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

Rick Mann rmann at latencyzero.com
Mon May 8 18:47:39 CDT 2017

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?


Rick Mann
rmann at latencyzero.com

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