package roomserver

import (
	"context"
	"fmt"
	"strconv"
	"strings"

	"github.com/dariuszjaniak/warp/server/internal/matrix/event"
	"github.com/dariuszjaniak/warp/server/internal/matrix/id"
	"github.com/dariuszjaniak/warp/server/internal/storage"
)

// MaxAliasLength is the spec's cap on a room alias, sigil and server name
// included.
const MaxAliasLength = id.MaxLength

// aliasFor builds a full local alias from the localpart a client supplied,
// and checks it.
func (r *API) aliasFor(localpart string) (string, error) {
	alias := "#" + localpart + ":" + r.serverName
	if err := r.validateLocalAlias(alias); err != nil {
		return "", err
	}
	return alias, nil
}

// validateLocalAlias checks an alias and insists it belongs to this server.
//
// The grammar is deliberately the loose one the spec gives for an alias —
// anything but a colon, and no control characters — rather than the
// localpart rules an account is held to. An alias is a label a room's
// members chose, and the spec puts no character restrictions on it beyond
// what parsing needs.
func (r *API) validateLocalAlias(alias string) error {
	if err := validateAliasGrammar(alias); err != nil {
		return err
	}
	_, serverName, _ := strings.Cut(alias[1:], ":")
	if serverName != r.serverName {
		return fmt.Errorf("%w: %s is not an alias of this server", ErrForbidden, alias)
	}
	return nil
}

// validateAliasGrammar checks an alias of any server.
func validateAliasGrammar(alias string) error {
	switch {
	case alias == "":
		return fmt.Errorf("%w: a room alias must not be empty", ErrInvalidContent)
	case alias[0] != '#':
		return fmt.Errorf("%w: a room alias must start with '#'", ErrInvalidContent)
	case len(alias) > MaxAliasLength:
		return fmt.Errorf("%w: a room alias may be at most %d bytes", ErrInvalidContent, MaxAliasLength)
	}

	localpart, serverName, found := strings.Cut(alias[1:], ":")
	switch {
	case !found:
		return fmt.Errorf("%w: %s has no server name; expected #alias:server", ErrInvalidContent, alias)
	case localpart == "":
		return fmt.Errorf("%w: %s has an empty localpart", ErrInvalidContent, alias)
	case serverName == "":
		return fmt.Errorf("%w: %s has an empty server name", ErrInvalidContent, alias)
	}
	// A control character in an alias would be invisible in every client
	// that shows it, which is exactly what makes it worth refusing.
	if strings.ContainsFunc(alias, func(c rune) bool { return c < 0x20 || c == 0x7f }) {
		return fmt.Errorf("%w: %s contains a control character", ErrInvalidContent, alias)
	}
	return nil
}

// isLocalAlias reports whether an alias belongs to this server.
func (r *API) isLocalAlias(alias string) bool {
	if validateAliasGrammar(alias) != nil {
		return false
	}
	_, serverName, _ := strings.Cut(alias[1:], ":")
	return serverName == r.serverName
}

// CreateAlias points a local alias at a room.
//
// The caller has to be in the room. The spec leaves the permission to the
// server, and "a member of the room may give it a name" is the rule that
// matches what an alias is for while keeping a stranger from squatting
// names against rooms they cannot see.
func (r *API) CreateAlias(ctx context.Context, alias, roomID, userID string) error {
	if err := r.validateLocalAlias(alias); err != nil {
		return err
	}
	if err := id.ValidateRoomID(roomID); err != nil {
		return fmt.Errorf("%w: %v", ErrInvalidContent, err)
	}
	if _, err := r.store.Room(ctx, roomID); err != nil {
		return err
	}
	if err := r.requireJoined(ctx, roomID, userID); err != nil {
		return err
	}
	return r.store.CreateAlias(ctx, storage.RoomAlias{
		Alias:       alias,
		RoomID:      roomID,
		Creator:     userID,
		CreatedAtMS: r.nowMS(),
	})
}

// DeleteAlias removes a local alias.
//
// Either the user who created it, or a user who could set the room's
// canonical alias, may delete it — the first because it is theirs, the
// second because the room's moderators have to be able to clean up after a
// member who has left.
func (r *API) DeleteAlias(ctx context.Context, alias, userID string, isAdmin bool) error {
	if err := r.validateLocalAlias(alias); err != nil {
		return err
	}

	stored, err := r.store.Alias(ctx, alias)
	if err != nil {
		return err
	}
	if !isAdmin && stored.Creator != userID {
		canSet, err := r.CanSendStateEvent(ctx, stored.RoomID, userID, TypeCanonicalAlias)
		if err != nil {
			return err
		}
		if !canSet {
			return fmt.Errorf("%w: %s may not delete the alias %s", ErrForbidden, userID, alias)
		}
	}
	return r.store.DeleteAlias(ctx, alias)
}

// ResolvedAlias is what a directory lookup returns.
type ResolvedAlias struct {
	RoomID string

	// Servers are the servers known to be in the room, which a client
	// passes back as ?server_name= when it joins. With federation off the
	// only one is this server.
	Servers []string
}

// ResolveAlias turns a room alias into a room ID.
//
// A remote alias has to be asked of the server that owns it, which needs
// federation; with it off the client is told that rather than being told
// the alias does not exist, because the two call for quite different
// reactions.
func (r *API) ResolveAlias(ctx context.Context, alias string) (ResolvedAlias, error) {
	if err := validateAliasGrammar(alias); err != nil {
		return ResolvedAlias{}, err
	}
	if !r.isLocalAlias(alias) {
		if !r.federationEnabled() {
			return ResolvedAlias{}, fmt.Errorf(
				"%w: %s belongs to another server, and resolving a remote alias",
				ErrFederationDisabled, alias)
		}
		return ResolvedAlias{}, fmt.Errorf(
			"%w: resolving an alias of another server is not implemented yet", ErrFederationDisabled)
	}

	stored, err := r.store.Alias(ctx, alias)
	if err != nil {
		return ResolvedAlias{}, err
	}
	return ResolvedAlias{RoomID: stored.RoomID, Servers: []string{r.serverName}}, nil
}

// RoomAliases lists the local aliases of a room, for a user who may see it.
func (r *API) RoomAliases(ctx context.Context, roomID, userID string) ([]string, error) {
	if _, err := r.requireReadable(ctx, roomID, userID); err != nil {
		return nil, err
	}
	aliases, err := r.store.AliasesForRoom(ctx, roomID)
	if err != nil {
		return nil, err
	}
	if aliases == nil {
		aliases = []string{}
	}
	return aliases, nil
}

// RoomVisibility reports whether a room is in this server's public
// directory.
func (r *API) RoomVisibility(ctx context.Context, roomID string) (string, error) {
	room, err := r.store.Room(ctx, roomID)
	if err != nil {
		return "", err
	}
	if room.Published {
		return VisibilityPublic, nil
	}
	return VisibilityPrivate, nil
}

// SetRoomVisibility publishes a room in this server's directory, or
// withdraws it.
//
// The permission is the one the spec hints at and Synapse settled on: a
// user who could set the room's canonical alias, or a server admin. An
// unpublished room that nobody can find is recoverable; a room published by
// a passer-by is not.
func (r *API) SetRoomVisibility(ctx context.Context, roomID, userID, visibility string, isAdmin bool) error {
	switch visibility {
	case VisibilityPublic, VisibilityPrivate:
	default:
		return fmt.Errorf("%w: visibility must be %q or %q", ErrInvalidContent,
			VisibilityPublic, VisibilityPrivate)
	}
	if _, err := r.store.Room(ctx, roomID); err != nil {
		return err
	}

	if !isAdmin {
		canSet, err := r.CanSendStateEvent(ctx, roomID, userID, TypeCanonicalAlias)
		if err != nil {
			return err
		}
		if !canSet {
			return fmt.Errorf("%w: %s may not change whether %s is published",
				ErrForbidden, userID, roomID)
		}
	}
	return r.store.SetPublished(ctx, roomID, visibility == VisibilityPublic)
}

// PublicRoomsRequest pages and filters the public room directory.
type PublicRoomsRequest struct {
	// Limit is how many rooms to return; 0 lets the store decide.
	Limit int

	// Since is the pagination token from a previous response.
	Since string

	// Search is the generic search term from the filter.
	Search string

	// Server is the ?server= parameter: another server's directory, which
	// needs federation.
	Server string
}

// PublicRoom is one entry in the public room directory.
type PublicRoom struct {
	RoomID           string   `json:"room_id"`
	Name             string   `json:"name,omitempty"`
	Topic            string   `json:"topic,omitempty"`
	CanonicalAlias   string   `json:"canonical_alias,omitempty"`
	AvatarURL        string   `json:"avatar_url,omitempty"`
	NumJoinedMembers int      `json:"num_joined_members"`
	WorldReadable    bool     `json:"world_readable"`
	GuestCanJoin     bool     `json:"guest_can_join"`
	JoinRule         string   `json:"join_rule,omitempty"`
	RoomType         string   `json:"room_type,omitempty"`
	Aliases          []string `json:"aliases,omitempty"`
}

// PublicRoomsResponse is the directory page.
type PublicRoomsResponse struct {
	Chunk []PublicRoom `json:"chunk"`

	// NextBatch and PrevBatch are the tokens for the pages either side,
	// and are absent at the ends.
	NextBatch string `json:"next_batch,omitempty"`
	PrevBatch string `json:"prev_batch,omitempty"`

	TotalRoomCountEstimate int `json:"total_room_count_estimate"`
}

// PublicRooms returns a page of this server's public room directory.
//
// The pagination token is the offset, written out as a decimal. That is
// enough for a directory ordered by member count and is honest about what
// it is: a token that encoded a cursor would imply a stability the ordering
// does not have, since a room gaining a member moves between pages whatever
// the token says.
func (r *API) PublicRooms(ctx context.Context, req PublicRoomsRequest) (PublicRoomsResponse, error) {
	if req.Server != "" && req.Server != r.serverName {
		if !r.federationEnabled() {
			return PublicRoomsResponse{}, fmt.Errorf(
				"%w: reading the room directory of %s", ErrFederationDisabled, req.Server)
		}
		return PublicRoomsResponse{}, fmt.Errorf(
			"%w: reading another server's room directory is not implemented yet", ErrFederationDisabled)
	}

	offset, err := parseDirectoryToken(req.Since)
	if err != nil {
		return PublicRoomsResponse{}, err
	}

	summaries, total, err := r.store.PublishedRooms(ctx, storage.RoomFilter{
		Search: req.Search,
		Limit:  req.Limit,
		Offset: offset,
	})
	if err != nil {
		return PublicRoomsResponse{}, err
	}

	resp := PublicRoomsResponse{
		Chunk:                  make([]PublicRoom, 0, len(summaries)),
		TotalRoomCountEstimate: total,
	}
	for _, summary := range summaries {
		aliases, err := r.store.AliasesForRoom(ctx, summary.ID)
		if err != nil {
			return PublicRoomsResponse{}, err
		}
		resp.Chunk = append(resp.Chunk, publicRoomOf(summary, aliases))
	}

	if offset > 0 {
		resp.PrevBatch = directoryToken(max(offset-len(summaries), 0))
	}
	if offset+len(summaries) < total {
		resp.NextBatch = directoryToken(offset + len(summaries))
	}
	return resp, nil
}

// publicRoomOf projects a stored room onto a directory entry.
func publicRoomOf(summary storage.RoomSummary, aliases []string) PublicRoom {
	return PublicRoom{
		RoomID:           summary.ID,
		Name:             summary.Name,
		Topic:            summary.Topic,
		CanonicalAlias:   summary.CanonicalAlias,
		AvatarURL:        summary.AvatarURL,
		NumJoinedMembers: summary.JoinedMembers,
		WorldReadable:    summary.HistoryVisibility == HistoryVisibilityWorldReadable,
		GuestCanJoin:     summary.GuestAccess == GuestAccessCanJoin,
		JoinRule:         summary.JoinRule,
		RoomType:         summary.RoomType,
		Aliases:          aliases,
	}
}

// containsString reports whether needle is in haystack.
func containsString(haystack []string, needle string) bool {
	for _, s := range haystack {
		if s == needle {
			return true
		}
	}
	return false
}

// membershipFilterValues are the memberships a client may filter a member
// list by.
var membershipFilterValues = []string{
	event.MembershipJoin, event.MembershipLeave,
	event.MembershipInvite, event.MembershipBan, event.MembershipKnock,
}

// ValidateMembershipFilter checks a ?membership= or ?not_membership= value.
func ValidateMembershipFilter(value string) error {
	if value == "" || containsString(membershipFilterValues, value) {
		return nil
	}
	return fmt.Errorf("%w: %q is not a membership", ErrInvalidContent, value)
}

// maxDirectoryOffset bounds how far into the directory a pagination token
// may point. The token comes back from a client, so it is input: without a
// bound, "since=99999999999999" is an invitation to make the database count
// past the end of a table.
const maxDirectoryOffset = 1 << 20

// directoryToken renders a directory offset as a pagination token.
func directoryToken(offset int) string {
	return strconv.Itoa(offset)
}

// parseDirectoryToken reads a pagination token back.
func parseDirectoryToken(token string) (int, error) {
	if token == "" {
		return 0, nil
	}
	offset, err := strconv.Atoi(token)
	if err != nil || offset < 0 || offset > maxDirectoryOffset {
		return 0, fmt.Errorf("%w: %q is not a pagination token from this server", ErrInvalidContent, token)
	}
	return offset, nil
}