.. # Copyright (C) 2020-2023 Intel Corporation
.. # SPDX-License-Identifier: Apache-2.0

.. _data_splitting:

*****************
Dataset Splitters
*****************


OpenFL allows you to specify custom data splits **for simulation runs on a single dataset**.

You may apply data splitters differently depending on the OpenFL workflow that you follow. 


OPTION 1: Use **Native Python API** (Aggregator-Based Workflow) Functions to Split the Data (Deprecated)
===========================================================================================

Predefined OpenFL data splitters functions are as follows:

- ``openfl.utilities.data_splitters.EqualNumPyDataSplitter`` (default)
- ``openfl.utilities.data_splitters.RandomNumPyDataSplitter``
- ``openfl.interface.aggregation_functions.LogNormalNumPyDataSplitter``, which assumes the ``data`` argument as ``np.ndarray`` of integers (labels)
- ``openfl.interface.aggregation_functions.DirichletNumPyDataSplitter``, which assumes the ``data`` argument as ``np.ndarray`` of integers (labels)

Alternatively, you can create an `implementation <https://github.com/intel/openfl/blob/develop/openfl/utilities/data_splitters/numpy.py>`_ of :class:`openfl.plugins.data_splitters.NumPyDataSplitter` and pass it to the :code:`FederatedDataset` function as either ``train_splitter`` or ``valid_splitter`` keyword argument.


OPTION 2: Use Dataset Splitters in your Shard Descriptor
========================================================

Apply one of previously mentioned splitting function on your data to perform a simulation. 

``NumPyDataSplitter`` requires a single ``split`` function. The :code:`split` function returns a list of indices which represents the collaborator-wise indices groups.

This function receives ``data`` - NumPy array required to build the subsets of data indices. It could be the whole dataset, or labels only, or anything else.


.. code-block:: python

    X_train, y_train = ... # train set
    X_valid, y_valid = ... # valid set
    train_splitter = RandomNumPyDataSplitter()
    valid_splitter = RandomNumPyDataSplitter()
    # collaborator_count value is passed to DataLoader constructor
    # shard_num can be evaluated from data_path
    train_idx = train_splitter.split(y_train, collaborator_count)[shard_num]
    valid_idx = valid_splitter.split(y_valid, collaborator_count)[shard_num]
    X_train_shard = X_train[train_idx]
    X_valid_shard = X_valid[valid_idx]

.. note::
    By default, the data is shuffled and split equally. See an `example <https://github.com/intel/openfl/blob/develop/openfl/utilities/data_splitters/numpy.py>`_ of :class:`openfl.utilities.data_splitters.EqualNumPyDataSplitter` for details.