// Package cchat is a set of stabilized interfaces for cchat implementations,
// joining the backend and frontend together.
//
// Backend
//
// Methods implemented by the backend that have frontend containers as arguments
// can do IO. Frontends must NOT rely on individual backend caches and should
// always assume that they will block.
//
// Methods that do not return an error must NOT do any IO to prevent blocking
// the main thread. Methods that do return an error may do IO, but they should
// be  documented per method. ID() and Name() must never do any IO.
//
// Backend implementations have certain conditions that should be adhered to:
//
//    - Storing MessagesContainer and ServersContainer are advised against;
//    however, they should be done if need be.
//    - Other containers such as LabelContainer and IconContainer should also
//    not be stored; however, the same rule as above applies.
//    - For the server list, icon updates and such that happen after their calls
//    should use SetServers().
//    - For the nickname of the current server, the backend can store the state
//    of the label container. It must, however, remove the container when the
//    stop callback from JoinServer() is called.
//    - Some methods that take in a container may take in a context as well.
//    Although implementations don't have to use this context, it should try to.
//
// Note: IO in most cases usually refer to networking, but they should files and
// anything that could block, such as mutexes or semaphores.
//
// Note: As mentioned above, contexts are optional for both the frontend and
// backend. The frontend may use it for cancellation, and the backend may ignore
// it.
//
// Frontend
//
// Frontend contains all interfaces that a frontend can or must implement. The
// backend may call these methods any time from any goroutine. Thus, they should
// be thread-safe. They should also not block the call by doing so, as backends
// may call these methods in its own main thread.
//
// It is worth pointing out that frontend container interfaces will not have an
// error handling API, as frontends can do that themselves. Errors returned by
// backend methods will be errors from the backend itself and never the frontend
// errors.
//
package cchat

import (
	"context"
	"io"
	"time"

	"github.com/diamondburned/cchat/text"
)

// A service is a complete service that's capable of multiple sessions. It has
// to implement the Authenticate() method, which returns an implementation of
// Authenticator.
//
// A service can implement SessionRestorer, which would indicate the frontend
// that it can restore past sessions. Sessions are saved using the SessionSaver
// interface that Session can implement.
//
// A service can also implement Configurator if it has additional
// configurations. The current API is a flat key-value map, which can be parsed
// by the backend itself into more meaningful data structures. All
// configurations must be optional, as frontends may not implement a
// configurator UI.
//
// Service can implement the following interfaces:
//
//    - Namer
//    - SessionRestorer (optional)
//    - Configurator (optional)
//    - Icon (optional)
//
type Service interface {
	// Namer returns the name of the service.
	Namer
	// Authenticate begins the authentication process. It's put into a method so
	// backends can easily restart the entire process.
	Authenticate() Authenticator
}

// SessionRestorer extends Service and is called by the frontend to restore a
// saved session. The frontend may call this at any time, but it's usually on
// startup.
//
// To save a session, refer to SessionSaver which extends Session.
type SessionRestorer interface {
	RestoreSession(map[string]string) (Session, error)
}

// Configurator is what the backend can implement for an arbitrary configuration
// API.
type Configurator interface {
	Configuration() (map[string]string, error)
	SetConfiguration(map[string]string) error
}

// ErrInvalidConfigAtField is the structure for an error at a specific
// configuration field. Frontends can use this and highlight fields if the
// backends support it.
type ErrInvalidConfigAtField struct {
	Key string
	Err error
}

func (err *ErrInvalidConfigAtField) Error() string {
	return "Error at " + err.Key + ": " + err.Err.Error()
}

func (err *ErrInvalidConfigAtField) Unwrap() error {
	return err.Err
}

// The authenticator interface allows for a multistage initial authentication
// API that the backend could use. Multistage is done by calling
// AuthenticateForm then Authenticate again forever until no errors are
// returned.
//
//    var s *cchat.Session
//    var err error
//
//    for {
//        // Pseudo-function to render the form and return the results of those forms
//        // when the user confirms it.
//        outputs := renderAuthForm(svc.AuthenticateForm())
//
//        s, err = svc.Authenticate(outputs)
//        if err != nil {
//            renderError(errors.Wrap(err, "Error while authenticating"))
//            continue // retry
//        }
//
//        break // success
//    }
//
type Authenticator interface {
	// AuthenticateForm should return a list of authentication entries for
	// the frontend to render.
	AuthenticateForm() []AuthenticateEntry
	// Authenticate will be called with a list of values with indices
	// correspond to the returned slice of AuthenticateEntry.
	Authenticate([]string) (Session, error)
}

// AuthenticateEntry represents a single authentication entry, usually an email
// or password prompt. Passwords or similar entries should have Secrets set to
// true, which should imply to frontends that the fields be masked.
type AuthenticateEntry struct {
	Name      string
	Secret    bool
	Multiline bool
}

// Identifier requires ID() to return a uniquely identifiable string for
// whatever this is embedded into. Typically, servers and messages have IDs. It
// is worth mentioning that IDs should be consistent throughout the lifespan of
// the program or maybe even forever.
type Identifier interface {
	ID() string
}

// Namer requires Name() to return the name of the object. Typically, this
// implies usernames for sessions or service names for services.
type Namer interface {
	Name() text.Rich
}

// A session is returned after authentication on the service. Session implements
// Name(), which should return the username most of the time. It also implements
// ID(), which might be used by frontends to check against MessageAuthor.ID()
// and other things.
//
// A session can implement SessionSaver, which would allow the frontend to save
// the session into its keyring at any time. Whether the keyring is completely
// secure or not is up to the frontend. For a Gtk client, that would be using
// the GNOME Keyring daemon.
//
// Session can implement the following interfaces:
//
//    - Identifier
//    - Namer
//    - ServerList
//    - Icon (optional)
//    - Commander (optional)
//    - SessionSaver (optional)
//
type Session interface {
	// Identifier should typically return the user ID.
	Identifier
	// Namer gives the name of the session, which is typically the username.
	Namer

	// Disconnect asks the service to disconnect. It does not necessarily mean
	// removing the service.
	//
	// The frontend must cancel the active ServerMessage before disconnecting.
	// The backend can rely on this behavior.
	//
	// The frontend will reuse the stored session data from SessionSaver to
	// reconnect.
	//
	// When this function fails, the frontend may display the error upfront.
	// However, it will treat the session as actually disconnected. If needed,
	// the backend must implement reconnection by itself.
	Disconnect() error

	ServerList
}

// SessionSaver extends Session and is called by the frontend to save the
// current session. This is typically called right after authentication, but a
// frontend may call this any time, including when it's closing.
//
// The frontend can ask to restore a session using SessionRestorer, which
// extends Service.
type SessionSaver interface {
	Save() (map[string]string, error)
}

// Commander is an optional interface that a session could implement for command
// support. This is different from just intercepting the SendMessage() API, as
// this extends globally to the entire session.
//
// Commander can implement the following interfaces:
//
//    - CommandCompleter (optional)
//
type Commander interface {
	// RunCommand executes the given command, with the slice being already split
	// arguments, similar to os.Args. The function could return an output
	// stream, in which the frontend must display it live and close it on EOF.
	//
	// The function must not do any IO; if it does, then they have to be in a
	// goroutine and stream their results to the ReadCloser.
	//
	// The client should make guarantees that an empty string (and thus a
	// zero-length string slice) should be ignored. The backend should be able
	// to assume that the argument slice is always length 1 or more.
	RunCommand([]string) (io.ReadCloser, error)
}

// CommandCompleter is an optional interface that a session could implement for
// completion support. This also depends on whether or not the frontend supports
// it.
type CommandCompleter interface {
	// CompleteCommand is called with the line and current word, which the
	// backend should return with a list of new words.
	CompleteCommand(words []string, wordIndex int) []string
}

// Server is a single server-like entity that could translate to a guild, a
// channel, a chat-room, and such. A server must implement at least ServerList
// or ServerMessage, else the frontend must treat it as a no-op.
//
// Server can implement the following interfaces:
//
//    - Identifier
//    - Namer
//    - ServerList and/or ServerMessage (and its interfaces)
//    - ServerNickname (optional)
//    - Icon (optional)
//
type Server interface {
	Identifier
	Namer

	// Implement ServerList and/or ServerMessage.
}

// ServerNickname extends Server to add a specific user nickname into a server.
// The frontend should not traverse up the server tree, and thus the backend
// must handle nickname inheritance. This also means that servers that don't
// implement ServerMessage also don't need to implement ServerNickname. By
// default, the session name should be used.
type ServerNickname interface {
	Nickname(context.Context, LabelContainer) (stop func(), err error)
}

// Icon is an extra interface that an interface could implement for an icon.
// Typically, Service would return the service logo, Session would return the
// user's avatar, and Server would return the server icon.
//
// For session, the avatar should be the same as the one returned by messages
// sent by the current user.
type Icon interface {
	Icon(context.Context, IconContainer) (stop func(), err error)
}

// ServerList is for servers that contain children servers. This is similar to
// guilds containing channels in Discord, or IRC servers containing channels.
//
// There isn't a similar stop callback API unlike other interfaces because all
// servers are expected to be listed. However, they could be hidden, such as
// collapsing a tree.
//
// The backend should call both the container and other icon and label
// containers, if any.
type ServerList interface {
	// Servers should call SetServers() on the given ServersContainer to render
	// all servers. This function can do IO, and the frontend should run this in
	// a goroutine.
	Servers(ServersContainer) error
}

// ServerMessage is for servers that contain messages. This is similar to
// Discord or IRC channels.
//
// ServerMessage can implement the following interfaces:
//
//    - ServerMessageSender (optional): adds message sending capability.
//    - ServerMessageSendCompleter (optional): adds message input completion
//    capability.
//    - ServerMessageAttachmentSender (optional): adds attachment sending
//    capability.
//    - ServerMessageEditor (optional): adds message editing capability.
//    - ServerMessageActioner (optional): adds custom actions capability.
//    - ServerMessageUnreadIndicator (optional): adds unread indication
//    capability.
//    - ServerMessageTypingIndicator (optional): adds typing indication
//    capability.
//    - ServerMessageMemberLister (optional): adds member listing capability.
//
type ServerMessage interface {
	// JoinServer joins a server that's capable of receiving messages. The
	// server may not necessarily support sending messages.
	JoinServer(context.Context, MessagesContainer) (stop func(), err error)
}

// ServerMessageUnreadIndicator is for servers that can contain messages and
// know from the state if that message makes the server unread and if it
// contains a message that mentions the user.
type ServerMessageUnreadIndicator interface {
	// UnreadIndicate subscribes the given unread indicator for unread and
	// mention events. Examples include when a new message is arrived and the
	// backend needs to indicate that it's unread.
	//
	// This function must provide a way to remove callbacks, as clients must
	// call this when the old server is destroyed, such as when Servers is
	// called.
	UnreadIndicate(UnreadIndicator) (stop func(), err error)
}

// ServerMessageSender optionally extends ServerMessage to add message sending
// capability to the server.
type ServerMessageSender interface {
	// SendMessage is called by the frontend to send a message to this channel.
	SendMessage(SendableMessage) error
}

// ServerMessageAttachmentSender optionally extends ServerMessageSender to
// indicate that the backend can accept attachments in its messages. The
// attachments will still be sent through SendMessage, though this interface
// will mostly be used to indicate the capability.
type ServerMessageAttachmentSender interface {
	ServerMessageSender
	// SendAttachments sends only message attachments. The frontend would
	// most of the time use SendableMessage that implements
	// SendableMessageAttachments, but this method is useful for detecting
	// capabilities.
	SendAttachments([]MessageAttachment) error
}

// ServerMessageEditor optionally extends ServerMessage to add message editing
// capability to the server. Only EditMessage can have IO.
type ServerMessageEditor interface {
	// MessageEditable returns whether or not a message can be edited by the
	// client.
	MessageEditable(id string) bool
	// RawMessageContent gets the original message text for editing. Backends
	// must not do IO.
	RawMessageContent(id string) (string, error)
	// EditMessage edits the message with the given ID to the given content,
	// which is the edited string from RawMessageContent. This method can do IO.
	EditMessage(id, content string) error
}

// ServerMessageActioner optionally extends ServerMessage to add custom message
// action capabilities to the server. Similarly to ServerMessageEditor, these
// functions can have IO.
type ServerMessageActioner interface {
	// MessageActions returns a list of possible actions in pretty strings that
	// the frontend will use to directly display. This function must not do any
	// IO.
	//
	// The string slice returned can be nil or empty.
	MessageActions(messageID string) []string
	// DoMessageAction executes a message action on the given messageID, which
	// would be taken from MessageHeader.ID(). This function is allowed to do
	// IO; the frontend should take care of running this asynchronously.
	DoMessageAction(action, messageID string) error
}

// ServerMessageTypingIndicator optionally extends ServerMessage to provide
// bidirectional typing indicating capabilities. This is similar to typing
// events on Discord and typing client tags on IRCv3.
//
// The client should remove a typer when a message is received with the same
// user ID, when RemoveTyper() is called by the backend or when the timeout
// returned from TypingTimeout() has been reached.
type ServerMessageTypingIndicator interface {
	// Typing is called by the client to indicate that the user is typing. This
	// function can do IO calls, and the client will take care of calling it in
	// a goroutine (or an asynchronous queue) as well as throttling it to
	// TypingTimeout.
	Typing() error
	// TypingTimeout returns the interval between typing events sent by the
	// client as well as the timeout before the client should remove the typer.
	// Typically, a constant should be returned.
	TypingTimeout() time.Duration
	// TypingSubscribe subscribes the given indicator to typing events sent by
	// the backend. The added event handlers have to be removed by the backend
	// when the stop() callback is called.
	//
	// This method does not take in a context, as it's supposed to only use
	// event handlers and not do any IO calls. Nonetheless, the client must
	// treat it like it does and call it asynchronously.
	TypingSubscribe(TypingIndicator) (stop func(), err error)
}

// Typer is an individual user that's typing. This interface is used
// interchangably in TypingIndicator and thus ServerMessageTypingIndicator as
// well.
type Typer interface {
	MessageAuthor
	Time() time.Time
}

// ServerMessageSendCompleter optionally extends ServerMessageSender to add
// autocompletion into the message composer. IO is not allowed and the backend
// should do that only in goroutines and update its state for future calls.
//
// Frontends could utilize the split package inside utils for splitting words
// and index.
type ServerMessageSendCompleter interface {
	// CompleteMessage returns the list of possible completion entries for the
	// given word list and the current word index. It takes in a list of
	// whitespace-split slice of string as well as the position of the cursor
	// relative to the given string slice.
	CompleteMessage(words []string, current int) []CompletionEntry
}

// CompletionEntry is a single completion entry returned by CompleteMessage. The
// icon URL field is optional.
type CompletionEntry struct {
	// Raw is the text to be replaced in the input box.
	Raw string
	// Text is the label to be displayed.
	Text text.Rich
	// Secondary is the label to be displayed on the second line, on the right
	// of Text, or not displayed at all. This should be optional. This text may
	// be dimmed out as styling.
	Secondary text.Rich
	// IconURL is the URL to the icon that will be displayed on the left of the
	// text. This field is optional.
	IconURL string
	// Image returns whether or not the icon URL is actually an image, which
	// indicates that the frontend should not do rounded corners.
	Image bool
}

// ServerMessageMemberLister optionally extends ServerMessage to add a member
// list into each channel. This function works similarly to ServerMessage's
// JoinServer.
type ServerMessageMemberLister interface {
	// ListMembers assigns the given container to the channel's member list.
	// The given context may be used to provide HTTP request cancellations, but
	// frontends must not rely solely on this, as the general context rules
	// applies.
	ListMembers(context.Context, MemberListContainer) (stop func(), err error)
}

// UserStatus represents a user's status. This might be used by the frontend to
// visually display the status.
type UserStatus uint8

const (
	UnknownStatus UserStatus = iota
	OnlineStatus
	IdleStatus
	BusyStatus // also known as Do Not Disturb
	AwayStatus
	OfflineStatus
	InvisibleStatus // reserved; currently unused
)

// String formats a user status as a title string, such as "Online" or
// "Unknown". It treats unknown constants as UnknownStatus.
func (s UserStatus) String() string {
	switch s {
	case OnlineStatus:
		return "Online"
	case IdleStatus:
		return "Idle"
	case BusyStatus:
		return "Busy"
	case AwayStatus:
		return "Away"
	case OfflineStatus:
		return "Offline"
	case InvisibleStatus:
		return "Invisible"
	case UnknownStatus:
		fallthrough
	default:
		return "Unknown"
	}
}

// ListMember represents a single member in the member list. This is a base
// interface that may implement more interfaces, such as Iconer for the user's
// avatar. The frontend may give everyone an avatar regardless, or it may not
// show any avatars at all.
//
// This interface works similarly to a slightly extended MessageAuthor
// interface.
type ListMember interface {
	// Identifier identifies the individual member. This works similarly to
	// MessageAuthor.
	Identifier
	// Namer returns the name of the member. This works similarly to a
	// MessageAuthor.
	Namer
	// Status returns the status of the member. The backend does not have to
	// show offline members with the offline status if it doesn't want to show
	// offline menbers at all.
	Status() UserStatus
	// Secondary returns the subtext of this member. This could be anything,
	// such as a user's custom status or away reason.
	Secondary() text.Rich
}

// MessageHeader implements the minimum interface for any message event.
type MessageHeader interface {
	Identifier
	Time() time.Time
}

// MessageCreate is the interface for an incoming message.
type MessageCreate interface {
	MessageHeader
	Author() MessageAuthor
	Content() text.Rich
}

// MessageUpdate is the interface for a message update (or edit) event. If the
// returned text.Rich returns true for Empty(), then the element shouldn't be
// changed.
type MessageUpdate interface {
	MessageHeader
	Author() MessageAuthor // optional (nilable)
	Content() text.Rich    // optional (rich.Content == "")
}

// MessageAuthor is the interface for an identifiable message author. The
// returned ID may or may not be used by the frontend, but clients must
// guarantee uniqueness for intended behaviors.
//
// The frontend may also use this to squash messages with the same author
// together.
type MessageAuthor interface {
	Identifier
	Namer
}

// MessageAuthorAvatar is an optional interface that MessageAuthor could
// implement. A frontend may optionally support this. A backend may return an
// empty string, in which the frontend must handle, perhaps by using a
// placeholder.
type MessageAuthorAvatar interface {
	Avatar() (url string)
}

// MessageDelete is the interface for a message delete event.
type MessageDelete interface {
	MessageHeader
}

// MessageNonce extends SendableMessage and MessageCreate to add nonce support.
// This is known in IRC as labeled responses. Clients could use this for
// various purposes, including knowing when a message has been sent
// successfully.
//
// Both the backend and frontend must implement this for the feature to work
// properly. The backend must check if SendableMessage implements MessageNonce,
// and the frontend must check if MessageCreate implements MessageNonce.
type MessageNonce interface {
	Nonce() string
}

// MessageMentioned extends MessageCreate to add mentioning support. The
// frontend may or may not implement this. If it does, the frontend will
// typically format the message into a notification and play a sound.
type MessageMentioned interface {
	// Mentioned returns whether or not the message mentions the current user.
	Mentioned() bool
}