// Package simplex runs the SimpleX Chat command-line client and talks
// to it over its WebSocket API.
//
// The protocol, documented in the simplex-chat repository under bots/:
// a command goes out as {"corrId": "...", "cmd": "..."}, and the client
// answers it with {"corrId": "...", "resp": {...}} carrying the same id.
// Everything it sends without a corrId is an event. The API has no
// authentication; the client binds it to localhost only.
package simplex

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"strconv"
	"strings"
	"sync"

	"github.com/gorilla/websocket"
)

// maxMessageSize bounds one message from the chat client. The largest
// thing it sends is a record carrying a contact's profile picture, well
// under this.
const maxMessageSize = 16 << 20

var (
	// ErrClosed is returned by Command once the connection has ended.
	ErrClosed = errors.New("connection to the chat client closed")

	errUnexpected = errors.New("unexpected response")
	errCommand    = errors.New("command failed")
)

// EventHandler receives each event the chat client sends. Events are
// delivered one at a time, on the goroutine that also delivers command
// responses: a handler may Send, but must never wait on Command, whose
// response could then never arrive.
type EventHandler func(c *Client, ev Event)

// Client is a connection to the chat client's WebSocket API.
type Client struct {
	conn    *websocket.Conn
	log     *slog.Logger
	onEvent EventHandler

	// writeMu serialises writes: the connection allows one writer at
	// a time, and Command and Send are called from different
	// goroutines.
	writeMu sync.Mutex

	mu      sync.Mutex
	lastID  uint64
	waiting map[string]chan Event

	done chan struct{}
	err  error // why the read loop ended; valid once done is closed
}

// Dial connects to the chat client's API at url and starts reading from
// it, passing every event to onEvent.
func Dial(
	ctx context.Context, url string, log *slog.Logger, onEvent EventHandler,
) (*Client, error) {
	conn, resp, err := websocket.DefaultDialer.DialContext(ctx, url, nil)
	if resp != nil {
		_ = resp.Body.Close()
	}

	if err != nil {
		return nil, fmt.Errorf("connecting to %s: %w", url, err)
	}

	conn.SetReadLimit(maxMessageSize)

	c := &Client{
		conn:    conn,
		log:     log,
		onEvent: onEvent,
		waiting: make(map[string]chan Event),
		done:    make(chan struct{}),
	}

	go c.read()

	return c, nil
}

// Done is closed when the connection ends; Err then says why.
func (c *Client) Done() <-chan struct{} {
	return c.done
}

// Err returns why the connection ended. Call it only after Done is
// closed.
func (c *Client) Err() error {
	return c.err
}

// Close ends the connection.
func (c *Client) Close() error {
	err := c.conn.Close()
	if err != nil {
		return fmt.Errorf("closing connection: %w", err)
	}

	return nil
}

// ActiveUser returns the chat client's active user profile.
func (c *Client) ActiveUser(ctx context.Context) (User, error) {
	var r struct {
		User User `json:"user"`
	}

	err := c.command(ctx, cmdShowActiveUser, TypeActiveUser, &r)

	return r.User, err
}

// Address returns the user's long-term contact address, and false if
// the user has none.
func (c *Client) Address(ctx context.Context, userID int64) (ConnLink, bool, error) {
	//nolint:tagliatelle // the chat client's wire format.
	var r struct {
		ContactLink struct {
			ConnLinkContact ConnLink `json:"connLinkContact"`
		} `json:"contactLink"`
	}

	err := c.command(ctx, cmdShowAddress(userID), TypeUserContactLink, &r)

	var cerr *CommandError
	if errors.As(err, &cerr) && cerr.Detail == "userContactLinkNotFound" {
		return ConnLink{}, false, nil
	}

	if err != nil {
		return ConnLink{}, false, err
	}

	return r.ContactLink.ConnLinkContact, true, nil
}

// CreateAddress creates the user's long-term contact address.
func (c *Client) CreateAddress(ctx context.Context, userID int64) (ConnLink, error) {
	//nolint:tagliatelle // the chat client's wire format.
	var r struct {
		ConnLinkContact ConnLink `json:"connLinkContact"`
	}

	err := c.command(ctx, cmdCreateAddress(userID), TypeUserContactLinkCreated, &r)

	return r.ConnLinkContact, err
}

// SetAddressSettings replaces the settings of the user's address.
func (c *Client) SetAddressSettings(
	ctx context.Context, userID int64, s AddressSettings,
) error {
	cmd, err := cmdSetAddressSettings(userID, s)
	if err != nil {
		return err
	}

	return c.command(ctx, cmd, TypeUserContactLinkUpdated, nil)
}

// SendText sends a text message to a contact, as a reply to the message
// quotedItemID (0 for none). It does not wait for the chat client to
// accept it; a failure is logged when the client's answer arrives.
func (c *Client) SendText(contactID, quotedItemID int64, text string) error {
	cmd, err := cmdSendText(contactID, quotedItemID, text)
	if err != nil {
		return err
	}

	c.mu.Lock()
	id := c.nextID()
	c.mu.Unlock()

	return c.write(id, cmd)
}

// CommandError is a command the chat client refused. Type and Detail
// are the discriminators of its chatError record, such as "errorStore"
// and "userContactLinkNotFound".
type CommandError struct {
	Type   string
	Detail string
}

func (e *CommandError) Error() string {
	return fmt.Sprintf("%s: %s/%s", errCommand, e.Type, e.Detail)
}

func (e *CommandError) Unwrap() error {
	return errCommand
}

// command sends cmd, waits for its response, and, if the response has
// type want, decodes it into out (unless out is nil).
func (c *Client) command(ctx context.Context, cmd, want string, out any) error {
	ch := make(chan Event, 1)

	c.mu.Lock()
	id := c.nextID()
	c.waiting[id] = ch
	c.mu.Unlock()

	defer func() {
		c.mu.Lock()
		delete(c.waiting, id)
		c.mu.Unlock()
	}()

	err := c.write(id, cmd)
	if err != nil {
		return err
	}

	var ev Event

	select {
	case ev = <-ch:
	case <-c.done:
		return fmt.Errorf("%w: %w", ErrClosed, c.err)
	case <-ctx.Done():
		return fmt.Errorf("waiting for a response to %q: %w", cmdName(cmd), ctx.Err())
	}

	switch ev.Type {
	case want:
		if out == nil {
			return nil
		}

		return ev.Decode(out)
	case TypeChatCmdError:
		return commandError(ev)
	default:
		return fmt.Errorf("%w to %q: %s", errUnexpected, cmdName(cmd), ev.Type)
	}
}

// nextID returns a fresh correlation id. The caller holds c.mu.
func (c *Client) nextID() string {
	c.lastID++

	return strconv.FormatUint(c.lastID, 10)
}

func (c *Client) write(id, cmd string) error {
	c.writeMu.Lock()
	defer c.writeMu.Unlock()

	err := c.conn.WriteJSON(command{CorrID: id, Cmd: cmd})
	if err != nil {
		return fmt.Errorf("sending %q: %w", cmdName(cmd), err)
	}

	return nil
}

// read is the only reader of the connection. It runs until the
// connection fails or is closed.
func (c *Client) read() {
	defer close(c.done)

	for {
		_, data, err := c.conn.ReadMessage()
		if err != nil {
			c.err = err

			return
		}

		c.dispatch(data)
	}
}

// dispatch routes one message: a response to whoever waits for it, an
// event to the handler. A message that does not parse is logged and
// skipped rather than ending the connection; the API documentation
// warns that records change between releases.
func (c *Client) dispatch(data []byte) {
	var env envelope

	err := json.Unmarshal(data, &env)
	if err != nil {
		c.log.Warn("undecodable message from the chat client", "error", err)

		return
	}

	var head tagged

	err = json.Unmarshal(env.Resp, &head)
	if err != nil {
		c.log.Warn("undecodable record from the chat client", "error", err)

		return
	}

	ev := Event{Type: head.Type, raw: env.Resp}

	if env.CorrID == "" {
		c.onEvent(c, ev)

		return
	}

	c.mu.Lock()
	ch, ok := c.waiting[env.CorrID]
	c.mu.Unlock()

	if ok {
		ch <- ev

		return
	}

	// The response to a SendText: nobody waits for it, so a failure
	// is reported here or nowhere.
	if ev.Type == TypeChatCmdError {
		c.log.Warn("sending a message failed", "error", commandError(ev))
	}
}

func commandError(ev Event) error {
	var r cmdError

	err := ev.Decode(&r)
	if err != nil {
		return err
	}

	e := &CommandError{Type: r.ChatError.Type}

	for _, detail := range []*tagged{
		r.ChatError.ErrorType, r.ChatError.StoreError, r.ChatError.AgentError,
	} {
		if detail != nil {
			e.Detail = detail.Type
		}
	}

	return e
}

// cmdName is a command without its arguments, for error messages: the
// arguments of /_send are a message someone wrote.
func cmdName(cmd string) string {
	name, _, _ := strings.Cut(cmd, " ")

	return name
}