'''A demo for defining data iterator.

    .. versionadded:: 1.2.0

The demo that defines a customized iterator for passing batches of data into
`xgboost.DeviceQuantileDMatrix` and use this `DeviceQuantileDMatrix` for
training.  The feature is used primarily designed to reduce the required GPU
memory for training on distributed environment.

Aftering going through the demo, one might ask why don't we use more native
Python iterator?  That's because XGBoost requires a `reset` function, while
using `itertools.tee` might incur significant memory usage according to:

  https://docs.python.org/3/library/itertools.html#itertools.tee.

'''

import xgboost
import cupy
import numpy

COLS = 64
ROWS_PER_BATCH = 1000            # data is splited by rows
BATCHES = 32


class IterForDMatrixDemo(xgboost.core.DataIter):
    '''A data iterator for XGBoost DMatrix.

    `reset` and `next` are required for any data iterator, other functions here
    are utilites for demonstration's purpose.

    '''
    def __init__(self):
        '''Generate some random data for demostration.

        Actual data can be anything that is currently supported by XGBoost.
        '''
        self.rows = ROWS_PER_BATCH
        self.cols = COLS
        rng = cupy.random.RandomState(1994)
        self._data = [rng.randn(self.rows, self.cols)] * BATCHES
        self._labels = [rng.randn(self.rows)] * BATCHES
        self._weights = [rng.uniform(size=self.rows)] * BATCHES

        self.it = 0             # set iterator to 0
        super().__init__()

    def as_array(self):
        return cupy.concatenate(self._data)

    def as_array_labels(self):
        return cupy.concatenate(self._labels)

    def as_array_weights(self):
        return cupy.concatenate(self._weights)

    def data(self):
        '''Utility function for obtaining current batch of data.'''
        return self._data[self.it]

    def labels(self):
        '''Utility function for obtaining current batch of label.'''
        return self._labels[self.it]

    def weights(self):
        return self._weights[self.it]

    def reset(self):
        '''Reset the iterator'''
        self.it = 0

    def next(self, input_data):
        '''Yield next batch of data.'''
        if self.it == len(self._data):
            # Return 0 when there's no more batch.
            return 0
        input_data(data=self.data(), label=self.labels(),
                   weight=self.weights())
        self.it += 1
        return 1


def main():
    rounds = 100
    it = IterForDMatrixDemo()

    # Use iterator, must be `DeviceQuantileDMatrix` for quantile DMatrix.
    m_with_it = xgboost.DeviceQuantileDMatrix(it)

    # Use regular DMatrix.
    m = xgboost.DMatrix(it.as_array(), it.as_array_labels(),
                        weight=it.as_array_weights())

    assert m_with_it.num_col() == m.num_col()
    assert m_with_it.num_row() == m.num_row()

    reg_with_it = xgboost.train({'tree_method': 'gpu_hist'}, m_with_it,
                                num_boost_round=rounds)
    predict_with_it = reg_with_it.predict(m_with_it)

    reg = xgboost.train({'tree_method': 'gpu_hist'}, m,
                        num_boost_round=rounds)
    predict = reg.predict(m)

    numpy.testing.assert_allclose(predict_with_it, predict,
                                  rtol=1e6)


if __name__ == '__main__':
    main()
