import torch


def class_reduce(num: torch.Tensor,
                 denom: torch.Tensor,
                 weights: torch.Tensor,
                 class_reduction: str = 'none') -> torch.Tensor:
    """
    Function used to reduce classification metrics of the form `num / denom * weights`.
    For example for calculating standard accuracy the num would be number of
    true positives per class, denom would be the support per class, and weights
    would be a tensor of 1s

    Args:
        num: numerator tensor
        decom: denominator tensor
        weights: weights for each class
        class_reduction: reduction method for multiclass problems

            - ``'micro'``: calculate metrics globally (default)
            - ``'macro'``: calculate metrics for each label, and find their unweighted mean.
            - ``'weighted'``: calculate metrics for each label, and find their weighted mean.
            - ``'none'``: returns calculated metric per class

    """
    valid_reduction = ('micro', 'macro', 'weighted', 'none')
    if class_reduction == 'micro':
        return torch.sum(num) / torch.sum(denom)

    # For the rest we need to take care of instances where the denom can be 0
    # for some classes which will produce nans for that class
    fraction = num / denom
    fraction[fraction != fraction] = 0
    if class_reduction == 'macro':
        return torch.mean(fraction)
    elif class_reduction == 'weighted':
        return torch.sum(fraction * (weights / torch.sum(weights)))
    elif class_reduction == 'none':
        return fraction

    raise ValueError(f'Reduction parameter {class_reduction} unknown.'
                     f' Choose between one of these: {valid_reduction}')