settled on using a [UInt8] as the buffer, because it let me avoid various .withUnsafePointer{} calls (I need the unsafe buffer pointer to live outside the scope of the closures). Unfortunately, When I go to shrink the buffer to its final size with:
> 
>    self.dataBuffer = Array(self.dataBuffer![0 ..< finalBufferSize])
> 
> This ends up taking over 2 minutes to complete (on an iPad Pro). finalBufferSize is very large, 240 MB, but I think it's doing a very naive copy.
> 
> I've since worked around this problem, but is there any way to improve on this?

`self.dataBuffer!.removeSubrange(finalBufferSize ..< self.dataBuffer!.endIndex)` might be more efficient. However, copying an array of bytes definitely shouldn't be that slow, so is worth a bug report.

-Joe