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How to compress and decompress data

Swift version: 5.1

Paul Hudson    @twostraws   

Apple gives us dedicated API for compressing binary data, although annoyingly it exists on NSData without being bridged neatly to Swift’s native Data type. Fortunately, that conversion is trivial, so this functionality isn’t hard to use.

Here’s how to use it:

do {
    let compressedData = try (yourData as NSData).compressed(using: .lzfse)
    // use your compressed data
} catch {
    print(error.localizedDescription)
}

Although it’s common for folks to call compression and decompression zipping and unzipping, zip is one particular file format and the code above uses the LZFSE compression algorithm instead. This an Apple-design compression algorithm that is recommended unless you specifically need one of the others. The alternatives are:

  • .lz4 compresses less effectively, but is significantly faster.
  • .zlib is the recommended general purpose algorithm to use if you want cross compatibility with non-Apple devices.
  • .lzma compresses the most effectively, but should only be used if memory usage and speed are not important. Apple specifically states that this might use a large amount of memory, and so it should be avoided if you’re trying to compress large amounts of data. This is about 10x slower than the alternatives.

If you want to decompress some data, you should use the counterpart method, decompressed(using:), which works in the same way.

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