package discover

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"net"
	"os"
	"time"

	"git.faercol.me/faercol/http-boot-config/config/config"
)

var ErrNoServer = errors.New("no server found")

var discoveryMsg = []byte("BOOT_DISCOVER")

type discoveryPayload struct {
	ManagementAddress string `json:"managementAddress"`
	Version           string `json:"version"`
}

func DiscoverServer(conf config.Autodiscovery) (string, error) {
	raddr, err := net.ResolveUDPAddr("udp", conf.DiscoveryAddr)
	if err != nil {
		return "", fmt.Errorf("invalid discovery address: %w", err)
	}

	conn, err := net.ListenUDP("udp", nil)
	if err != nil {
		return "", fmt.Errorf("failed to dial UDP multicast group: %w", err)
	}
	defer conn.Close()

	n, err := conn.WriteToUDP(discoveryMsg, raddr)
	if err != nil {
		return "", fmt.Errorf("failed to write to UDP connection: %w", err)
	}
	if n < len(discoveryMsg) {
		return "", fmt.Errorf("failed to write the entire message (%d/%d)", n, len(discoveryMsg))
	}

	conn.SetReadDeadline(time.Now().Add(conf.Timeout))
	resp := make([]byte, 2048)
	if _, err := conn.Read(resp); err != nil {
		if errors.Is(err, os.ErrDeadlineExceeded) {
			return "", ErrNoServer
		}
		return "", fmt.Errorf("failed to read response from server: %w", err)
	}

	var payload discoveryPayload
	if err := json.Unmarshal(bytes.Trim(resp, "\x00"), &payload); err != nil {
		return "", fmt.Errorf("failed to parse response from server: %w", err)
	}

	return payload.ManagementAddress, nil
}