// Package metrics exposes Prometheus metrics: the Go runtime and process metrics that the
// default registry collects, HTTP request metrics and MusicServer's own metrics.
package metrics

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/http"
	"time"

	"music-server/internal/logging"

	"github.com/prometheus/client_golang/prometheus/promhttp"
	"go.uber.org/zap"
)

// Server serves /metrics on a port of its own, so it is never reachable through the
// public port.
type Server struct {
	httpServer *http.Server
}

// NewServer creates a metrics server for the given port.
func NewServer(port int) *Server {
	mux := http.NewServeMux()
	mux.Handle("GET /metrics", promhttp.Handler())
	return &Server{httpServer: &http.Server{
		Addr:              fmt.Sprintf(":%d", port),
		Handler:           mux,
		ReadHeaderTimeout: 5 * time.Second,
	}}
}

// Handler returns the handler that serves /metrics.
func (s *Server) Handler() http.Handler {
	return s.httpServer.Handler
}

// Start listens on the port and serves metrics in the background. It returns an error
// when the port cannot be opened.
func (s *Server) Start() error {
	ln, err := net.Listen("tcp", s.httpServer.Addr)
	if err != nil {
		return err
	}
	logging.GetLogger().Info("Metrics server starting", zap.String("address", s.httpServer.Addr))
	go func() {
		if err := s.httpServer.Serve(ln); !errors.Is(err, http.ErrServerClosed) {
			logging.GetLogger().Error("Metrics server error", zap.Error(err))
		}
	}()
	return nil
}

// Shutdown stops the metrics server.
func (s *Server) Shutdown(ctx context.Context) error {
	return s.httpServer.Shutdown(ctx)
}