// Package metrics provides in-process Prometheus-format instrumentation.
// All counters and gauges use atomic operations and are safe for concurrent use.
package metrics

import (
	"fmt"
	"net/http"
	"sync/atomic"
	"time"
)

var startTime = time.Now()

// Counters and gauges — updated by callers in ws.go and api.go.
var (
	wsConns       atomic.Int64 // current open WebSocket connections (gauge)
	msgIn         atomic.Int64 // total WS messages received (counter)
	msgOut        atomic.Int64 // total WS messages sent (counter)
	writeOps      atomic.Int64 // total signal write operations (counter)
	historyReqs   atomic.Int64 // total history requests served (counter)
)

// IncWsConns increments the active WebSocket connection gauge.
func IncWsConns() { wsConns.Add(1) }

// DecWsConns decrements the active WebSocket connection gauge.
func DecWsConns() { wsConns.Add(-1) }

// IncMsgIn increments the total inbound WS message counter.
func IncMsgIn() { msgIn.Add(1) }

// IncMsgOut increments the total outbound WS message counter.
func IncMsgOut() { msgOut.Add(1) }

// IncWrites increments the signal write operation counter.
func IncWrites() { writeOps.Add(1) }

// IncHistoryReqs increments the history request counter.
func IncHistoryReqs() { historyReqs.Add(1) }

// Handler returns an http.HandlerFunc that renders Prometheus-format metrics.
// activeSubs is called each request to read the current number of unique signal
// subscriptions from the broker; pass nil to omit the metric.
func Handler(activeSubs func() int) http.HandlerFunc {
	return func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")

		uptime := time.Since(startTime).Seconds()
		subs := 0
		if activeSubs != nil {
			subs = activeSubs()
		}

		fmt.Fprintf(w, "# HELP uopi_uptime_seconds Seconds elapsed since the server started\n")
		fmt.Fprintf(w, "# TYPE uopi_uptime_seconds counter\n")
		fmt.Fprintf(w, "uopi_uptime_seconds %.3f\n\n", uptime)

		fmt.Fprintf(w, "# HELP uopi_ws_connections Current number of open WebSocket connections\n")
		fmt.Fprintf(w, "# TYPE uopi_ws_connections gauge\n")
		fmt.Fprintf(w, "uopi_ws_connections %d\n\n", wsConns.Load())

		fmt.Fprintf(w, "# HELP uopi_signal_subscriptions Current number of unique upstream signal subscriptions\n")
		fmt.Fprintf(w, "# TYPE uopi_signal_subscriptions gauge\n")
		fmt.Fprintf(w, "uopi_signal_subscriptions %d\n\n", subs)

		fmt.Fprintf(w, "# HELP uopi_ws_messages_in_total Total WebSocket messages received from clients\n")
		fmt.Fprintf(w, "# TYPE uopi_ws_messages_in_total counter\n")
		fmt.Fprintf(w, "uopi_ws_messages_in_total %d\n\n", msgIn.Load())

		fmt.Fprintf(w, "# HELP uopi_ws_messages_out_total Total WebSocket messages sent to clients\n")
		fmt.Fprintf(w, "# TYPE uopi_ws_messages_out_total counter\n")
		fmt.Fprintf(w, "uopi_ws_messages_out_total %d\n\n", msgOut.Load())

		fmt.Fprintf(w, "# HELP uopi_write_ops_total Total signal write operations\n")
		fmt.Fprintf(w, "# TYPE uopi_write_ops_total counter\n")
		fmt.Fprintf(w, "uopi_write_ops_total %d\n\n", writeOps.Load())

		fmt.Fprintf(w, "# HELP uopi_history_requests_total Total historical data requests served\n")
		fmt.Fprintf(w, "# TYPE uopi_history_requests_total counter\n")
		fmt.Fprintf(w, "uopi_history_requests_total %d\n\n", historyReqs.Load())
	}
}