package main

import (
	"crypto/tls"
	"fmt"
	"log"
	"net"
	"net/smtp"
	"os"
	"strings"
	"time"
)

func main() {
	smtpHost := getenv("SMTP_HOST", "smtp.163.com")
	smtpPort := getenv("SMTP_PORT", "465")
	smtpUser := strings.TrimSpace(os.Getenv("SMTP_USER"))
	smtpPass := strings.TrimSpace(os.Getenv("SMTP_PASS"))
	mailTo := strings.TrimSpace(os.Getenv("MAIL_TO"))
	subject := getenv("MAIL_SUBJECT", "SuperTodo SMTP test")
	body := getenv("MAIL_BODY", "This is a SMTP connectivity test email from SuperTodo.")

	if smtpUser == "" || smtpPass == "" || mailTo == "" {
		log.Fatal("missing required env: SMTP_USER, SMTP_PASS, MAIL_TO")
	}

	if err := sendMailTLS(smtpHost, smtpPort, smtpUser, smtpPass, mailTo, subject, body); err != nil {
		log.Fatalf("send mail failed: %v", err)
	}

	log.Printf("mail sent successfully: from=%s to=%s via %s:%s", smtpUser, mailTo, smtpHost, smtpPort)
}

func sendMailTLS(host, port, user, pass, to, subject, body string) error {
	addr := net.JoinHostPort(host, port)
	dialer := &net.Dialer{Timeout: 10 * time.Second}
	conn, err := tls.DialWithDialer(dialer, "tcp", addr, &tls.Config{
		ServerName: host,
		MinVersion: tls.VersionTLS12,
	})
	if err != nil {
		return fmt.Errorf("tls dial failed: %w", err)
	}
	defer conn.Close()

	client, err := smtp.NewClient(conn, host)
	if err != nil {
		return fmt.Errorf("smtp client failed: %w", err)
	}
	defer client.Close()

	auth := smtp.PlainAuth("", user, pass, host)
	if err := client.Auth(auth); err != nil {
		return fmt.Errorf("smtp auth failed: %w", err)
	}
	if err := client.Mail(user); err != nil {
		return fmt.Errorf("mail from failed: %w", err)
	}
	if err := client.Rcpt(to); err != nil {
		return fmt.Errorf("rcpt to failed: %w", err)
	}

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

	msg := buildMessage(user, to, subject, body)
	if _, err := w.Write([]byte(msg)); err != nil {
		return fmt.Errorf("write message failed: %w", err)
	}
	if err := w.Close(); err != nil {
		return fmt.Errorf("close writer failed: %w", err)
	}
	if err := client.Quit(); err != nil {
		return fmt.Errorf("smtp quit failed: %w", err)
	}
	return nil
}

func buildMessage(from, to, subject, body string) string {
	var b strings.Builder
	b.WriteString("From: " + from + "\r\n")
	b.WriteString("To: " + to + "\r\n")
	b.WriteString("Subject: " + subject + "\r\n")
	b.WriteString("MIME-Version: 1.0\r\n")
	b.WriteString("Content-Type: text/plain; charset=UTF-8\r\n")
	b.WriteString("\r\n")
	b.WriteString(body)
	if !strings.HasSuffix(body, "\n") {
		b.WriteString("\r\n")
	}
	return b.String()
}

func getenv(key, fallback string) string {
	v := strings.TrimSpace(os.Getenv(key))
	if v == "" {
		return fallback
	}
	return v
}