package smtp

import (
	"context"
	"crypto/rand"
	"crypto/tls"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"net"
	"net/smtp"
	"strings"
	"time"
)

type SMTPMode string

const (
	SMTPModeSSL         SMTPMode = "ssl"
	SMTPModeTLS         SMTPMode = "tls"
	SMTPModeUnencrypted SMTPMode = "unencrypted"
)

type SMTPConfig struct {
	Host     string
	Port     int
	Username string
	Password string
	From     string
	Mode     SMTPMode
	Auth     string
}

type Mailer interface {
	Send(ctx context.Context, to string, subject string, htmlBody string, textBody string) error
}

type SMTPMailer struct {
	cfg SMTPConfig
}

const defaultSMTPOperationTimeout = 30 * time.Second

func NewSMTPMailer(cfg SMTPConfig) *SMTPMailer {
	return &SMTPMailer{cfg: cfg}
}

func (m *SMTPMailer) Send(ctx context.Context, to string, subject string, htmlBody string, textBody string) error {
	if strings.TrimSpace(m.cfg.Host) == "" || m.cfg.Port == 0 || strings.TrimSpace(m.cfg.From) == "" {
		return fmt.Errorf("smtp is not configured")
	}
	if ctx == nil {
		ctx = context.Background()
	}
	if err := ctx.Err(); err != nil {
		return err
	}

	now := time.Now()
	addr := fmt.Sprintf("%s:%d", m.cfg.Host, m.cfg.Port)
	deadline := smtpSendDeadline(ctx, now)
	if !deadline.After(now) {
		if err := ctx.Err(); err != nil {
			return err
		}
		return context.DeadlineExceeded
	}

	var client *smtp.Client
	var err error

	switch m.cfg.Mode {
	case SMTPModeSSL:
		client, err = dialSSL(ctx, addr, m.cfg.Host, deadline)
	case SMTPModeUnencrypted:
		client, err = dialPlain(ctx, addr, m.cfg.Host, deadline)
	case SMTPModeTLS:
		fallthrough
	default:
		client, err = dialStartTLS(ctx, addr, m.cfg.Host, deadline)
	}
	if err != nil {
		return err
	}
	defer client.Close()
	done := make(chan struct{})
	go func() {
		select {
		case <-ctx.Done():
			_ = client.Close()
		case <-done:
		}
	}()
	defer close(done)

	if m.cfg.Username != "" {
		if err := authenticate(client, m.cfg); err != nil {
			if ctx.Err() != nil {
				return ctx.Err()
			}
			return err
		}
	}

	if err := client.Mail(m.cfg.From); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("smtp mail from: %w", err)
	}
	if err := client.Rcpt(to); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("smtp rcpt to: %w", err)
	}

	writer, err := client.Data()
	if err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("smtp data: %w", err)
	}

	message := buildMIMEMessage(m.cfg.From, to, subject, textBody, htmlBody)
	if _, err := writer.Write([]byte(message)); err != nil {
		_ = writer.Close()
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("smtp write body: %w", err)
	}
	if err := writer.Close(); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("smtp close body: %w", err)
	}
	if err := client.Quit(); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("smtp quit: %w", err)
	}
	return nil
}

func smtpSendDeadline(ctx context.Context, now time.Time) time.Time {
	if ctx != nil {
		if deadline, ok := ctx.Deadline(); ok {
			return deadline
		}
	}
	return now.Add(defaultSMTPOperationTimeout)
}

func authenticate(client *smtp.Client, cfg SMTPConfig) error {
	authMethod := strings.ToLower(strings.TrimSpace(cfg.Auth))
	if authMethod == "" {
		authMethod = "auto"
	}

	_, methodsRaw := client.Extension("AUTH")
	methods := strings.ToUpper(methodsRaw)

	tryLogin := strings.Contains(methods, "LOGIN")
	tryPlain := strings.Contains(methods, "PLAIN")

	switch authMethod {
	case "none":
		return nil
	case "login":
		return authWithLogin(client, cfg)
	case "plain":
		return authWithPlain(client, cfg)
	case "auto":
		if tryLogin {
			if err := authWithLogin(client, cfg); err == nil {
				return nil
			}
		}
		if tryPlain {
			if err := authWithPlain(client, cfg); err == nil {
				return nil
			}
		}
		if !tryLogin && !tryPlain {
			// Last fallback if server did not advertise methods.
			if err := authWithLogin(client, cfg); err == nil {
				return nil
			}
			if err := authWithPlain(client, cfg); err == nil {
				return nil
			}
		}
		return fmt.Errorf("smtp auth failed for available methods")
	default:
		return fmt.Errorf("unsupported smtp auth method: %s", authMethod)
	}
}

func authWithPlain(client *smtp.Client, cfg SMTPConfig) error {
	auth := smtp.PlainAuth("", cfg.Username, cfg.Password, cfg.Host)
	if err := client.Auth(auth); err != nil {
		return fmt.Errorf("smtp plain auth: %w", err)
	}
	return nil
}

func authWithLogin(client *smtp.Client, cfg SMTPConfig) error {
	if err := client.Auth(loginAuth{username: cfg.Username, password: cfg.Password}); err != nil {
		return fmt.Errorf("smtp login auth: %w", err)
	}
	return nil
}

type loginAuth struct {
	username string
	password string
}

func (a loginAuth) Start(_ *smtp.ServerInfo) (string, []byte, error) {
	return "LOGIN", []byte{}, nil
}

func (a loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
	if !more {
		return nil, nil
	}
	challengeRaw := strings.TrimSpace(string(fromServer))
	challenge := strings.ToLower(challengeRaw)
	if decoded, err := base64.StdEncoding.DecodeString(challengeRaw); err == nil {
		challenge = strings.ToLower(string(decoded))
	}
	switch {
	case strings.Contains(challenge, "username"):
		return []byte(a.username), nil
	case strings.Contains(challenge, "password"):
		return []byte(a.password), nil
	}
	return nil, fmt.Errorf("unexpected smtp login challenge")
}

func dialSSL(ctx context.Context, addr string, host string, deadline time.Time) (*smtp.Client, error) {
	timeout := time.Until(deadline)
	if timeout <= 0 {
		if err := ctx.Err(); err != nil {
			return nil, err
		}
		return nil, context.DeadlineExceeded
	}

	dialer := &tls.Dialer{
		NetDialer: &net.Dialer{Timeout: timeout},
		Config:    &tls.Config{ServerName: host},
	}
	conn, err := dialer.DialContext(ctx, "tcp", addr)
	if err != nil {
		if ctx.Err() != nil {
			return nil, ctx.Err()
		}
		return nil, fmt.Errorf("smtp ssl dial: %w", err)
	}
	_ = conn.SetDeadline(deadline)
	client, err := smtp.NewClient(conn, host)
	if err != nil {
		return nil, fmt.Errorf("smtp new client ssl: %w", err)
	}
	return client, nil
}

func dialPlain(ctx context.Context, addr string, host string, deadline time.Time) (*smtp.Client, error) {
	timeout := time.Until(deadline)
	if timeout <= 0 {
		if err := ctx.Err(); err != nil {
			return nil, err
		}
		return nil, context.DeadlineExceeded
	}

	dialer := &net.Dialer{Timeout: timeout}
	conn, err := dialer.DialContext(ctx, "tcp", addr)
	if err != nil {
		if ctx.Err() != nil {
			return nil, ctx.Err()
		}
		return nil, fmt.Errorf("smtp plain dial: %w", err)
	}
	_ = conn.SetDeadline(deadline)
	client, err := smtp.NewClient(conn, host)
	if err != nil {
		return nil, fmt.Errorf("smtp new client plain: %w", err)
	}
	return client, nil
}

func dialStartTLS(ctx context.Context, addr string, host string, deadline time.Time) (*smtp.Client, error) {
	client, err := dialPlain(ctx, addr, host, deadline)
	if err != nil {
		return nil, err
	}
	if ok, _ := client.Extension("STARTTLS"); !ok {
		_ = client.Close()
		return nil, fmt.Errorf("smtp server does not support STARTTLS")
	}
	if err := client.StartTLS(&tls.Config{ServerName: host}); err != nil {
		_ = client.Close()
		if ctx.Err() != nil {
			return nil, ctx.Err()
		}
		return nil, fmt.Errorf("smtp starttls: %w", err)
	}
	return client, nil
}

func buildMIMEMessage(from, to, subject, textBody, htmlBody string) string {
	boundary := randomMIMEBoundary()
	return strings.Join([]string{
		fmt.Sprintf("From: %s", from),
		fmt.Sprintf("To: %s", to),
		fmt.Sprintf("Subject: %s", subject),
		"MIME-Version: 1.0",
		fmt.Sprintf("Content-Type: multipart/alternative; boundary=%s", boundary),
		"",
		fmt.Sprintf("--%s", boundary),
		"Content-Type: text/plain; charset=UTF-8",
		"",
		textBody,
		fmt.Sprintf("--%s", boundary),
		"Content-Type: text/html; charset=UTF-8",
		"",
		htmlBody,
		fmt.Sprintf("--%s--", boundary),
		"",
	}, "\r\n")
}

func randomMIMEBoundary() string {
	raw := make([]byte, 12)
	if _, err := rand.Read(raw); err != nil {
		return fmt.Sprintf("mime-boundary-%d", time.Now().UnixNano())
	}
	return "mime-boundary-" + hex.EncodeToString(raw)
}