universal in-place decompression interface

[Moelf]

by "in-place" I mean the user would pre-allocate a Vector{UInt8} or something as the sink

sometimes decompression is needed for low-level stuff, such as handling "buffers" in a file spec, and multiple decompression together assemble the entire data blob.

it would be nice if there's a in-place interface that support multiple algorithms through this central package.

[svilupp]

I've drafted a PR around Buffers to support faster decompression when loading Arrow files.

Would that support your use case as well? Or would you need the sink to be ByteData (instead of buffer)?

[mkitti]

Closed by #132 and released as 0.9.12

[nhz2]

I'm reopening this issue because I think there are some issues to work on with the current interface.

1. The Buffer type used in #136 is internal. See #202
2. For use in Zarr.jl it would be helpful to be able to decompress directly into for example a Vector{Float64} to avoid an extra copy.
3. The underlying codec should be informed somehow that it is doing a fully in-place operation, so it can internally avoid extra buffering and copies. Ref: https://github.com/JuliaIO/CodecZstd.jl/pull/52

[Moelf]

somehow I miseed @mkitti 's original comment since the "closed by" refer to this very issue, what was the PR that supposedly fixed this?

[Moelf]

For use in Zarr.jl it would be helpful to be able to decompress directly into for example a Vector{Float64} to avoid an extra copy.

this can't work directly, the two possibilities are you have a buffer = reinterpret(UInt8, ...) and you give this buffer to TranscodingStreams.

Or, you have data = reinterpret(Float64, buffer) and give the buffer to TranscodingStreams

[nhz2]

Yes, I think this would require a new unsafe_transcode! function that works directly with pointers.

[nhz2]

Also, a general unsafe_transcode! interface could be useful for other packages that don't support or need a streaming API like Blosc.jl, LibDeflate.jl, JLD2.jl, HDF5.jl, Zarr.jl... so maybe it should go in a separate LosslessChunkCompressors.jl package, and be added as a dependency here.

[nhz2]

@Moelf @mkitti I have a draft interface for in-place encoding and decoding defined in https://github.com/nhz2/ChunkCodecs.jl/blob/main/ChunkCodecCore/src/interface.jl

The interface currently doesn't directly use pointers, which is nice for avoiding GC issues, but sometimes things don't work as expected, for example decoding a view of a PyArray: https://github.com/JuliaPy/PythonCall.jl/issues/579

[mkitti]

That's interesting. I will try to take a closer look next week.