import torch


def penalty_builder(penalty_config):
    if penalty_config == '':
        return lambda x, y: y
    pen_type, alpha = penalty_config.split('_')
    alpha = float(alpha)
    if pen_type == 'wu':
        return lambda x, y: length_wu(x, y, alpha)
    if pen_type == 'avg':
        return lambda x, y: length_average(x, y, alpha)


def length_wu(length, logprobs, alpha=0.):
    """
    NMT length re-ranking score from
    "Google's Neural Machine Translation System" :cite:`wu2016google`.
    """

    modifier = (((5 + length) ** alpha) /
                ((5 + 1) ** alpha))
    return logprobs / modifier


def length_average(length, logprobs, alpha=0.):
    """
    Returns the average probability of tokens in a sequence.
    """
    return logprobs / length


def split_tensors(n, x):
    if torch.is_tensor(x):
        assert x.shape[0] % n == 0
        x = x.reshape(x.shape[0] // n, n, *x.shape[1:]).unbind(1)
    elif type(x) is list or type(x) is tuple:
        x = [split_tensors(n, _) for _ in x]
    elif x is None:
        x = [None] * n
    return x


def repeat_tensors(n, x):
    """
    For a tensor of size Bx..., we repeat it n times, and make it Bnx...
    For collections, do nested repeat
    """
    if torch.is_tensor(x):
        x = x.unsqueeze(1)  # Bx1x...
        x = x.expand(-1, n, *([-1] * len(x.shape[2:])))  # Bxnx...
        x = x.reshape(x.shape[0] * n, *x.shape[2:])  # Bnx...
    elif type(x) is list or type(x) is tuple:
        x = [repeat_tensors(n, _) for _ in x]
    return x
