package aperture

import (
	"fmt"
	"net/http"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

const streamIDLabel = "streamID"

var (
	// mailboxCount tracks the current number of active mailboxes.
	mailboxCount = prometheus.NewGauge(prometheus.GaugeOpts{
		Namespace: "hashmail",
		Name:      "mailbox_count",
	})

	// mailboxReadCount counts each time a mailbox pair is being used.
	// A session consists of a bidirectional stream each using a mailbox
	// with an ID that overlaps for the first 63 bytes and differ for the
	// last bit. So in order to obtain accurate data about a specific
	// mailbox session, the stream ID that will be recorded is the first
	// 16 bytes of the session ID and we will only record the odd stream's
	// reads so that we don't duplicate the data.
	mailboxReadCount = prometheus.NewCounterVec(
		prometheus.CounterOpts{
			Namespace: "hashmail",
			Name:      "mailbox_read_count",
		}, []string{streamIDLabel},
	)
)

// PrometheusConfig is the set of configuration data that specifies if
// Prometheus metric exporting is activated, and if so the listening address of
// the Prometheus server.
type PrometheusConfig struct {
	// Enabled, if true, then Prometheus metrics will be exported.
	Enabled bool `long:"enabled" description:"if true prometheus metrics will be exported"`

	// ListenAddr is the listening address that we should use to allow the
	// main Prometheus server to scrape our metrics.
	ListenAddr string `long:"listenaddr" description:"the interface we should listen on for prometheus"`
}

// StartPrometheusExporter registers all relevant metrics with the Prometheus
// library, then launches the HTTP server that Prometheus will hit to scrape
// our metrics.
func StartPrometheusExporter(cfg *PrometheusConfig) error {
	// If we're not active, then there's nothing more to do.
	if !cfg.Enabled {
		return nil
	}

	// Next, we'll register all our metrics.
	prometheus.MustRegister(mailboxCount)
	prometheus.MustRegister(mailboxReadCount)

	// Finally, we'll launch the HTTP server that Prometheus will use to
	// scape our metrics.
	go func() {
		log.Infof("Prometheus metrics http endpoint being served on "+
			"%s", cfg.ListenAddr)

		http.Handle("/metrics", promhttp.Handler())
		fmt.Println(http.ListenAndServe(cfg.ListenAddr, nil))
	}()

	return nil
}