def _roles_acls(key_name, roles):
    acls = []
    for role in roles:
        role_permissions = role.permissions.get(key_name)
        if role_permissions:
            acls.append(role_permissions)
    return acls


def sum_acls(result_acl, acls=None, roles=None, key=None, **permissions):
    if bool(roles) != bool(key):
        if key:
            raise ValueError(
                'You have provided "key" but omited "roles" argument')
        else:
            raise ValueError(
                'You have provided "roles" but omited "key" argument')

    if acls and roles:
        raise ValueError(
            'You can not provide both "acls" and "roles" arguments')

    if roles:
        acls = _roles_acls(key, roles)

    for permission, compare in permissions.items():
        try:
            permission_value = result_acl[permission]
        except KeyError:
            message = 'Default value for permission "%s" is not provided'
            raise ValueError(message % permission)

        for acl in acls:
            try:
                permission_value = compare(permission_value, acl[permission])
            except KeyError:
                pass
        result_acl[permission] = permission_value

    return result_acl


# Common comparisions
def greater(a, b):
    return a if a > b else b


def greater_or_zero(a, b):
    if a == 0:
        return a
    elif b == 0:
        return b
    else:
        return greater(a, b)


def lower(a, b):
    return a if a < b else b