forked from toolshed/abra
90 lines
5.4 KiB
Markdown
90 lines
5.4 KiB
Markdown
# Huff0 entropy compression
|
|
|
|
This package provides Huff0 encoding and decoding as used in zstd.
|
|
|
|
[Huff0](https://github.com/Cyan4973/FiniteStateEntropy#new-generation-entropy-coders),
|
|
a Huffman codec designed for modern CPU, featuring OoO (Out of Order) operations on multiple ALU
|
|
(Arithmetic Logic Unit), achieving extremely fast compression and decompression speeds.
|
|
|
|
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/huff0)
|
|
|
|
## News
|
|
|
|
This is used as part of the [zstandard](https://github.com/klauspost/compress/tree/master/zstd#zstd) compression and decompression package.
|
|
|
|
This ensures that most functionality is well tested.
|
|
|
|
# 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 [`Compress1X`](https://godoc.org/github.com/klauspost/compress/huff0#Compress1X) and
|
|
[`Compress4X`](https://godoc.org/github.com/klauspost/compress/huff0#Compress4X) functions.
|
|
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 |
|
|
| `ErrTooBig` | Returned if the input block exceeds the maximum allowed size (128 Kib) |
|
|
| `(error)` | An internal error occurred. |
|
|
|
|
|
|
As can be seen above some of 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/huff0#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.
|
|
|
|
The `Scratch` object will retain state that allows to re-use previous tables for encoding and decoding.
|
|
|
|
## Tables and re-use
|
|
|
|
Huff0 allows for reusing tables from the previous block to save space if that is expected to give better/faster results.
|
|
|
|
The Scratch object allows you to set a [`ReusePolicy`](https://godoc.org/github.com/klauspost/compress/huff0#ReusePolicy)
|
|
that controls this behaviour. See the documentation for details. This can be altered between each block.
|
|
|
|
Do however note that this information is *not* stored in the output block and it is up to the users of the package to
|
|
record whether [`ReadTable`](https://godoc.org/github.com/klauspost/compress/huff0#ReadTable) should be called,
|
|
based on the boolean reported back from the CompressXX call.
|
|
|
|
If you want to store the table separate from the data, you can access them as `OutData` and `OutTable` on the
|
|
[`Scratch`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch) object.
|
|
|
|
## Decompressing
|
|
|
|
The first part of decoding is to initialize the decoding table through [`ReadTable`](https://godoc.org/github.com/klauspost/compress/huff0#ReadTable).
|
|
This will initialize the decoding tables.
|
|
You can supply the complete block to `ReadTable` and it will return the data part of the block
|
|
which can be given to the decompressor.
|
|
|
|
Decompressing is done by calling the [`Decompress1X`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch.Decompress1X)
|
|
or [`Decompress4X`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch.Decompress4X) function.
|
|
|
|
For concurrently decompressing content with a fixed table a stateless [`Decoder`](https://godoc.org/github.com/klauspost/compress/huff0#Decoder) can be requested which will remain correct as long as the scratch is unchanged. The capacity of the provided slice indicates the expected output size.
|
|
|
|
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.
|
|
|
|
# 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.
|