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dev reference documentation for stream compression

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Alex Coyte 2017-05-02 13:16:41 +10:00 committed by Matthew Barr
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doc/dev-reference/runtime.rst
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@ -80,6 +80,42 @@ functions for the management of streams:
another, resetting the destination stream first. This call avoids the another, resetting the destination stream first. This call avoids the
allocation done by :c:func:`hs_copy_stream`. allocation done by :c:func:`hs_copy_stream`.
==================
Stream Compression
==================
A stream object is allocated as a fixed size region of memory which has been
sized to ensure that no memory allocations are required during scan
operations. When the system is under memory pressure, it may be useful to reduce
the memory consumed by streams that are not expected to be used soon. The
Hyperscan API provides calls for translating a stream to and from a compressed
representation for this purpose. The compressed representation differs from the
full stream object as it does not reserve space for components which are not
required given the current stream state. The Hyperscan API functions for this
functionality are:
* :c:func:`hs_compress_stream`: fills the provided buffer with a compressed
representation of the stream and returns the number of bytes consumed by the
compressed representation. If the buffer is not large enough to hold the
compressed representation, :c:member:`HS_INSUFFICIENT_SPACE` is returned along
with the required size. This call does not modify the original stream in any
way: it may still be written to with :c:func:`hs_scan_stream`, used as part of
the various reset calls to reinitialise its state, or
:c:func:`hs_close_stream` may be called to free its resources.
* :c:func:`hs_expand_stream`: creates a new stream based on a buffer containing
a compressed representation.
* :c:func:`hs_reset_and_expand_stream`: constructs a stream based on a buffer
containing a compressed representation on top of an existing stream, resetting
the existing stream first. This call avoids the allocation done by
:c:func:`hs_expand_stream`.
Note: it is not recommended to use stream compression between every call to scan
for performance reasons as it takes time to convert between the compressed
representation and a standard stream.
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