def map_quicktree_dirs(struct):
    """Maps ``struct.dirs`` to alphanumeric keys.

    ``struct.dirs`` is mapped in alphanumeric order. Keys are selected
    for assignment in alphanumeric order as well, and are looped over.

    :param QuickTreeStruct struct:
    :return: ``struct.dirs`` mapped to alphanumeric keys
    :rtype: dict[str, list[str]]
    """
    ret_keys = [
        "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d",
        "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r",
        "s", "t", "u", "v", "w", "x", "y", "z"
    ]
    ret = {x: [] for x in ret_keys}

    sorted_dirs_list = sorted(struct["dirs"])
    for i in range(len(sorted_dirs_list)):
        ret_key = ret_keys[i % len(ret_keys)]
        ret[ret_key] += [sorted_dirs_list[i]]

    return ret