=====================
Blosc filter for HDF5
=====================

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This is a filter for HDF5 that uses the Blosc compressor; by installing this
filter, you can read and write HDF5 files with Blosc-compressed datasets.

You need to be a bit careful before using this filter because you
should not activate the shuffle right in HDF5, but rather from Blosc
itself.  This is because Blosc uses an SIMD shuffle internally which
is much faster.

Installing the Blosc filter plugin
==================================

Instead of just linking this Blosc filter into your HDF5 application, it is possible to install
it as a system-wide HDF5 plugin (with HDF5 1.8.11 or later).  This is useful because it allows
*every* HDF5-using program on your system to transparently read Blosc-compressed HDF5 files.

As described in the `HDF5 plugin documentation <https://portal.hdfgroup.org/display/HDF5/HDF5+Dynamically+Loaded+Filters>`_, you just need to compile the Blosc plugin into a shared library and
copy it to the plugin directory (which defaults to ``/usr/local/hdf5/lib/plugin`` on non-Windows systems).

Following the ``cmake`` instructions below produces a ``libH5Zblosc.so`` shared library 
file (or ``.dylib``/``.dll`` on Mac/Windows), that you can copy to the HDF5 plugin directory.

To *write* Blosc-compressed HDF5 files, on the other hand, an HDF5 using program must be
specially modified to enable the Blosc filter when writing HDF5 datasets, as described below.


Linking the Blosc filter directly into your program
===================================================

Instead of (or in addition to) installing the Blosc plugin system-wide as
described above, you can also link the Blosc filter directly into your
application.  Although this only makes the Blosc filter available in
your application (as opposed to other HDF5-using applications), it
is useful in cases where installing the plugin is inconvenient.  Compile
the Blosc filter as described above, but link ``libblosc_filter.a``
(generated by ``make``) directly into your program.

In order to register Blosc in your HDF5 application, you then need
to call a function in blosc_filter.h, with the following signature::

    int register_blosc(char **version, char **date)

Calling this will register the filter with the HDF5 library and will
return info about the Blosc release in `**version` and `**date`
char pointers.

A non-negative return value indicates success.  If the registration
fails, an error is pushed onto the current error stack and a negative
value is returned.

An example C program ('src/example.c') is included which demonstrates
the proper use of the filter.

This filter has been tested against HDF5 versions 1.6.5 through
1.8.10.  It is released under the MIT license (see LICENSE.txt for
details).

Using the Blosc filter in your application
==========================================

Assuming the filter is installed (either by a system-wide plugin or registered
directly in your program as described above), your application can transparently
*read* HDF5 files with Blosc-compressed datasets.  (The HDF5 library will detect
that the dataset is Blosc-compressed and invoke the filter automatically).

To *write* an HDF5 file with a Blosc-compressed dataset, you call the
`H5Pset_filter <https://www.hdfgroup.org/HDF5/doc/RM/RM_H5P.html#Property-SetFilter>`_ function
on the property list of the dataset you are creating, and pass ``FILTER_BLOSC``
(defined in ``blosc_filter.h``) for the ``filter_id`` parameter.   In addition, HDF5
only supports compression for "chunked" datasets; this just means that you need to
call `H5Pset_chunk <https://www.hdfgroup.org/HDF5/doc/RM/RM_H5P.html#Property-SetChunk>`_ to
specify a chunk size (e.g. 1MB chunks), and the subsequent chunking of the dataset I/O
is performed transparently by HDF5.

Compiling
=========

The filter consists of a single 'src/blosc_filter.c' source file and
'src/blosc_filter.h' header, which will need the Blosc library
installed to work.   It is simplest to just use the provided ``cmake``
build scripts, which compile and both the filter and the Blosc library
into a library for you

Assuming you have `cmake <http://www.cmake.org/>`_ and other standard
Unix build tools installed, do::

    mkdir build
    cd build
    cmake ..
    make

This generates the library/plugin files required above in the ``build``
directory.

Acknowledgments
===============

See THANKS.rst.


----

  **Enjoy data!**
