def _decoded_str_len(l):
    """
    Compute how long an encoded string of length *l* becomes.
    """
    rem = l % 4
    if rem == 3:
        last_group_len = 2
    elif rem == 2:
        last_group_len = 1
    else:
        last_group_len = 0
    return l // 4 * 3 + last_group_len
