How to use the xhistogram.duck_array_ops.ravel_multi_index function in xhistogram

hist_shapes = [nb+1 for nb in nbins]

    # a marginally faster implementation would be to use searchsorted,
    # like numpy histogram itself does
    # https://github.com/numpy/numpy/blob/9c98662ee2f7daca3f9fae9d5144a9a8d3cabe8c/numpy/lib/histograms.py#L864-L882
    # for now we stick with `digitize` because it's easy to understand how it works

    # the maximum possible value of of digitize is nbins
    # for right=False:
    #   - 0 corresponds to a < b[0]
    #   - i corresponds to bins[i-1] <= a < b[i]
    #   - nbins corresponds to a a >= b[1]
    each_bin_indices = [digitize(a, b) for a, b in zip(args, bins)]
    # product of the bins gives the joint distribution
    if N_inputs > 1:
        bin_indices = ravel_multi_index(each_bin_indices, hist_shapes)
    else:
        bin_indices = each_bin_indices[0]
    # total number of unique bin indices
    N = reduce(lambda x, y: x*y, hist_shapes)

    bin_counts = _dispatch_bincount(bin_indices, weights, N, hist_shapes,
                                    block_size=block_size)

    # just throw out everything outside of the bins, as np.histogram does
    # TODO: make this optional?
    slices = (slice(None),) + (N_inputs * (slice(1, -1),))
    return bin_counts[slices]