package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"net/http"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"
)

const (
	configDir      = "/tmp/nexavpn-vpn-dns"
	corefilePath   = configDir + "/Corefile"
	overridesPath  = configDir + "/service-overrides.hosts"
)

type dnsResponse struct {
	Records []dnsRecord `json:"records"`
}

type dnsRecord struct {
	Domain   string `json:"domain"`
	TargetIP string `json:"target_ip"`
}

func main() {
	ctx := context.Background()

	if err := os.MkdirAll(configDir, 0o755); err != nil {
		log.Fatalf("unable to create coredns config dir: %v", err)
	}

	if err := writeCorefile(); err != nil {
		log.Fatalf("unable to write Corefile: %v", err)
	}
	if err := refreshOverrides(ctx); err != nil {
		log.Printf("initial dns override sync failed: %v", err)
	}

	go func() {
		ticker := time.NewTicker(15 * time.Second)
		defer ticker.Stop()
		for range ticker.C {
			if err := refreshOverrides(ctx); err != nil {
				log.Printf("dns override sync failed: %v", err)
			}
		}
	}()

	cmd := exec.Command("/coredns", "-conf", corefilePath)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	log.Printf("starting coredns on %s", envOrDefault("NEXAVPN_VPN_DNS_ADDR", ":53"))
	if err := cmd.Run(); err != nil {
		log.Fatalf("coredns exited: %v", err)
	}
}

func writeCorefile() error {
	upstreams := parseList(envOrDefault("NEXAVPN_VPN_DNS_UPSTREAMS", "172.16.0.100,172.16.0.105"))
	if len(upstreams) == 0 {
		return errors.New("no upstream dns servers configured")
	}

	corefile := fmt.Sprintf(`%s {
    errors
    hosts ` + overridesPath + ` {
        ttl 30
        reload 15s
        fallthrough
    }
    forward . %s
    cache 30
}
`, envOrDefault("NEXAVPN_VPN_DNS_ADDR", ":53"), strings.Join(upstreams, " "))

	return os.WriteFile(corefilePath, []byte(corefile), 0o644)
}

func refreshOverrides(ctx context.Context) error {
	syncURL := strings.TrimRight(envOrDefault("NEXAVPN_DNS_SYNC_URL", "http://127.0.0.1:8080/api/v1/gateway-agent/dns/services"), "/")
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, syncURL, nil)
	if err != nil {
		return err
	}
	req.Header.Set("X-Gateway-Bootstrap-Token", os.Getenv("GATEWAY_BOOTSTRAP_TOKEN"))

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return fmt.Errorf("service dns sync failed with status %s", resp.Status)
	}

	var payload dnsResponse
	if err := json.NewDecoder(resp.Body).Decode(&payload); err != nil {
		return err
	}

	lines := make([]string, 0, len(payload.Records))
	for _, record := range payload.Records {
		domain := normalizeDomain(record.Domain)
		targetIP := strings.TrimSpace(record.TargetIP)
		if domain == "" || targetIP == "" {
			continue
		}
		lines = append(lines, targetIP+" "+domain)
	}

	content := strings.Join(lines, "\n")
	if content != "" {
		content += "\n"
	}

	return os.WriteFile(filepath.Clean(overridesPath), []byte(content), 0o644)
}

func parseList(raw string) []string {
	seen := make(map[string]struct{})
	values := make([]string, 0)
	for _, part := range strings.Split(raw, ",") {
		value := strings.TrimSpace(part)
		if value == "" {
			continue
		}
		if _, ok := seen[value]; ok {
			continue
		}
		seen[value] = struct{}{}
		values = append(values, value)
	}
	return values
}

func normalizeDomain(value string) string {
	value = strings.TrimSpace(strings.ToLower(value))
	value = strings.TrimSuffix(value, ".")
	return value
}

func envOrDefault(key string, fallback string) string {
	if value := strings.TrimSpace(os.Getenv(key)); value != "" {
		return value
	}
	return fallback
}