// Package pskr subscribes to PSK Reporter's MQTT feed and turns it into the
// spots the band-opening detector already eats.
//
// Why this exists at all: the detector was fed from the DX cluster and the RBN,
// and on VHF that is a few hundred skimmers, nearly all of them on HF. A 6 m
// opening carrying 869 stations reached OpsLog as a handful of spots, or none.
// PSK Reporter is every ordinary station running WSJT-X and reporting what it
// decodes — the difference is two orders of magnitude, not a threshold.
//
// The feed's shape happens to suit us exactly:
//
//	topic   pskr/filter/v2/<band>/<mode>/<tx call>/<rx call>/...
//	payload {"f":50313000,"md":"FT8","rp":-12,"sc":"F4BPO","sl":"JN36",
//	         "rc":"OH5CX","rl":"KP30","b":"6m"}
//
// BOTH grids are in the message, so distance and bearing are arithmetic. No
// lookup, no DXCC-centre approximation, no extra network call — which is what
// made the cluster path's bearings coarse.
//
// Volume is the real design constraint. Six metres open is thousands of
// messages a minute, and OpsLog runs on some very old PCs. So: no history is
// kept here, nothing is persisted, each message is parsed and handed on or
// dropped, and the callback does the deciding.
package pskr

import (
	"encoding/json"
	"fmt"
	"strings"
	"sync"
	"time"

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

// DefaultBroker is PSK Reporter's public MQTT endpoint, TLS.
const DefaultBroker = "tls://mqtt.pskreporter.info:1884"

// Bands offered. HF below 12 m is deliberately absent: an "opening" on 20 m is
// the normal state of the band and announcing it says nothing. These are the
// bands where an opening is an event worth interrupting an operator for.
var Bands = []string{"12m", "10m", "6m", "4m", "2m"}

// Spot is one decode, already reduced to what a detector needs.
type Spot struct {
	Call    string // the transmitting station
	Band    string
	Mode    string
	Grid    string // transmitter's grid, 4 characters
	DistKm  int    // from the operator
	Bearing int    // degrees from the operator, short path
	At      time.Time
}

// Config is what the watcher needs to run.
type Config struct {
	Broker string
	Bands  []string
	// OpLat/OpLon are the operator's position: every spot is measured from it,
	// so with no position there is nothing to measure and the watcher stays down.
	OpLat, OpLon float64
	// Geo turns two grids into distance and bearing. Injected rather than
	// implemented here so it stays the SAME arithmetic the cluster path uses —
	// two answers for one question is how a bearing quietly becomes wrong.
	Geo func(grid string) (distKm int, bearing int, ok bool)
	// OnSpot receives every accepted decode. Called from the MQTT goroutine, so
	// it must not block: the broker's buffer is what pays for it if it does.
	OnSpot func(Spot)
	Logf   func(string, ...any)
}

// Watcher owns the MQTT connection and its subscriptions.
type Watcher struct {
	mu      sync.Mutex
	cfg     Config
	client  mqtt.Client
	running bool

	// received counts accepted spots since start, for the status panel: a
	// connection that is up but silent looks identical to one that is working
	// until you can see a number moving.
	received uint64
	lastAt   time.Time
	lastErr  string
}

func New(cfg Config) *Watcher {
	if cfg.Broker == "" {
		cfg.Broker = DefaultBroker
	}
	if len(cfg.Bands) == 0 {
		cfg.Bands = Bands
	}
	if cfg.Logf == nil {
		cfg.Logf = func(string, ...any) {}
	}
	return &Watcher{cfg: cfg}
}

// Start connects and subscribes. Safe to call when already running.
func (w *Watcher) Start() error {
	w.mu.Lock()
	defer w.mu.Unlock()
	if w.running {
		return nil
	}
	if w.cfg.Geo == nil || w.cfg.OnSpot == nil {
		return fmt.Errorf("pskr: Geo and OnSpot are required")
	}

	opts := mqtt.NewClientOptions().
		AddBroker(w.cfg.Broker).
		// A stable client id would collide with another OpsLog on the same
		// account; the broker is anonymous, so uniqueness is ours to provide.
		SetClientID(fmt.Sprintf("opslog-%d", time.Now().UnixNano())).
		SetCleanSession(true).
		SetAutoReconnect(true).
		SetConnectRetry(true).
		SetConnectRetryInterval(30 * time.Second).
		SetConnectTimeout(15 * time.Second).
		// No message is worth keeping if we cannot handle it now: an opening is
		// a thing happening at this moment, and a queue of stale decodes would
		// announce one that finished twenty minutes ago.
		SetOrderMatters(false)

	opts.OnConnect = func(c mqtt.Client) {
		w.cfg.Logf("pskr: connected to %s", w.cfg.Broker)
		for _, b := range w.cfg.Bands {
			// Every mode, every pair of stations, on this band. That firehose IS
			// the point: the detector's job is to find the shape in it.
			topic := "pskr/filter/v2/" + b + "/#"
			if tok := c.Subscribe(topic, 0, w.handle); tok.Wait() && tok.Error() != nil {
				w.cfg.Logf("pskr: subscribe %s failed: %v", topic, tok.Error())
				continue
			}
			w.cfg.Logf("pskr: watching %s", topic)
		}
	}
	opts.OnConnectionLost = func(_ mqtt.Client, err error) {
		w.mu.Lock()
		w.lastErr = err.Error()
		w.mu.Unlock()
		w.cfg.Logf("pskr: connection lost: %v (will retry)", err)
	}

	c := mqtt.NewClient(opts)
	// Deliberately NOT waiting on the connect token: the broker may be slow or
	// unreachable and startup must not hang on a feature that is decoration.
	// ConnectRetry brings it up in the background when it can.
	c.Connect()
	w.client = c
	w.running = true
	return nil
}

// Stop disconnects. Safe to call when already stopped.
func (w *Watcher) Stop() {
	w.mu.Lock()
	c, running := w.client, w.running
	w.client, w.running = nil, false
	w.mu.Unlock()
	if running && c != nil {
		c.Disconnect(250)
	}
}

// wire is the payload as PSK Reporter sends it — short keys, no nesting.
type wire struct {
	Freq   int64  `json:"f"`
	Mode   string `json:"md"`
	SNR    int    `json:"rp"`
	TxCall string `json:"sc"`
	TxGrid string `json:"sl"`
	RxCall string `json:"rc"`
	RxGrid string `json:"rl"`
	Band   string `json:"b"`
}

func (w *Watcher) handle(_ mqtt.Client, m mqtt.Message) {
	var p wire
	if err := json.Unmarshal(m.Payload(), &p); err != nil {
		return // malformed payloads are not worth a log line at this rate
	}
	call := strings.ToUpper(strings.TrimSpace(p.TxCall))
	grid := strings.ToUpper(strings.TrimSpace(p.TxGrid))
	if call == "" || len(grid) < 4 {
		return
	}
	// The TRANSMITTER is the station on the air; the receiver is whoever
	// happened to be listening. An opening is described by where the signals are
	// coming from, so it is the transmitter's grid that is measured.
	dist, brg, ok := w.cfg.Geo(grid[:4])
	if !ok {
		return
	}
	s := Spot{
		Call: call, Band: strings.ToLower(strings.TrimSpace(p.Band)),
		Mode: strings.ToUpper(strings.TrimSpace(p.Mode)), Grid: grid[:4],
		DistKm: dist, Bearing: brg,
		// Stamped on receipt: the broker's own timestamps vary between payload
		// versions, and the window this feeds is measured in minutes.
		At: time.Now(),
	}
	w.mu.Lock()
	w.received++
	w.lastAt = s.At
	w.mu.Unlock()
	w.cfg.OnSpot(s)
}

// Status is the snapshot the settings panel shows.
type Status struct {
	Running  bool      `json:"running"`
	Received uint64    `json:"received"`
	LastAt   time.Time `json:"last_at"`
	LastErr  string    `json:"last_err,omitempty"`
	Broker   string    `json:"broker"`
	Bands    []string  `json:"bands"`
}

func (w *Watcher) Status() Status {
	w.mu.Lock()
	defer w.mu.Unlock()
	st := Status{
		Running: w.running, Received: w.received, LastAt: w.lastAt,
		LastErr: w.lastErr, Broker: w.cfg.Broker,
	}
	st.Bands = append(st.Bands, w.cfg.Bands...)
	return st
}