package resolv

import (
	"bufio"
	"errors"
	"fmt"
	"net"
	"os"
	"slices"
	"strings"
	"sync"
)

const DefaultPath = "/run/dns/resolv.conf"

var (
	ErrInvalidAddress = errors.New("invalid nameserver address")
	ErrAlreadyExists  = errors.New("nameserver already exists")
	ErrNotFound       = errors.New("nameserver not found")
)

type Manager struct {
	path string
	mu   sync.Mutex
}

func New(path string) *Manager {
	if path == "" {
		path = DefaultPath
	}
	return &Manager{path: path}
}

func (m *Manager) List() ([]string, error) {
	m.mu.Lock()
	defer m.mu.Unlock()

	_, servers, err := m.read()
	if err != nil {
		return nil, err
	}
	return servers, nil
}

func (m *Manager) Add(addr string) error {
	addr = strings.TrimSpace(addr)
	if !isValidIP(addr) {
		return fmt.Errorf("%w: %q", ErrInvalidAddress, addr)
	}

	m.mu.Lock()
	defer m.mu.Unlock()

	lines, servers, err := m.read()
	if err != nil {
		return err
	}
	if slices.Contains(servers, addr) {
		return fmt.Errorf("%w: %s", ErrAlreadyExists, addr)
	}
	lines = append(lines, "nameserver "+addr)
	return m.write(lines)
}

func (m *Manager) Remove(addr string) error {
	addr = strings.TrimSpace(addr)
	if !isValidIP(addr) {
		return fmt.Errorf("%w: %q", ErrInvalidAddress, addr)
	}

	m.mu.Lock()
	defer m.mu.Unlock()

	lines, _, err := m.read()
	if err != nil {
		return err
	}

	out := make([]string, 0, len(lines))
	removed := false
	for _, line := range lines {
		if !removed && parseNameserver(line) == addr {
			removed = true
			continue
		}
		out = append(out, line)
	}
	if !removed {
		return fmt.Errorf("%w: %s", ErrNotFound, addr)
	}
	return m.write(out)
}

func (m *Manager) read() ([]string, []string, error) {
	f, err := os.Open(m.path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil, nil
		}
		return nil, nil, err
	}
	defer f.Close()

	var lines, servers []string
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		line := sc.Text()
		lines = append(lines, line)
		if ns := parseNameserver(line); ns != "" {
			servers = append(servers, ns)
		}
	}
	if err := sc.Err(); err != nil {
		return nil, nil, err
	}
	return lines, servers, nil
}

func (m *Manager) write(lines []string) error {
	dir := dirOf(m.path)
	tmp, err := os.CreateTemp(dir, ".resolv.conf.tmp.*")
	if err != nil {
		return err
	}
	tmpName := tmp.Name()
	cleanup := func() { _ = os.Remove(tmpName) }

	w := bufio.NewWriter(tmp)
	for _, line := range lines {
		if _, err := w.WriteString(line); err != nil {
			tmp.Close()
			cleanup()
			return err
		}
		if _, err := w.WriteString("\n"); err != nil {
			tmp.Close()
			cleanup()
			return err
		}
	}
	if err := w.Flush(); err != nil {
		tmp.Close()
		cleanup()
		return err
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		cleanup()
		return err
	}
	if err := tmp.Close(); err != nil {
		cleanup()
		return err
	}
	if err := os.Rename(tmpName, m.path); err != nil {
		cleanup()
		return err
	}
	return nil
}

func parseNameserver(line string) string {
	s := strings.TrimSpace(line)
	if s == "" || strings.HasPrefix(s, "#") || strings.HasPrefix(s, ";") {
		return ""
	}
	fields := strings.Fields(s)
	if len(fields) < 2 || fields[0] != "nameserver" {
		return ""
	}
	return fields[1]
}

func isValidIP(s string) bool {
	return s != "" && net.ParseIP(s) != nil
}

func dirOf(path string) string {
	if i := strings.LastIndex(path, "/"); i >= 0 {
		return path[:i]
	}
	return "."
}