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Add README detail around file format compatibility

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Toby Corkindale 2018-08-06 11:03:32 +10:00
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@ -83,9 +83,20 @@ Stream-based compressor/decompressor `SnappyOutputStream`/`SnappyInputStream` ar
* See also [Javadoc API](https://oss.sonatype.org/service/local/repositories/releases/archive/org/xerial/snappy/snappy-java/1.1.3-M1/snappy-java-1.1.3-M1-javadoc.jar/!/index.html)
#### Compatibility Notes
* `SnappyOutputStream` and `SnappyInputStream` use `[magic header:16 bytes]([block size:int32][compressed data:byte array])*` format. You can read the result of `Snappy.compress` with `SnappyInputStream`, but you cannot read the compressed data generated by `SnappyOutputStream` with `Snappy.uncompress`. Here is the data format compatibility matrix:
The original Snappy format definition did not define a file format. It later added
a "framing" format to define a file format, but by this point major software was
already using an industry standard instead -- represented in this library by the
`SnappyOutputStream` and `SnappyInputStream` methods.
For interoperability with other libraries, check that compatible formats are used.
Note that not all libraries support all variants.
* `SnappyOutputStream` and `SnappyInputStream` use `[magic header:16 bytes]([block size:int32][compressed data:byte array])*` format. You can read the result of `Snappy.compress` with `SnappyInputStream`, but you cannot read the compressed data generated by `SnappyOutputStream` with `Snappy.uncompress`.
* `SnappyHadoopCompatibleOutputStream` does not emit a file header but write out the current block size as a preemble to each block
#### Data format compatibility matrix:
| Write\Read | `Snappy.uncompress` | `SnappyInputStream` | `SnappyFramedInputStream` | `org.apache.hadoop.io.compress.SnappyCodec` |
| --------------- |:-------------------:|:------------------:|:-----------------------:|:-------------------------------------------:|
| `Snappy.compress` | ok | ok | x | x |