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The ARC/WARC factory and reader methods support both compressed and uncompressed files in varying combinations.
Basically you can create a reader for compressed or uncompressed files, but the factory also includes an auto-detection mechanism which can determine the correct reader on a per file basis.
Writing compressed ARC/WARC files is also possible though the use of different methods in the writer factories.
GZip compression is only supported on ARC/WARC files where each record is compressed individually and concatenated into one file and not the case where the whole ARC/WARC file and all it's records are GZip'ed as a whole. The later mostly because this makes random access to individual record highly ineffective.
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