import collections


def check_zeros(nexus_obj, absences=None, missing=None):
    """
    Checks for sites in the nexus that are coded as all empty.

    Returns a list of sites that are completely empty. Note that
    this is zero-indexed (i.e. the first site is site 0 not 1)
    to enable indexing of nexus.data.matrix/.character lists

    :param nexus_obj: A `NexusReader` instance
    :type nexus_obj: NexusReader

    :param absences: A list of values to be marked as absent.
        Default = ["0"]
    :type char: list

    :param missing: A list of values to be marked as missing.
        Default = ["-", "?"]
    :type char: list

    :return: A list of site indexes
    :raises ValueError: if any of the states in the
        `char` dictionary is not a string (i.e.
        integer or None values)
    """
    absences = absences if absences else ['0']
    missing = missing if missing else ['-', '?']

    bad = []
    for site_idx in range(0, nexus_obj.data.nchar):
        states = collections.Counter(
            [nexus_obj.data.matrix[t][site_idx] for t in nexus_obj.data.matrix])
        zeros = sum([states[k] for k in states if k in absences or k in missing])
        total = sum(states.values())
        if zeros == total:
            bad.append(site_idx)
    return bad