"""Utility functions used throughout Megatron core"""
from functools import reduce
import operator

import torch


def ensure_divisibility(numerator, denominator):
    """Ensure that numerator is divisible by the denominator."""
    assert numerator % denominator == 0, "{} is not divisible by {}".format(
        numerator, denominator
    )


def divide(numerator, denominator):
    """Ensure that numerator is divisible by the denominator and return
    the division value."""
    ensure_divisibility(numerator, denominator)
    return numerator // denominator


class GlobalMemoryBuffer:
    """Global buffer to avoid dynamic memory allocations.
    Caller should ensure that buffers of the same name
    are not used concurrently."""

    def __init__(self):
        self.buffer = {}

    def get_tensor(self, tensor_shape, dtype, name):
        required_len = reduce(operator.mul, tensor_shape, 1)
        if self.buffer.get((name, dtype), None) is None or \
                self.buffer[(name, dtype)].numel() < required_len:
            self.buffer[(name, dtype)] = \
                torch.empty(required_len,
                            dtype=dtype,
                            device=torch.cuda.current_device(),
                            requires_grad=False)

        return self.buffer[(name, dtype)][0:required_len].view(*tensor_shape)

def _kernel_make_viewless_tensor(inp, requires_grad):
    '''Make a viewless tensor.

    View tensors have the undesirable side-affect of retaining a reference
    to the originally-viewed tensor, even after manually setting the '.data'
    field. This method creates a new tensor that links to the old tensor's
    data, without linking the viewed tensor, referenced via the '._base'
    field.
    '''
    out = torch.empty(
        (1,),
        dtype = inp.dtype,
        device = inp.device,
        requires_grad = requires_grad,
    )
    out.data = inp.data
    return out

class MakeViewlessTensor(torch.autograd.Function):
    '''
    Autograd function to make a viewless tensor.

    This function should be used in cases where the computation graph needs
    to be propagated, but we only want a viewless tensor (e.g.,
    ParallelTransformer's hidden_states). Call this function by passing
    'keep_graph = True' to 'make_viewless_tensor()'.
    '''
    @staticmethod
    def forward(ctx, inp, requires_grad):
        return _kernel_make_viewless_tensor(inp, requires_grad)
    @staticmethod
    def backward(ctx, grad_output):
        return grad_output, None

def make_viewless_tensor(inp, requires_grad, keep_graph):
    '''
    Entry-point for creating viewless tensors.

    This method should be used, rather than calling 'MakeViewlessTensor'
    or '_kernel_make_viewless_tensor' directly. This method acts as a
    switch for determining if an autograd function or a regular method
    should be used to create the tensor.
    '''

    # return tensor as-is, if not a 'view'
    if inp._base is None:
        return inp

    # create viewless tensor
    if keep_graph:
        return MakeViewlessTensor.apply(inp, requires_grad)
    else:
        return _kernel_make_viewless_tensor(inp, requires_grad)

def assert_viewless_tensor(tensor, extra_msg = None):
    '''Assert that a tensor is not a view (i.e., its '._base' field is
    not set).'''
    if isinstance(tensor, list):
        [ assert_viewless_tensor(t) for t in tensor ]
        return tensor
    if not isinstance(tensor, torch.Tensor):
        return tensor
    assert tensor._base is None, (
        "Ensure tensor._base is None before setting tensor.data or storing "
        "tensor to memory buffer. Otherwise, a memory leak will occur (and "
        "likely accumulate over iterations). %s"
    ) % extra_msg
    return tensor

def safely_set_viewless_tensor_data(tensor, new_data_tensor):
    '''Safely set tensor's '.data' field.

    Check first that the tensor is viewless (i.e., '._base' not set). If not,
    raise an exception.
    '''
    assert_viewless_tensor(tensor, extra_msg = "FYI, tensor._base has shape %s, and new_data_tensor has shape %s." % ("--" if tensor._base is None else tensor._base.shape, new_data_tensor.shape))
    tensor.data = new_data_tensor
