// Broker client: the thin adapter behind the Publisher interface.
//
// Everything that decides *what to send and when* is in pump.go and is tested
// without a broker. This file only knows how to put bytes on a topic, which is
// why it is the one part that needs a real connection to exercise.
//
// Targets Mosquitto. No clustering, no shared subscriptions, no broker-side
// rules — a store publishes its own events under its own prefix and that is
// the whole interaction.
package mqtt

import (
	"context"
	"crypto/tls"
	"crypto/x509"
	"errors"
	"fmt"
	"log"
	"net"
	"net/url"
	"os"
	"strings"
	"time"

	paho "github.com/eclipse/paho.mqtt.golang"
)

// ClientOptions configures a broker connection.
type ClientOptions struct {
	// BrokerURL is tls://host:8883 in production, tcp://host:1883 for local
	// testing only. Credentials and footfall must never cross the internet in
	// the clear, so Connect refuses tcp:// to a non-loopback host.
	BrokerURL string
	ClientID  string
	Username  string
	Password  string
	// CAFile pins a private CA. Empty uses the system roots, which is what a
	// Let's Encrypt certificate on the broker needs.
	CAFile string
	// InsecureSkipVerify disables certificate checking. Only ever for a
	// self-signed staging box, and it is logged loudly when set, because a
	// forgotten one silently removes the protection TLS was added for.
	InsecureSkipVerify bool
	// PublishTimeout bounds a single publish. Without it a half-open
	// connection blocks the pump indefinitely and the queue grows behind it.
	PublishTimeout time.Duration
	Log            *log.Logger
}

// Client implements Publisher.
type Client struct {
	opts   ClientOptions
	client paho.Client
}

// NewClient dials the broker. It returns as soon as the connection is
// established; reconnection afterwards is automatic and the pump reads
// Connected() to decide whether to try.
func NewClient(opts ClientOptions) (*Client, error) {
	if opts.BrokerURL == "" {
		return nil, errors.New("mqtt: no broker url")
	}
	if opts.PublishTimeout <= 0 {
		opts.PublishTimeout = 10 * time.Second
	}
	if err := checkTransport(opts.BrokerURL); err != nil {
		return nil, err
	}

	po := paho.NewClientOptions().
		AddBroker(opts.BrokerURL).
		SetClientID(opts.ClientID).
		SetUsername(opts.Username).
		SetPassword(opts.Password).
		// The broker holds no state for us: every event is already durable on
		// our own disk, so a clean session avoids the broker queueing a
		// second copy we would then have to de-duplicate.
		SetCleanSession(true).
		SetAutoReconnect(true).
		SetConnectRetry(true).
		SetConnectRetryInterval(5 * time.Second).
		SetMaxReconnectInterval(2 * time.Minute).
		SetKeepAlive(30 * time.Second).
		SetConnectTimeout(15 * time.Second).
		// Publishes must fail fast rather than pile up in memory while the
		// link is down; the spool is what holds them, not the client.
		SetMessageChannelDepth(1).
		SetOrderMatters(true)

	if strings.HasPrefix(opts.BrokerURL, "tls://") ||
		strings.HasPrefix(opts.BrokerURL, "ssl://") {
		cfg, err := tlsConfig(opts)
		if err != nil {
			return nil, err
		}
		po.SetTLSConfig(cfg)
	}

	c := &Client{opts: opts}
	po.OnConnect = func(paho.Client) { c.logf("broker connected: %s", opts.BrokerURL) }
	po.OnConnectionLost = func(_ paho.Client, err error) {
		c.logf("broker connection lost: %v", err)
	}
	c.client = paho.NewClient(po)

	tok := c.client.Connect()
	if !tok.WaitTimeout(20 * time.Second) {
		// SetConnectRetry means paho retries internally and this token never
		// completes, so a REFUSED connection and an UNREACHABLE broker both
		// arrive here as a timeout. They need opposite actions - re-link this
		// PC, or go and look at the network - and reporting both as "timed
		// out" sent the diagnosis to the wrong place. Measured: mosquitto
		// logged "not authorised" while the agent logged a timeout.
		return c, fmt.Errorf("mqtt: %s", describeStall(opts.BrokerURL))
	}
	if err := tok.Error(); err != nil {
		return c, fmt.Errorf("mqtt: connect to %s: %w", opts.BrokerURL, err)
	}
	return c, nil
}

// describeStall says which of the two failures this is, by asking the one
// question that separates them: can we open a socket to the broker at all?
//
// It cannot name the exact reason - the broker does not tell a rejected client
// why, and a TLS failure looks the same from here - so it says what is known
// and what to check, rather than guessing. Being reachable but not accepted is
// overwhelmingly a credential this PC no longer has, which is what happens when
// a site is re-provisioned.
func describeStall(brokerURL string) string {
	host := brokerHostPort(brokerURL)
	if host == "" {
		return fmt.Sprintf("connect to %s timed out", brokerURL)
	}
	conn, err := net.DialTimeout("tcp", host, 5*time.Second)
	if err != nil {
		return fmt.Sprintf("cannot reach the broker at %s: %v - check the "+
			"network and that the broker is running", host, err)
	}
	_ = conn.Close()
	return fmt.Sprintf("the broker at %s is reachable but did not accept this "+
		"PC - usually its credentials are no longer valid; re-link it with "+
		"`behavision-agent claim <code>`", host)
}

// brokerHostPort extracts host:port for the reachability probe. Parsed with
// net/url, never by scanning for the first ":" - an IPv6 literal is bracketed
// and full of them.
func brokerHostPort(brokerURL string) string {
	u, err := url.Parse(brokerURL)
	if err != nil || u.Host == "" {
		return ""
	}
	if u.Port() != "" {
		return u.Host
	}
	if strings.HasPrefix(brokerURL, "tls://") || strings.HasPrefix(brokerURL, "ssl://") {
		return net.JoinHostPort(u.Hostname(), "8883")
	}
	return net.JoinHostPort(u.Hostname(), "1883")
}

// Publish sends one message at QoS 1 and waits for the broker's PUBACK.
//
// QoS 1, not 0 or 2. At QoS 0 the broker never confirms, so the pump would ack
// and delete an event that was dropped on the wire. QoS 2 costs two extra
// round trips to remove a duplicate the server can drop itself from the event
// id — at-least-once with idempotent consumers is the cheaper contract.
func (c *Client) Publish(ctx context.Context, topic string, payload []byte) error {
	if c.client == nil {
		return errors.New("mqtt: no client")
	}
	if !c.client.IsConnected() {
		return errors.New("mqtt: not connected")
	}
	tok := c.client.Publish(topic, 1, false, payload)

	// Honour both the caller's context and a hard timeout: a half-open TCP
	// connection can leave a token that never completes, which would stall the
	// pump forever with the queue growing behind it.
	done := make(chan struct{})
	go func() { tok.Wait(); close(done) }()
	select {
	case <-ctx.Done():
		return ctx.Err()
	case <-time.After(c.opts.PublishTimeout):
		return fmt.Errorf("mqtt: publish to %s timed out", topic)
	case <-done:
		return tok.Error()
	}
}

// Connected reports whether the broker link is up.
func (c *Client) Connected() bool {
	return c.client != nil && c.client.IsConnected()
}

// Close disconnects cleanly, giving in-flight publishes a moment to land.
func (c *Client) Close() {
	if c.client != nil && c.client.IsConnected() {
		c.client.Disconnect(1000)
	}
}

func (c *Client) logf(format string, args ...any) {
	if c.opts.Log != nil {
		c.opts.Log.Printf(format, args...)
	}
}

// checkTransport refuses plaintext MQTT to anywhere but the local machine.
//
// The payloads carry customer visit records and the connection carries the
// tenant's broker password. A tcp:// URL to a public host is not a
// configuration choice, it is a mistake, and it is one that works — which is
// exactly why it has to be rejected here rather than noticed later.
func checkTransport(raw string) error {
	if !strings.HasPrefix(raw, "tcp://") && !strings.HasPrefix(raw, "mqtt://") {
		return nil
	}
	if os.Getenv("BEHAVISION_ALLOW_PLAINTEXT_MQTT") == "1" {
		return nil
	}
	// url.Parse, not hand-rolled splitting: an IPv6 literal is bracketed and
	// full of colons, so scanning for the first ":" turns "[::1]:1883" into
	// "[" and refuses a perfectly good loopback address.
	u, err := url.Parse(raw)
	if err != nil {
		return fmt.Errorf("mqtt: cannot parse broker url %q: %w", raw, err)
	}
	switch u.Hostname() {
	case "localhost", "127.0.0.1", "::1", "":
		return nil
	}
	return fmt.Errorf("mqtt: refusing plaintext connection to %q - use tls:// "+
		"(set BEHAVISION_ALLOW_PLAINTEXT_MQTT=1 only for local testing)",
		u.Hostname())
}

func tlsConfig(opts ClientOptions) (*tls.Config, error) {
	cfg := &tls.Config{MinVersion: tls.VersionTLS12}
	if opts.InsecureSkipVerify {
		cfg.InsecureSkipVerify = true
		if opts.Log != nil {
			opts.Log.Print("WARNING: MQTT certificate verification is DISABLED")
		}
		return cfg, nil
	}
	if opts.CAFile == "" {
		return cfg, nil // system roots
	}
	pem, err := os.ReadFile(opts.CAFile)
	if err != nil {
		return nil, fmt.Errorf("mqtt: ca file: %w", err)
	}
	pool := x509.NewCertPool()
	if !pool.AppendCertsFromPEM(pem) {
		return nil, fmt.Errorf("mqtt: no certificates found in %s", opts.CAFile)
	}
	cfg.RootCAs = pool
	return cfg, nil
}

// osWriteFile is indirected so tests can build without importing os twice.
var osWriteFile = os.WriteFile