package dns

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

const (
	SettingsStorePath = "/etc/debtik/dns/settings.json"
	DnsmasqDNSConf    = "/etc/dnsmasq.d/debtik_dns.conf"
	SystemResolvConf  = "/etc/resolv.conf"
)

// Manager coordinates Router DNS forwarder, upstream resolvers, and static DNS mapping.
type Manager struct {
	mu             sync.RWMutex
	settings       DNSSettings
	dynamicServers []string
}

func NewManager() *Manager {
	m := &Manager{
		dynamicServers: make([]string, 0),
		settings: DNSSettings{
			Servers:             []string{"1.1.1.1", "8.8.8.8"},
			DynamicServers:      make([]string, 0),
			AllowRemoteRequests: true,
			CacheSize:           2048,
			Domain:              "",
			StaticRecords:       make([]StaticRecord, 0),
		},
	}

	m.load()
	if err := m.applyLocked(); err != nil {
		log.Printf("[DNS] Error applying initial DNS configuration: %v", err)
	}
	return m
}

func (m *Manager) load() {
	m.mu.Lock()
	defer m.mu.Unlock()

	data, err := os.ReadFile(SettingsStorePath)
	if err != nil {
		// If no settings file, read existing resolv.conf nameservers if any
		if curNameservers := readSystemNameservers(); len(curNameservers) > 0 {
			m.settings.Servers = curNameservers
		}
		return
	}

	var s DNSSettings
	if err := json.Unmarshal(data, &s); err == nil {
		if len(s.Servers) > 0 {
			m.settings.Servers = s.Servers
		}
		m.settings.AllowRemoteRequests = s.AllowRemoteRequests
		if s.CacheSize > 0 {
			m.settings.CacheSize = s.CacheSize
		}
		m.settings.Domain = s.Domain
		m.settings.DoHServer = s.DoHServer
		m.settings.VerifyDoHCert = s.VerifyDoHCert
		if s.StaticRecords != nil {
			m.settings.StaticRecords = s.StaticRecords
		}
	}
}

func (m *Manager) saveLocked() error {
	dir := filepath.Dir(SettingsStorePath)
	if err := os.MkdirAll(dir, 0755); err != nil {
		return err
	}

	data, err := json.MarshalIndent(m.settings, "", "  ")
	if err != nil {
		return err
	}

	return os.WriteFile(SettingsStorePath, data, 0644)
}

// applyLocked renders the dnsmasq DNS configuration and updates /etc/resolv.conf.
func (m *Manager) applyLocked() (result error) {
	oldConfig, configErr := os.ReadFile(DnsmasqDNSConf)
	oldResolver, resolverErr := os.ReadFile(SystemResolvConf)
	wasRunning := exec.Command("systemctl", "is-active", "--quiet", "dnsmasq").Run() == nil
	defer func() {
		if result != nil {
			if configErr == nil {
				_ = os.WriteFile(DnsmasqDNSConf, oldConfig, 0644)
			} else if os.IsNotExist(configErr) {
				_ = os.Remove(DnsmasqDNSConf)
			}
			if resolverErr == nil {
				_ = os.WriteFile(SystemResolvConf, oldResolver, 0644)
			}
			if wasRunning {
				if err := exec.Command("systemctl", "restart", "dnsmasq").Run(); err != nil {
					result = fmt.Errorf("DNS apply failed: %v; previous config restart failed: %w", result, err)
				}
			}
		}
	}()

	var sb strings.Builder
	sb.WriteString("# ==================================================================\n")
	sb.WriteString("# Enterprise Router DNS Forwarder & Cache Configuration\n")
	sb.WriteString("# Auto-generated by Network Appliance Controller. DO NOT EDIT DIRECTLY.\n")
	sb.WriteString("# ==================================================================\n\n")

	cacheSize := m.settings.CacheSize
	if cacheSize <= 0 {
		cacheSize = 2048
	}
	sb.WriteString(fmt.Sprintf("cache-size=%d\n", cacheSize))

	// Do not read /etc/resolv.conf in dnsmasq (prevents loop with nameserver 127.0.0.1)
	sb.WriteString("no-resolv\n")

	// Upstream DNS Servers (Configured + Dynamic)
	allUpstreams := make([]string, 0)
	seen := make(map[string]bool)

	for _, s := range m.settings.Servers {
		trimmed := strings.TrimSpace(s)
		if trimmed != "" && !seen[trimmed] {
			allUpstreams = append(allUpstreams, trimmed)
			seen[trimmed] = true
		}
	}
	for _, s := range m.dynamicServers {
		trimmed := strings.TrimSpace(s)
		if trimmed != "" && !seen[trimmed] {
			allUpstreams = append(allUpstreams, trimmed)
			seen[trimmed] = true
		}
	}

	if len(allUpstreams) == 0 {
		allUpstreams = append(allUpstreams, "1.1.1.1", "8.8.8.8")
	}

	sb.WriteString("\n# Upstream DNS Forwarders\n")
	for _, s := range allUpstreams {
		sb.WriteString(fmt.Sprintf("server=%s\n", s))
	}

	// Domain
	domain := strings.TrimSpace(m.settings.Domain)
	if domain != "" {
		sb.WriteString(fmt.Sprintf("\ndomain=%s\n", domain))
		sb.WriteString(fmt.Sprintf("local=/%s/\n", domain))
	}

	// Remote requests restriction
	if !m.settings.AllowRemoteRequests {
		sb.WriteString("\n# Remote requests disabled: only listen on loopback for host\n")
		sb.WriteString("listen-address=127.0.0.1\n")
	}

	// Static DNS Records
	sb.WriteString("\n# Local Static DNS Records (/ip dns static)\n")
	for _, rec := range m.settings.StaticRecords {
		if rec.Disabled {
			continue
		}
		name := strings.TrimSpace(rec.Name)
		addr := strings.TrimSpace(rec.Address)
		if name == "" || addr == "" {
			continue
		}

		recType := strings.ToUpper(strings.TrimSpace(rec.Type))
		switch recType {
		case "CNAME":
			sb.WriteString(fmt.Sprintf("cname=%s,%s\n", name, addr))
		case "TXT":
			sb.WriteString(fmt.Sprintf("txt-record=%s,%q\n", name, addr))
		default: // "A" or "AAAA"
			sb.WriteString(fmt.Sprintf("address=/%s/%s\n", name, addr))
		}
	}

	// Write dnsmasq config
	_ = os.MkdirAll(filepath.Dir(DnsmasqDNSConf), 0755)
	if err := os.WriteFile(DnsmasqDNSConf, []byte(sb.String()), 0644); err != nil {
		return fmt.Errorf("failed to write %s: %w", DnsmasqDNSConf, err)
	}

	// Test dnsmasq syntax
	testOut, testErr := exec.Command("dnsmasq", "--test").CombinedOutput()
	if testErr != nil {
		log.Printf("[DNS] dnsmasq syntax check failed: %v, output: %s", testErr, string(testOut))
		return fmt.Errorf("dnsmasq configuration test failed: %s", string(testOut))
	}

	// Update host /etc/resolv.conf
	var rsb strings.Builder
	rsb.WriteString("# ==================================================================\n")
	rsb.WriteString("# Network Appliance System Resolver\n")
	rsb.WriteString("# Managed by DebTik DNS Controller\n")
	rsb.WriteString("# ==================================================================\n\n")
	rsb.WriteString("nameserver 127.0.0.1\n")
	for _, s := range allUpstreams {
		rsb.WriteString(fmt.Sprintf("nameserver %s\n", s))
	}
	if domain != "" {
		rsb.WriteString(fmt.Sprintf("search %s\n", domain))
	}

	if err := os.WriteFile(SystemResolvConf, []byte(rsb.String()), 0644); err != nil {
		return err
	}

	// Restart dnsmasq to re-read all configuration files
	if err := exec.Command("systemctl", "restart", "dnsmasq").Run(); err != nil {
		return fmt.Errorf("failed to restart dnsmasq: %w", err)
	}

	return nil
}

// GetSettings returns a snapshot of current DNS settings with dynamic servers populated.
func (m *Manager) GetSettings() DNSSettings {
	m.mu.RLock()
	defer m.mu.RUnlock()

	s := m.settings
	s.DynamicServers = make([]string, len(m.dynamicServers))
	copy(s.DynamicServers, m.dynamicServers)
	return s
}

// SaveSettings updates the DNS configuration and applies it immediately.
func (m *Manager) SaveSettings(newSettings DNSSettings) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	cleanedServers := make([]string, 0)
	for _, s := range newSettings.Servers {
		s = strings.TrimSpace(s)
		if s != "" {
			if ip := net.ParseIP(s); ip == nil {
				return fmt.Errorf("invalid upstream DNS IP address %q", s)
			}
			cleanedServers = append(cleanedServers, s)
		}
	}

	if len(cleanedServers) == 0 {
		return fmt.Errorf("at least one valid upstream DNS server is required")
	}

	m.settings.Servers = cleanedServers
	m.settings.AllowRemoteRequests = newSettings.AllowRemoteRequests
	if newSettings.CacheSize > 0 {
		if newSettings.CacheSize > 50000 {
			m.settings.CacheSize = 50000
		} else {
			m.settings.CacheSize = newSettings.CacheSize
		}
	}
	m.settings.Domain = strings.TrimSpace(newSettings.Domain)
	m.settings.DoHServer = strings.TrimSpace(newSettings.DoHServer)
	m.settings.VerifyDoHCert = newSettings.VerifyDoHCert

	if err := m.saveLocked(); err != nil {
		return err
	}

	return m.applyLocked()
}

// SetDynamicServers sets peer DNS servers received dynamically from DHCP or PPP.
func (m *Manager) SetDynamicServers(servers []string) {
	m.mu.Lock()
	defer m.mu.Unlock()

	changed := false
	if len(m.dynamicServers) != len(servers) {
		changed = true
	} else {
		for i := range servers {
			if m.dynamicServers[i] != servers[i] {
				changed = true
				break
			}
		}
	}

	if changed {
		m.dynamicServers = make([]string, 0, len(servers))
		for _, s := range servers {
			trimmed := strings.TrimSpace(s)
			if trimmed != "" {
				m.dynamicServers = append(m.dynamicServers, trimmed)
			}
		}
		_ = m.applyLocked()
	}
}

// ListStaticRecords returns all user-defined static DNS host mappings.
func (m *Manager) ListStaticRecords() []StaticRecord {
	m.mu.RLock()
	defer m.mu.RUnlock()

	res := make([]StaticRecord, len(m.settings.StaticRecords))
	copy(res, m.settings.StaticRecords)
	return res
}

// SaveStaticRecord adds or updates a static DNS record.
func (m *Manager) SaveStaticRecord(rec StaticRecord) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	rec.Name = strings.TrimSpace(rec.Name)
	rec.Address = strings.TrimSpace(rec.Address)
	if rec.Name == "" {
		return fmt.Errorf("DNS name (hostname) is required")
	}
	if rec.Address == "" {
		return fmt.Errorf("target address is required")
	}

	rec.Type = strings.ToUpper(strings.TrimSpace(rec.Type))
	if rec.Type == "" {
		rec.Type = "A"
	}
	if rec.Type != "A" && rec.Type != "AAAA" && rec.Type != "CNAME" && rec.Type != "TXT" {
		return fmt.Errorf("unsupported record type %q: must be A, AAAA, CNAME, or TXT", rec.Type)
	}

	if rec.Type == "A" || rec.Type == "AAAA" {
		if ip := net.ParseIP(rec.Address); ip == nil {
			return fmt.Errorf("invalid IP address %q for record type %s", rec.Address, rec.Type)
		}
	}

	if rec.ID == "" {
		rec.ID = fmt.Sprintf("dns-rec-%d", time.Now().UnixNano())
		m.settings.StaticRecords = append(m.settings.StaticRecords, rec)
	} else {
		found := false
		for i, existing := range m.settings.StaticRecords {
			if existing.ID == rec.ID {
				m.settings.StaticRecords[i] = rec
				found = true
				break
			}
		}
		if !found {
			m.settings.StaticRecords = append(m.settings.StaticRecords, rec)
		}
	}

	if err := m.saveLocked(); err != nil {
		return err
	}

	return m.applyLocked()
}

// DeleteStaticRecord removes a static DNS record by ID.
func (m *Manager) DeleteStaticRecord(id string) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	newRecords := make([]StaticRecord, 0, len(m.settings.StaticRecords))
	found := false
	for _, rec := range m.settings.StaticRecords {
		if rec.ID == id {
			found = true
			continue
		}
		newRecords = append(newRecords, rec)
	}

	if !found {
		return fmt.Errorf("static DNS record %q not found", id)
	}

	m.settings.StaticRecords = newRecords
	if err := m.saveLocked(); err != nil {
		return err
	}

	return m.applyLocked()
}

// ToggleStaticRecord enables or disables a static DNS record.
func (m *Manager) ToggleStaticRecord(id string, disabled bool) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	found := false
	for i := range m.settings.StaticRecords {
		if m.settings.StaticRecords[i].ID == id {
			m.settings.StaticRecords[i].Disabled = disabled
			found = true
			break
		}
	}
	if !found {
		return fmt.Errorf("static DNS record %q not found", id)
	}

	if err := m.saveLocked(); err != nil {
		return err
	}

	return m.applyLocked()
}

// FlushCache clears dnsmasq DNS cache by sending SIGHUP.
func (m *Manager) FlushCache() error {
	cmd := exec.Command("pkill", "-HUP", "dnsmasq")
	if out, err := cmd.CombinedOutput(); err != nil {
		// Fallback to reload
		_ = exec.Command("systemctl", "reload", "dnsmasq").Run()
		log.Printf("[DNS] pkill -HUP result: %v (%s), fallback reload triggered", err, string(out))
	}
	return nil
}

// GetStatus returns the operational status of the DNS subsystem.
func (m *Manager) GetStatus() DNSStatus {
	m.mu.RLock()
	defer m.mu.RUnlock()

	resolvBytes, _ := os.ReadFile(SystemResolvConf)

	// Check if dnsmasq is running
	isRunning := false
	if err := exec.Command("systemctl", "is-active", "--quiet", "dnsmasq").Run(); err == nil {
		isRunning = true
	}

	activeServers := make([]string, 0, len(m.settings.Servers)+len(m.dynamicServers))
	activeServers = append(activeServers, m.settings.Servers...)
	activeServers = append(activeServers, m.dynamicServers...)

	return DNSStatus{
		ActiveServers:       activeServers,
		AllowRemoteRequests: m.settings.AllowRemoteRequests,
		CacheSize:           m.settings.CacheSize,
		StaticCount:         len(m.settings.StaticRecords),
		Domain:              m.settings.Domain,
		ResolvConfContent:   string(resolvBytes),
		ServiceRunning:      isRunning,
	}
}

// Helper: read existing nameservers from /etc/resolv.conf
func readSystemNameservers() []string {
	data, err := os.ReadFile(SystemResolvConf)
	if err != nil {
		return nil
	}

	var servers []string
	lines := strings.Split(string(data), "\n")
	for _, l := range lines {
		l = strings.TrimSpace(l)
		if strings.HasPrefix(l, "nameserver") {
			parts := strings.Fields(l)
			if len(parts) >= 2 && parts[1] != "127.0.0.1" && parts[1] != "::1" {
				servers = append(servers, parts[1])
			}
		}
	}
	return servers
}