mirror of https://github.com/go-gitea/gitea.git
79 lines
4.5 KiB
Markdown
79 lines
4.5 KiB
Markdown
|
# Finite State Entropy
|
||
|
|
|
||
|
|
This package provides Finite State Entropy encoding and decoding.
|
||
|
|
|
||
|
|
Finite State Entropy (also referenced as [tANS](https://en.wikipedia.org/wiki/Asymmetric_numeral_systems#tANS))
|
||
|
|
encoding provides a fast near-optimal symbol encoding/decoding
|
||
|
|
for byte blocks as implemented in [zstandard](https://github.com/facebook/zstd).
|
||
|
|
|
||
|
|
This can be used for compressing input with a lot of similar input values to the smallest number of bytes.
|
||
|
|
This does not perform any multi-byte [dictionary coding](https://en.wikipedia.org/wiki/Dictionary_coder) as LZ coders,
|
||
|
|
but it can be used as a secondary step to compressors (like Snappy) that does not do entropy encoding.
|
||
|
|
|
||
|
|
* [Godoc documentation](https://godoc.org/github.com/klauspost/compress/fse)
|
||
|
|
|
||
|
|
## News
|
||
|
|
|
||
|
|
* Feb 2018: First implementation released. Consider this beta software for now.
|
||
|
|
|
||
|
|
# Usage
|
||
|
|
|
||
|
|
This package provides a low level interface that allows to compress single independent blocks.
|
||
|
|
|
||
|
|
Each block is separate, and there is no built in integrity checks.
|
||
|
|
This means that the caller should keep track of block sizes and also do checksums if needed.
|
||
|
|
|
||
|
|
Compressing a block is done via the [`Compress`](https://godoc.org/github.com/klauspost/compress/fse#Compress) function.
|
||
|
|
You must provide input and will receive the output and maybe an error.
|
||
|
|
|
||
|
|
These error values can be returned:
|
||
|
|
|
||
|
|
| Error | Description |
|
||
|
|
|---------------------|-----------------------------------------------------------------------------|
|
||
|
|
| `<nil>` | Everything ok, output is returned |
|
||
|
|
| `ErrIncompressible` | Returned when input is judged to be too hard to compress |
|
||
|
|
| `ErrUseRLE` | Returned from the compressor when the input is a single byte value repeated |
|
||
|
|
| `(error)` | An internal error occurred. |
|
||
|
|
|
||
|
|
As can be seen above there are errors that will be returned even under normal operation so it is important to handle these.
|
||
|
|
|
||
|
|
To reduce allocations you can provide a [`Scratch`](https://godoc.org/github.com/klauspost/compress/fse#Scratch) object
|
||
|
|
that can be re-used for successive calls. Both compression and decompression accepts a `Scratch` object, and the same
|
||
|
|
object can be used for both.
|
||
|
|
|
||
|
|
Be aware, that when re-using a `Scratch` object that the *output* buffer is also re-used, so if you are still using this
|
||
|
|
you must set the `Out` field in the scratch to nil. The same buffer is used for compression and decompression output.
|
||
|
|
|
||
|
|
Decompressing is done by calling the [`Decompress`](https://godoc.org/github.com/klauspost/compress/fse#Decompress) function.
|
||
|
|
You must provide the output from the compression stage, at exactly the size you got back. If you receive an error back
|
||
|
|
your input was likely corrupted.
|
||
|
|
|
||
|
|
It is important to note that a successful decoding does *not* mean your output matches your original input.
|
||
|
|
There are no integrity checks, so relying on errors from the decompressor does not assure your data is valid.
|
||
|
|
|
||
|
|
For more detailed usage, see examples in the [godoc documentation](https://godoc.org/github.com/klauspost/compress/fse#pkg-examples).
|
||
|
|
|
||
|
|
# Performance
|
||
|
|
|
||
|
|
A lot of factors are affecting speed. Block sizes and compressibility of the material are primary factors.
|
||
|
|
All compression functions are currently only running on the calling goroutine so only one core will be used per block.
|
||
|
|
|
||
|
|
The compressor is significantly faster if symbols are kept as small as possible. The highest byte value of the input
|
||
|
|
is used to reduce some of the processing, so if all your input is above byte value 64 for instance, it may be
|
||
|
|
beneficial to transpose all your input values down by 64.
|
||
|
|
|
||
|
|
With moderate block sizes around 64k speed are typically 200MB/s per core for compression and
|
||
|
|
around 300MB/s decompression speed.
|
||
|
|
|
||
|
|
The same hardware typically does Huffman (deflate) encoding at 125MB/s and decompression at 100MB/s.
|
||
|
|
|
||
|
|
# Plans
|
||
|
|
|
||
|
|
At one point, more internals will be exposed to facilitate more "expert" usage of the components.
|
||
|
|
|
||
|
|
A streaming interface is also likely to be implemented. Likely compatible with [FSE stream format](https://github.com/Cyan4973/FiniteStateEntropy/blob/dev/programs/fileio.c#L261).
|
||
|
|
|
||
|
|
# Contributing
|
||
|
|
|
||
|
|
Contributions are always welcome. Be aware that adding public functions will require good justification and breaking
|
||
|
|
changes will likely not be accepted. If in doubt open an issue before writing the PR.
|