87 lines
5.0 KiB
Markdown
87 lines
5.0 KiB
Markdown
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# Huff0 entropy compression
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This package provides Huff0 encoding and decoding as used in zstd.
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[Huff0](https://github.com/Cyan4973/FiniteStateEntropy#new-generation-entropy-coders),
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a Huffman codec designed for modern CPU, featuring OoO (Out of Order) operations on multiple ALU
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(Arithmetic Logic Unit), achieving extremely fast compression and decompression speeds.
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This can be used for compressing input with a lot of similar input values to the smallest number of bytes.
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This does not perform any multi-byte [dictionary coding](https://en.wikipedia.org/wiki/Dictionary_coder) as LZ coders,
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but it can be used as a secondary step to compressors (like Snappy) that does not do entropy encoding.
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* [Godoc documentation](https://godoc.org/github.com/klauspost/compress/huff0)
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THIS PACKAGE IS NOT CONSIDERED STABLE AND API OR ENCODING MAY CHANGE IN THE FUTURE.
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## News
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* Mar 2018: First implementation released. Consider this beta software for now.
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# Usage
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This package provides a low level interface that allows to compress single independent blocks.
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Each block is separate, and there is no built in integrity checks.
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This means that the caller should keep track of block sizes and also do checksums if needed.
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Compressing a block is done via the [`Compress1X`](https://godoc.org/github.com/klauspost/compress/huff0#Compress1X) and
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[`Compress4X`](https://godoc.org/github.com/klauspost/compress/huff0#Compress4X) functions.
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You must provide input and will receive the output and maybe an error.
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These error values can be returned:
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| Error | Description |
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|---------------------|-----------------------------------------------------------------------------|
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| `<nil>` | Everything ok, output is returned |
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| `ErrIncompressible` | Returned when input is judged to be too hard to compress |
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| `ErrUseRLE` | Returned from the compressor when the input is a single byte value repeated |
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| `ErrTooBig` | Returned if the input block exceeds the maximum allowed size (128 Kib) |
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| `(error)` | An internal error occurred. |
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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.
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To reduce allocations you can provide a [`Scratch`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch) object
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that can be re-used for successive calls. Both compression and decompression accepts a `Scratch` object, and the same
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object can be used for both.
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Be aware, that when re-using a `Scratch` object that the *output* buffer is also re-used, so if you are still using this
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you must set the `Out` field in the scratch to nil. The same buffer is used for compression and decompression output.
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The `Scratch` object will retain state that allows to re-use previous tables for encoding and decoding.
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## Tables and re-use
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Huff0 allows for reusing tables from the previous block to save space if that is expected to give better/faster results.
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The Scratch object allows you to set a [`ReusePolicy`](https://godoc.org/github.com/klauspost/compress/huff0#ReusePolicy)
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that controls this behaviour. See the documentation for details. This can be altered between each block.
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Do however note that this information is *not* stored in the output block and it is up to the users of the package to
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record whether [`ReadTable`](https://godoc.org/github.com/klauspost/compress/huff0#ReadTable) should be called,
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based on the boolean reported back from the CompressXX call.
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If you want to store the table separate from the data, you can access them as `OutData` and `OutTable` on the
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[`Scratch`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch) object.
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## Decompressing
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The first part of decoding is to initialize the decoding table through [`ReadTable`](https://godoc.org/github.com/klauspost/compress/huff0#ReadTable).
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This will initialize the decoding tables.
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You can supply the complete block to `ReadTable` and it will return the data part of the block
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which can be given to the decompressor.
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Decompressing is done by calling the [`Decompress1X`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch.Decompress1X)
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or [`Decompress4X`](https://godoc.org/github.com/klauspost/compress/huff0#Scratch.Decompress4X) function.
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You must provide the output from the compression stage, at exactly the size you got back. If you receive an error back
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your input was likely corrupted.
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It is important to note that a successful decoding does *not* mean your output matches your original input.
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There are no integrity checks, so relying on errors from the decompressor does not assure your data is valid.
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# Contributing
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Contributions are always welcome. Be aware that adding public functions will require good justification and breaking
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changes will likely not be accepted. If in doubt open an issue before writing the PR.
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