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Lossless Data Compression
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Some lossless data compression algorithms are available in botan, currently all
via third party libraries - these include zlib (including deflate and gzip
formats), bzip2, and lzma.
.. note::
You should always compress *before* you encrypt, because encryption seeks to
hide the redundancy that compression is supposed to try to find and remove.
Compression is done through the ``Compression_Algorithm`` and
``Decompression_Algorithm`` classes, both defined in `compression.h`
Compression and decompression both work in three stages: starting a
message (``start``), continuing to process it (``update``), and then
finally completing processing the stream (``finish``).
.. cpp:class:: Compression_Algorithm
.. cpp:function:: void start(size_t level)
Initialize the compression engine. This must be done before calling
``update`` or ``finish``. The meaning of the `level` parameter varies by
the algorithm but generally takes a value between 1 and 9, with higher
values implying typically better compression from and more memory and/or
CPU time consumed by the compression process. The decompressor can always
handle input from any compressor.
.. cpp:function:: void update(secure_vector<byte>& buf, \
size_t offset = 0, bool flush = false)
Compress the material in the in/out parameter ``buf``. The leading
``offset`` bytes of ``buf`` are ignored and remain untouched; this can be
useful for ignoring packet headers. If ``flush`` is true, the
compression state is flushed, allowing the decompressor to recover the
entire message up to this point without having the see the rest of the
compressed stream.
.. cpp::function:: void finish(secure_vector<byte>& buf, size_t offset = 0)
Finish compressing a message. The ``buf`` and ``offset`` parameters are
treated as in ``update``. It is acceptable to call ``start`` followed by
``finish`` with the entire message, without any intervening call to
``update``.
.. cpp:class:: Decompression_Algorithm
.. cpp:function:: void start()
Initialize the decompression engine. This must be done before calling
``update`` or ``finish``. No level is provided here; the decompressor
can accept input generated by any compression parameters.
.. cpp:function:: void update(secure_vector<byte>& buf, \
size_t offset = 0)
Decompress the material in the in/out parameter ``buf``. The leading
``offset`` bytes of ``buf`` are ignored and remain untouched; this can be
useful for ignoring packet headers.
This function may throw if the data seems to be invalid.
.. cpp::function:: void finish(secure_vector<byte>& buf, size_t offset = 0)
Finish decompressing a message. The ``buf`` and ``offset`` parameters are
treated as in ``update``. It is acceptable to call ``start`` followed by
``finish`` with the entire message, without any intervening call to
``update``.
This function may throw if the data seems to be invalid.
The easiest way to get a compressor is via the functions
.. cpp:function:: Compression_Algorithm* make_compressor(std::string type)
.. cpp:function:: Decompression_Algorithm* make_decompressor(std::string type)
Supported values for `type` include `zlib` (raw zlib with no checksum),
`deflate` (zlib's deflate format), `gzip`, `bz2`, and `lzma`. A null pointer
will be returned if the algorithm is unavailable.
To use a compression algorithm in a `Pipe` use the adaptor types
`Compression_Filter` and `Decompression_Filter` from `comp_filter.h`. The
constructors of both filters take a `std::string` argument (passed to
`make_compressor` or `make_decompressor`), the compression filter also takes a
`level` parameter. Finally both constructors have a parameter `buf_sz` which
specifies the size of the internal buffer that will be used - inputs will be
broken into blocks of this size. The default is 4096.
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