// Package bot is the calculator: it runs the SimpleX Chat client, makes
// sure the bot has a contact address that accepts everyone, and answers
// every text message with the value of the arithmetic in it.
package bot

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"strconv"
	"time"

	"sneak.berlin/go/simplexcalc/internal/calc"
	"sneak.berlin/go/simplexcalc/internal/simplex"
)

// DisplayName is the name of the bot's SimpleX profile, given to it
// when the profile is created on the first start.
const DisplayName = "calc"

// Welcome is sent to everyone whose contact request the bot accepts.
const Welcome = "Send me arithmetic, such as 2 + 2 or 5 * 5/2, " +
	"and I will reply with the result."

const (
	// chatPort is where the chat client serves its API, on localhost
	// inside the bot's own container.
	chatPort = 5225

	// connectTimeout bounds the wait for a freshly started chat client
	// to open its API, which includes creating or migrating the
	// database.
	connectTimeout = 60 * time.Second

	// setupTimeout bounds each setup command. Creating an address
	// talks to SimpleX relays over the network.
	setupTimeout = 2 * time.Minute

	// retryInterval paces the connection attempts.
	retryInterval = 250 * time.Millisecond

	dataDirMode = 0o700
)

var errExited = errors.New("simplex-chat exited")

// Run starts the chat client with its database in dataDir, connects to
// it, sets up the bot's address, and answers messages until ctx is
// cancelled — which is a clean stop and returns nil — or until the chat
// client or the connection to it fails, which returns the error.
func Run(ctx context.Context, log *slog.Logger, dataDir string) error {
	err := os.MkdirAll(dataDir, dataDirMode)
	if err != nil {
		return fmt.Errorf("creating data directory: %w", err)
	}

	// Cancelling this stops the chat client; the deferred wait makes
	// Run return only once it has exited, whatever path Run takes.
	cliCtx, stopCLI := context.WithCancel(ctx)

	cli, err := simplex.StartCLI(cliCtx, log, filepath.Join(dataDir, "simplex"),
		DisplayName, chatPort)
	if err != nil {
		stopCLI()

		return err
	}

	defer func() {
		stopCLI()
		<-cli.Done()
	}()

	client, err := connect(ctx, log, cli)
	if err != nil {
		return err
	}

	defer func() { _ = client.Close() }()

	err = setUp(ctx, log, client)
	if err != nil {
		return err
	}

	select {
	case <-ctx.Done():
		return nil
	case <-client.Done():
		return fmt.Errorf("%w: %w", simplex.ErrClosed, client.Err())
	case <-cli.Done():
		return fmt.Errorf("%w: %w", errExited, cli.Err())
	}
}

// connect waits for the chat client to open its API and connects to it.
func connect(
	ctx context.Context, log *slog.Logger, cli *simplex.CLI,
) (*simplex.Client, error) {
	ctx, cancel := context.WithTimeout(ctx, connectTimeout)
	defer cancel()

	url := "ws://127.0.0.1:" + strconv.Itoa(chatPort)

	for {
		client, err := simplex.Dial(ctx, url, log, handle(log))
		if err == nil {
			return client, nil
		}

		select {
		case <-cli.Done():
			return nil, fmt.Errorf("%w before opening its API: %w", errExited, cli.Err())
		case <-ctx.Done():
			return nil, fmt.Errorf("waiting for simplex-chat: %w (last error: %w)",
				ctx.Err(), err)
		case <-time.After(retryInterval):
		}
	}
}

// setUp gives the bot a long-term contact address, creating it on the
// first start, and sets it to accept every contact request and to greet
// each new contact. The settings are written on every start, so an
// address whose settings were changed by hand is put right.
func setUp(ctx context.Context, log *slog.Logger, client *simplex.Client) error {
	ctx, cancel := context.WithTimeout(ctx, setupTimeout)
	defer cancel()

	user, err := client.ActiveUser(ctx)
	if err != nil {
		return fmt.Errorf("reading the bot's profile: %w", err)
	}

	link, ok, err := client.Address(ctx, user.UserID)
	if err != nil {
		return fmt.Errorf("reading the bot's address: %w", err)
	}

	if !ok {
		log.Info("creating the bot's address")

		link, err = client.CreateAddress(ctx, user.UserID)
		if err != nil {
			return fmt.Errorf("creating the bot's address: %w", err)
		}
	}

	err = client.SetAddressSettings(ctx, user.UserID, simplex.AddressSettings{
		AutoAccept: &simplex.AutoAccept{AcceptIncognito: false},
		AutoReply:  &simplex.MsgContent{Type: "text", Text: Welcome},
	})
	if err != nil {
		return fmt.Errorf("setting the bot's address to accept everyone: %w", err)
	}

	log.Info("ready",
		"display_name", user.Profile.DisplayName,
		"address", link.ShortLink,
		"full_address", link.FullLink,
	)

	return nil
}

// handle answers each text message a contact sends.
func handle(log *slog.Logger) simplex.EventHandler {
	return func(c *simplex.Client, ev simplex.Event) {
		switch ev.Type {
		case simplex.TypeNewChatItems:
			var r simplex.NewChatItems

			err := ev.Decode(&r)
			if err != nil {
				log.Warn("ignoring an event", "error", err)

				return
			}

			for _, item := range r.ChatItems {
				msg, ok := item.Message()
				if !ok {
					continue
				}

				err = c.SendText(msg.ContactID, msg.ItemID, Reply(msg.Text))
				if err != nil {
					log.Error("replying", "contact_id", msg.ContactID, "error", err)
				}
			}
		case simplex.TypeContactConnected:
			var r simplex.ContactConnected

			err := ev.Decode(&r)
			if err != nil {
				log.Warn("ignoring an event", "error", err)

				return
			}

			log.Info("contact connected", "contact_id", r.Contact.ContactID)
		default:
			log.Debug("event", "type", ev.Type)
		}
	}
}

// Reply is the bot's answer to a message: the value of the arithmetic
// in it, or a short explanation of why there is none.
func Reply(text string) string {
	result, err := calc.Evaluate(text)

	switch {
	case err == nil:
		return result
	case errors.Is(err, calc.ErrTooLong):
		return fmt.Sprintf("That is too long for me: at most %d characters, please.",
			calc.MaxInputLength)
	case errors.Is(err, calc.ErrDivisionByZero):
		return "I cannot divide by zero."
	case errors.Is(err, calc.ErrTooLarge):
		return "The result is too large for me."
	default:
		return "I only understand arithmetic: numbers, + - * / and " +
			"parentheses, such as 5 * 5/2."
	}
}