// Package metrics implements a standalone HTTP server for serving pprof
// profiles and Prometheus metrics.
package metrics

import (
	"context"
	"errors"
	"net/http"
	"net/http/pprof"
	"net/netip"
	"sync/atomic"
	"time"

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

	"github.com/sot-tech/mochi/pkg/log"
)

const (
	readTimeout  = 5 * time.Second
	writeTimeout = readTimeout * 2
)

var (
	logger        = log.NewLogger("metrics")
	serverCounter = new(int32)
)

// Enabled indicates that configured at least one metrics server
func Enabled() bool {
	return atomic.LoadInt32(serverCounter) > 0
}

// Server represents a standalone HTTP server for serving a Prometheus metrics
// endpoint.
type Server struct {
	srv *http.Server
}

// AddressFamily returns the label value for reporting the address family of an IP address.
func AddressFamily(ip netip.Addr) string {
	switch {
	case ip.Is4(), ip.Is4In6():
		return "IPv4"
	case ip.Is6():
		return "IPv6"
	default:
		return "<unknown>"
	}
}

// Close shuts down the server.
func (s *Server) Close() error {
	return s.srv.Shutdown(context.Background())
}

// NewServer creates a new instance of a Prometheus server that asynchronously
// serves requests.
func NewServer(addr string) *Server {
	mux := http.NewServeMux()

	mux.Handle("/metrics", promhttp.Handler())
	mux.HandleFunc("/debug/pprof/", pprof.Index)
	mux.HandleFunc("/debug/pprof/cmdline", pprof.Cmdline)
	mux.HandleFunc("/debug/pprof/profile", pprof.Profile)
	mux.HandleFunc("/debug/pprof/symbol", pprof.Symbol)
	mux.HandleFunc("/debug/pprof/trace", pprof.Trace)

	s := &Server{
		srv: &http.Server{
			Addr:              addr,
			Handler:           mux,
			ReadTimeout:       readTimeout,
			ReadHeaderTimeout: readTimeout,
			WriteTimeout:      writeTimeout,
		},
	}

	go func() {
		atomic.AddInt32(serverCounter, 1)
		defer atomic.AddInt32(serverCounter, -1)
		if err := s.srv.ListenAndServe(); !errors.Is(err, http.ErrServerClosed) {
			logger.Error().Err(err).Msg("failed while serving prometheus")
		}
	}()

	return s
}