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# compressionStream(using:writingTo:blockSize:flags:threadCount:)

Creates a compression sequential output stream.

```
static func compressionStream(using compressionAlgorithm: ArchiveCompression, writingTo compressedStream: ArchiveByteStream, blockSize: Int = (1<<20), flags: ArchiveFlags = [], threadCount: Int = 0) -> ArchiveByteStream?
```

## Parameters

`compressionAlgorithm`

The compression algorithm.

`compressedStream`

An output stream that receives compressed data, the operation only calls write methods.

`blockSize`

The compression block size, in bytes.

`flags`

Flags that control the behavior of the operation.

`threadCount`

The number of worker threads that the operation uses, set to `0` for default.

## Return Value

A new archive byte stream.

## Discussion

The new stream writes compressed data to the supplied archive byte stream.

The stream that the function returns only implements [`write(from:)`](/documentation/AppleArchive/ArchiveByteStreamProtocol/write(from:)) and [`write(from:atOffset:)`](/documentation/AppleArchive/ArchiveByteStreamProtocol/write(from:atOffset:)).

During compression the operation splits the data into blocks of `blockSize` bytes, compressing each block independently. Larger blocks provide better compression, but require more memory (for compression and decompression) and increase latency in random access.

Good values for `blockSize` are between 256 KB and 16 MB. The 1 MB default value provides a good compromise between compression ratio and memory requirement.

---

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