What you have is probably ok in common usage; but there is a chance that it
would not properly do a copy on write change as you expect if the Data was
somehow obtained from a multi-threaded environment. Here is a slight refactor
(with a few extra fixes and comments for you that might save you some time
later)
import Foundation
import Compression
public enum CompressionAlgorithm {
case lz4 // speed is critical
case lz4a // space is critical
case zlib // reasonable speed and space
case lzfse // better speed and space
}
private enum CompressionOperation {
case compression
case decompression
}
private func perform(_ operation: CompressionOperation,
on input: Data,
using algorithm: CompressionAlgorithm,
workingBufferSize: Int = 2000) -> Data? {
// set the algorithm
let streamAlgorithm: compression_algorithm
switch algorithm {
case .lz4: streamAlgorithm = COMPRESSION_LZ4
case .lz4a: streamAlgorithm = COMPRESSION_LZMA
case .zlib: streamAlgorithm = COMPRESSION_ZLIB
case .lzfse: streamAlgorithm = COMPRESSION_LZFSE
}
// set the stream operation, and flags
let streamOperation: compression_stream_operation
let flags: Int32
switch operation {
case .compression:
streamOperation = COMPRESSION_STREAM_ENCODE
flags = Int32(COMPRESSION_STREAM_FINALIZE.rawValue)
case .decompression:
streamOperation = COMPRESSION_STREAM_DECODE
flags = 0
}
let dstSize = workingBufferSize
let dstPointer = UnsafeMutablePointer<UInt8>.allocate(capacity: dstSize) //
you could technically use a Data for this too by accessing the mutable bytes ;)
defer {
dstPointer.deallocate(capacity: dstSize)
}
let inputCount = input.count
return input.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) in // the
closure here can return a Data so the entire compression operation can be
encapsulated in the lambda
var output = Data()
var stream = compression_stream(dst_ptr: dstPointer, dst_size: dstSize,
src_ptr: bytes, src_size: inputCount, state: nil)
var status = compression_stream_init(&stream, streamOperation,
streamAlgorithm)
guard status != COMPRESSION_STATUS_ERROR else {
return nil // perhaps instead of making this nullable a throw?
}
defer {
compression_stream_destroy(&stream)
}
while status == COMPRESSION_STATUS_OK {
// process the stream
status = compression_stream_process(&stream, flags)
// collect bytes from the stream and reset
switch status {
case COMPRESSION_STATUS_OK:
output.append(dstPointer, count: dstSize)
stream.dst_ptr = dstPointer
stream.dst_size = dstSize
break // I am certain you probably meant to break here?
case COMPRESSION_STATUS_ERROR:
return nil // perhaps instead of making this nullable a throw?
case COMPRESSION_STATUS_END:
output.append(dstPointer, count: stream.dst_ptr - dstPointer)
break // here too, this should break right?
default:
break // this probably should be a fatal error since it is an
unexpected result.
}
}
return output
}
}
// Compressed keeps the compressed data and the algorithm
// together as one unit, so you never forget how the data was
// compressed.
public struct Compressed {
public let data: Data // this was what was saving your state of the pointer
public let algorithm: CompressionAlgorithm
public init(data: Data, algorithm: CompressionAlgorithm) {
self.data = data
self.algorithm = algorithm
}
// Compress the input with the specified algorith. Returns nil if it fails.
public static func compress(input: Data,
with algorithm: CompressionAlgorithm) ->
Compressed? {
guard let data = perform(.compression, on: input, using: algorithm)
else {
return nil
}
return Compressed(data: data, algorithm: algorithm)
}
// Factory method to return uncompressed data. Returns nil if it cannot be
decompressed.
public func makeDecompressed() -> Data? {
return perform(.decompression, on: data, using: algorithm)
}
}
// For discoverability, add a compressed method to Data
extension Data {
// Factory method to make compressed data or nil if it fails.
public func makeCompressed(with algorithm: CompressionAlgorithm) ->
Compressed? {
return Compressed.compress(input: self, with: algorithm)
}
}
// Example usage:
let input = Data(bytes: Array(repeating: UInt8(123), count: 10000))
let compressed = input.makeCompressed(with: .lzfse)
compressed?.data.count // in most cases much less than orginal input count
let restoredInput = compressed?.makeDecompressed()
input == restoredInput // true
> On Dec 9, 2016, at 9:04 AM, Ray Fix <ray...@gmail.com> wrote:
>
> Hi Philippe,
>
> Thank you for your response!
>
>> On Dec 9, 2016, at 8:44 AM, Philippe Hausler <phaus...@apple.com
>> <mailto:phaus...@apple.com>> wrote:
>>
>> So letting the bytes escape from the closure could potentially be in that
>> territory of “bad things may happen”. If you know for certain that the Data
>> will not be mutated and it’s underlying storage will last for the duration
>> of the usage then you should be able to get away with it.
>>
>
> So far it is looking like I am getting away with it. I believe the conditions
> that you state hold.
>
>> The bytes pointer is an inner pointer return from the encapsulating object
>> backing the struct Data. Since that backing object can potentially be
>> deallocated via mutation or the buffer may be reallocated (which very often
>> can cause the base address of the pointer to change) we decided it would be
>> safer to use a closure to ensure the lifespan of the bytes over the access
>> to the bytes.
>>
>> Perhaps it might be best to understand more of what you are trying to do to
>> offer a better solution.
>
> I am trying to use the compression_stream C API in the Compression framework
> and wrap it so it is a bit more Swifty. :]
>
> A full gist of the playground is here:
> https://gist.github.com/rayfix/a286bb55accffef09249ba3535993782
> <https://gist.github.com/rayfix/a286bb55accffef09249ba3535993782>
>
> See line 91.
>
> Appreciate the hints.
>
> Ray
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