from ..layers.merge import concatenate
from .. import backend as K
from ..layers.core import Lambda
from ..engine.training import Model


def _get_available_devices():
    from tensorflow.python.client import device_lib
    local_device_protos = device_lib.list_local_devices()
    return [x.name for x in local_device_protos]


def _normalize_device_name(name):
    name = name.lower().replace('device:', '')
    return name


def multi_gpu_model(model, gpus):
    """Replicates a model on different GPUs.

    Specifically, this function implements single-machine
    multi-GPU data parallelism. It works in the following way:

    - Divide the model's input(s) into multiple sub-batches.
    - Apply a model copy on each sub-batch. Every model copy
        is executed on a dedicated GPU.
    - Concatenate the results (on CPU) into one big batch.

    E.g. if your `batch_size` is 64 and you use `gpus=2`,
    then we will divide the input into 2 sub-batches of 32 samples,
    process each sub-batch on one GPU, then return the full
    batch of 64 processed samples.

    This induces quasi-linear speedup on up to 8 GPUs.

    This function is only available with the TensorFlow backend
    for the time being.

    # Arguments
        model: A Keras model instance. To avoid OOM errors,
            this model could have been built on CPU, for instance
            (see usage example below).
        gpus: Integer >= 2, number of on GPUs on which to create
            model replicas.

    # Returns
        A Keras `Model` instance which can be used just like the initial
        `model` argument, but which distributes its workload on multiple GPUs.

    # Example

    ```python
        import tensorflow as tf
        from keras.applications import Xception
        from keras.utils import multi_gpu_model
        import numpy as np

        num_samples = 1000
        height = 224
        width = 224
        num_classes = 1000

        # Instantiate the base model
        # (here, we do it on CPU, which is optional).
        with tf.device('/cpu:0'):
            model = Xception(weights=None,
                             input_shape=(height, width, 3),
                             classes=num_classes)

        # Replicates the model on 8 GPUs.
        # This assumes that your machine has 8 available GPUs.
        parallel_model = multi_gpu_model(model, gpus=8)
        parallel_model.compile(loss='categorical_crossentropy',
                               optimizer='rmsprop')

        # Generate dummy data.
        x = np.random.random((num_samples, height, width, 3))
        y = np.random.random((num_samples, num_classes))

        # This `fit` call will be distributed on 8 GPUs.
        # Since the batch size is 256, each GPU will process 32 samples.
        parallel_model.fit(x, y, epochs=20, batch_size=256)
    ```
    """
    if K.backend() != 'tensorflow':
        raise ValueError('`multi_gpu_model` is only available '
                         'with the TensorFlow backend.')
    if gpus <= 1:
        raise ValueError('For multi-gpu usage to be effective, '
                         'call `multi_gpu_model` with `gpus >= 2`. '
                         'Received: `gpus=%d`' % gpus)

    import tensorflow as tf

    target_devices = ['/cpu:0'] + ['/gpu:%d' % i for i in range(gpus)]
    available_devices = _get_available_devices()
    available_devices = [_normalize_device_name(name) for name in available_devices]
    for device in target_devices:
        if device not in available_devices:
            raise ValueError(
                'To call `multi_gpu_model` with `gpus=%d`, '
                'we expect the following devices to be available: %s. '
                'However this machine only has: %s. '
                'Try reducing `gpus`.' % (gpus,
                                          target_devices,
                                          available_devices))

    def get_slice(data, i, parts):
        shape = tf.shape(data)
        batch_size = shape[:1]
        input_shape = shape[1:]
        step = batch_size // parts
        if i == gpus - 1:
            size = batch_size - step * i
        else:
            size = step
        size = tf.concat([size, input_shape], axis=0)
        stride = tf.concat([step, input_shape * 0], axis=0)
        start = stride * i
        return tf.slice(data, start, size)

    all_outputs = []
    for i in range(len(model.outputs)):
        all_outputs.append([])

    # Place a copy of the model on each GPU,
    # each getting a slice of the inputs.
    for i in range(gpus):
        with tf.device('/gpu:%d' % i):
            with tf.name_scope('replica_%d' % i):
                inputs = []
                # Retrieve a slice of the input.
                for x in model.inputs:
                    input_shape = tuple(x.get_shape().as_list())[1:]
                    slice_i = Lambda(get_slice,
                                     output_shape=input_shape,
                                     arguments={'i': i,
                                                'parts': gpus})(x)
                    inputs.append(slice_i)

                # Apply model on slice
                # (creating a model replica on the target device).
                outputs = model(inputs)
                if not isinstance(outputs, list):
                    outputs = [outputs]

                # Save the outputs for merging back together later.
                for o in range(len(outputs)):
                    all_outputs[o].append(outputs[o])

    # Merge outputs on CPU.
    with tf.device('/cpu:0'):
        merged = []
        for outputs in all_outputs:
            merged.append(concatenate(outputs,
                                      axis=0))
        return Model(model.inputs, merged)
