// NewIdentityMapping takes a requested username and
// using the data from /etc/sub{uid,gid} ranges, creates the
// proper uid and gid remapping ranges for that user/group pair
func NewIdentityMapping(name string) (*IdentityMapping, error) {
	usr, err := LookupUser(name)
	if err != nil {
		return nil, fmt.Errorf("Could not get user for username %s: %v", name, err)
	}

	subuidRanges, err := lookupSubUIDRanges(usr)
	if err != nil {
		return nil, err
	}
	subgidRanges, err := lookupSubGIDRanges(usr)
	if err != nil {
		return nil, err
	}

	return &IdentityMapping{
		uids: subuidRanges,
		gids: subgidRanges,
	}, nil
}

func lookupSubUIDRanges(usr user.User) ([]IDMap, error) {
	r, err := parseSubuid(strconv.Itoa(usr.Uid))
	if err != nil {
		return nil, err
	}
	if len(r) == 0 {
		r, err = parseSubuid(usr.Name)
		if err != nil {
			return nil, err
		}
	}
	if len(r) == 0 {
		return nil, errors.Errorf("no subuid ranges found for user %q", usr.Name)
	}
	return createIDMap(r), nil
}

func lookupSubGIDRanges(usr user.User) ([]IDMap, error) {
	r, err := parseSubgid(strconv.Itoa(usr.Uid))
	if err != nil {
		return nil, err
	}
	if len(r) == 0 {
		r, err = parseSubgid(usr.Name)
		if err != nil {
			return nil, err
		}
	}
	if len(r) == 0 {
		return nil, errors.Errorf("no subgid ranges found for user %q", usr.Name)
	}
	return createIDMap(r), nil
}