docs: added operators and zfp section

   components/components

   engines/virtual_engines

   engines/engines

   operators/operators

   api_full/api_full

   api_high/api_high

*************

ZFPCompressor

*************

The ``ZFPCompressor`` Operator is compressor that uses a lossy but optionally

error-bounded compression to achieve high compression ratios.

ZFP provides compressed-array classes that support high throughput read and

write random access to individual array elements. ZFP also supports serial and

parallel (OpenMP and CUDA) compression of whole arrays, e.g., for applications

that read and write large data sets to and from disk.

ADIOS2 provides a ``ZFPCompressor`` operator that lets you compress an

decompress variables. Below there is an example of how to invoke

``ZFPCompressor`` operator:

.. code-block:: c++

    adios2::IO io = adios.DeclareIO("Output");

    auto ZFPOp    = adios.DefineOperator("ZFPCompressor", adios2::ops::LossyZFP);

    auto var_r32 = io.DefineVariable<float>("r32", shape, start, count);

    var_r32.AddOperation(ZFPOp, {{adios2::ops::zfp::key::rate, rate}});

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ZFPCompressor Specific parameters

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``ZFPCompressor`` operator accepts the following operator specific

parameters:

+-------------------+---------------------------------------------+

| ``ZFPCompressor`` available parameters                          |

+===================+=============================================+

| ``accuracy``      | Fixed absolute error tolerance              |

+-------------------+---------------------------------------------+

| ``rate``          | Fixed number of bits in a compression unit  |

+-------------------+---------------------------------------------+

| ``precision``     | Fixed number of uncompressed bits per value |

+-------------------+---------------------------------------------+

| ``backend``       | Backend device: ``cuda`` ``omp`` ``serial`` |

+-------------------+---------------------------------------------+

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ZFPCompressor Execution Policy

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``ZFPCompressor`` can run in multiple backend devices: GPUs (CUDA), OpenMP, and

in the host CPU. By default ``ZFPCompressor`` will choose its backend following

the above order upon the availability of the device adapter.

Exceptionally, if its corresponding ADIOS2 variable contains a CUDA memory

address, this is a CUDA buffer, the CUDA backend will be called if available.

In any case, the user can manually set the backend using the ZFPOperator

specific parameter ``backend``.

###################

Supported Operators

###################

The Operator abstraction allows ADIOS2 to act upon the user application data,

either from a ``adios2::Variable`` or a set of Variables in an ``adios2::IO``

object.  Current supported operations are:

1. Data compression/decompression, lossy and lossless.

2. Callback functions (C++11 bindings only) supported by specific engines

This section provides a description of the :ref:`Available Operators` in ADIOS2

and their specific parameters to allow extra-control from the user. Parameters

are passed in key-value pairs for:

1. Operator general supported parameters.

2. Operator specific supported parameters.

Parameters are passed at:

1. Compile time: using the second parameter of the method

   ``ADIOS2::DefineOperator``

2. :ref:`Runtime Configuration Files` in the :ref:`ADIOS` component.

.. include:: ZFPCompressor.rst