My mistake. If I create a new array I get the problem. EG: import Foundation
func elapsed(s: DispatchTime, e: DispatchTime) -> Double { return Double(e.uptimeNanoseconds - s.uptimeNanoseconds) / 1_000_000_000 } let s = 250_000_000 var a = [UInt8]() a.reserveCapacity(s) let sa = DispatchTime.now() for i in 1 ... s { a.append(0) } let ea = DispatchTime.now() print("Append time: \(elapsed(s: sa, e: ea)) s") let st = DispatchTime.now() a = Array(a[0 ..< (s >> 1)]) let et = DispatchTime.now() print("Trim time: \(elapsed(s: st, e: et)) s") print("a count: \(a.count), capacity: \(a.capacity)") Prints: Append time: 2.65726525 s Trim time: 36.954417356 s a count: 125000000, capacity: 125001696 -- Howard. On 9 May 2017 at 17:53, Howard Lovatt <howard.lov...@gmail.com> wrote: > I find trimming relative to appending OK on my 6 year old MacBook Pro. EG: > > import Foundation > > func elapsed(s: DispatchTime, e: DispatchTime) -> Double { > return Double(e.uptimeNanoseconds - s.uptimeNanoseconds) / > 1_000_000_000 > } > > let s = 250_000_000 > var a = [UInt8]() > a.reserveCapacity(s) > > let sa = DispatchTime.now() > for i in 1 ... s { > a.append(0) > } > let ea = DispatchTime.now() > print("Append time: \(elapsed(s: sa, e: ea)) s") > > let st = DispatchTime.now() > let ha = a[0 ..< (s >> 1)] > let et = DispatchTime.now() > print("Trim time: \(elapsed(s: st, e: et)) s") > > print("ha count: \(ha.count), capacity: \(ha.capacity)") > > > Prints: > > Append time: 2.612397389 s > Trim time: 0.000444132 s > ha count: 125000000, capacity: 125000000 > > > -- Howard. > > On 9 May 2017 at 12:56, Kelvin Ma via swift-users <swift-users@swift.org> > wrote: > >> Depending on what you’re trying to do with the data, you might be better >> off using an UnsafeBufferPointer and allocating and reallocating that, >> C-style. >> >> On Mon, May 8, 2017 at 7:01 PM, Philippe Hausler via swift-users < >> swift-users@swift.org> wrote: >> >>> I wonder if Data might be a better tool for the job here since it is >>> it’s own slice type and would avoid copying large amounts of data into >>> temporary buffers. >>> >>> > On May 8, 2017, at 16:47, Rick Mann via swift-users < >>> swift-users@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? >>> > >>> > Thanks, >>> > >>> > -- >>> > Rick Mann >>> > rm...@latencyzero.com >>> > >>> > >>> > _______________________________________________ >>> > swift-users mailing list >>> > swift-users@swift.org >>> > https://lists.swift.org/mailman/listinfo/swift-users >>> >>> _______________________________________________ >>> swift-users mailing list >>> swift-users@swift.org >>> https://lists.swift.org/mailman/listinfo/swift-users >>> >> >> >> _______________________________________________ >> swift-users mailing list >> swift-users@swift.org >> https://lists.swift.org/mailman/listinfo/swift-users >> >> >
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