elm-flate

An implementation of DEFLATE compression.

The deflate format is used in common file formats like gzip, png, and woff. This package implements vanilla deflate/inflate, as well as gzip and zlib variants. It additionally exposes LZ77 compression and the Adler32 and Crc32 checksum algorithms.

Example

import Bytes
import Bytes.Encode as Encode
import Bytes.Decode as Decode

text : String
text =
    "Dyn flam dôve nea"

decodeAsString : Bytes -> Maybe String
decodeAsString buffer =
    let 
        decoder = Decode.string (Bytes.width buffer)
    in 
        Decode.decode decoder buffer

inflate (deflate (Encode.encode (Encode.string text)))
    |> Maybe.andThen decodeAsString
    --> Just "Dyn flam dôve nea"

Performance

Performance is not great for the moment, in particular using LZ77 compression is very slow for large data. If all you need is data that looks like deflated data, then raw encoding is the fastest

deflateWithOptions Flate.Raw

If you actually want some compression but are worried about speed, then you can use the NoCompression argument. This skips LZ77 compression, but still uses a huffman table to make the data a little smaller at relatively low cost.

deflateWithOptions (Flate.Static Flate.NoCompression)

Of course, if you really want compression and speed is less of a factor, the default deflate will use a dynamic (custom to your data) huffman table and LZ77 compression.

deflateWithOptions (Flate.Dynamic (Flate.WithWindowSize LZ77.maxWindowSize))

With this package as a testing ground, hopefully we can make better primitives that will make this package faster in the future. For now it at least works, and we can build thinks on top of it. For instance PNG encoding is now possible.

Acknowledgements

• The decoder is based on the libflate rust crate by Takeru Ohta
• The encoder is based on tiny-inflate written by Devon Govett
• Which is a JS port of the tinf c library by Jørgen Ibsen