// Package mail provides email sending functionality for password reset emails.
// Uses Go's net/smtp package for sending emails via SMTP protocol.
package mail

import (
	"crypto/rand"  // cryptographically secure random number generator
	"crypto/tls"   // TLS/SSL encryption for secure email sending
	"encoding/hex" // hex encoding for token generation
	"fmt"          // string formatting for URLs and messages
	"net/smtp"     // simple mail transfer protocol package

	"github.com/imc-vibe/backend/internal/config"
)

// EmailService handles sending emails through SMTP.
// Stores SMTP configuration and provides methods for different email types.
type EmailService struct {
	host     string // SMTP server hostname (e.g., "smtp.example.com")
	port     string // SMTP server port (e.g., "587" for TLS, "465" for SSL)
	username string // SMTP authentication username
	password string // SMTP authentication password
	from     string // Sender email address (e.g., "noreply@example.com")
	baseURL  string // Base URL of the web application (for building reset links)
}

// NewEmailService creates a new EmailService instance from configuration.
// cfg: application configuration containing SMTP settings and base URL.
func NewEmailService(cfg *config.Config) *EmailService {
	return &EmailService{
		host:     cfg.SMTPHost,
		port:     cfg.SMTPPort,
		username: cfg.SMTPUser,
		password: cfg.SMTPPassword,
		from:     cfg.SMTPFrom,
		baseURL:  cfg.BaseURL,
	}
}

// SendPasswordReset sends a password reset email to the user.
// to: recipient email address
// token: unique reset token that was generated and stored in the database
// The email contains a clickable link to reset the password.
func (s *EmailService) SendPasswordReset(to, token string) error {
	// Build the reset URL with the token as a query parameter.
	// Example: https://mail.example.com/auth/reset-password?token=abc123...
	resetURL := fmt.Sprintf("%s/auth/reset-password?token=%s", s.baseURL, token)

	// Compose the email subject and body.
	subject := "Password Reset Request"
	body := fmt.Sprintf(`Hi,

You requested a password reset for your account.

Click the link below to reset your password:
%s

This link will expire in 1 hour.

If you didn't request this, please ignore this email.

Best regards,
IMC Vibe Mail Server Admin
`, resetURL)

	// Send the email using the send method.
	return s.send(to, subject, body)
}

// send is the internal method that actually sends the email via SMTP.
// Handles both port 587 (TLS) and port 465 (SSL) connections.
func (s *EmailService) send(to, subject, body string) error {
	// Check if SMTP is configured. If not, return an error.
	if s.host == "" || s.username == "" {
		return fmt.Errorf("SMTP not configured")
	}

	// Build the server address in host:port format.
	addr := fmt.Sprintf("%s:%s", s.host, s.port)

	// Build the email message with headers.
	// \r\n is the standard line ending for email headers and body.
	msg := fmt.Sprintf("From: %s\r\n"+
		"To: %s\r\n"+
		"Subject: %s\r\n"+
		"MIME-Version: 1.0\r\n"+
		"Content-Type: text/plain; charset=\"UTF-8\"\r\n"+
		"\r\n"+
		"%s", s.from, to, subject, body)

	// Port 465 uses implicit SSL/TLS, port 587 uses STARTTLS.
	// Both are secure, but they work differently.
	var auth smtp.Auth
	if s.port == "465" {
		// For port 465, we need to establish a TLS connection first
		// before sending anything, then upgrade the connection.
		err := s.sendWithTLS(addr, to, msg)
		return err
	} else {
		// For other ports (usually 587), use STARTTLS which upgrades
		// a plain connection to TLS after connecting.
		auth = smtp.PlainAuth("", s.username, s.password, s.host)
		err := smtp.SendMail(addr, auth, s.from, []string{to}, []byte(msg))
		return err
	}
}

// sendWithTLS sends an email using an explicit TLS connection on port 465.
// Port 465 uses implicit TLS - the entire connection is encrypted from the start.
func (s *EmailService) sendWithTLS(addr, to, msg string) error {
	// Create a TLS configuration for the secure connection.
	// ServerName must match the SMTP server's certificate.
	tlsConfig := &tls.Config{
		ServerName: s.host,
	}

	// Establish a TLS connection to the SMTP server.
	// This creates an encrypted tunnel from the start.
	conn, err := tls.Dial("tcp", addr, tlsConfig)
	if err != nil {
		return err
	}
	defer conn.Close() // Ensure connection is closed when we're done

	// Create an SMTP client on top of the TLS connection.
	client, err := smtp.NewClient(conn, s.host)
	if err != nil {
		return err
	}
	defer client.Close()

	// Authenticate with the SMTP server using PLAIN authentication.
	// The empty string "" is the identity (usually same as username).
	auth := smtp.PlainAuth("", s.username, s.password, s.host)
	if err := client.Auth(auth); err != nil {
		return err
	}

	// Set the sender (MAIL FROM command).
	if err := client.Mail(s.from); err != nil {
		return err
	}

	// Set the recipient (RCPT TO command).
	if err := client.Rcpt(to); err != nil {
		return err
	}

	// Start sending the message body (DATA command).
	w, err := client.Data()
	if err != nil {
		return err
	}

	// Write the email body to the data stream.
	_, err = w.Write([]byte(msg))
	if err != nil {
		return err
	}

	// Close the data writer to finish the message.
	err = w.Close()
	if err != nil {
		return err
	}

	// Send the QUIT command to gracefully close the connection.
	return client.Quit()
}

// GenerateToken creates a cryptographically random token for password reset links.
// Returns a 64-character hexadecimal string (32 bytes encoded as hex).
// Uses crypto/rand for security-sensitive token generation.
func GenerateToken() string {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		panic("crypto/rand failed: " + err.Error())
	}
	return hex.EncodeToString(b)
}

// ValidateToken checks if a token has the correct format.
// Returns true if the token is exactly 64 characters of valid hex.
// This is a quick validation before checking the database.
func ValidateToken(token string) bool {
	if len(token) != 64 {
		return false
	}
	_, err := hex.DecodeString(token)
	return err == nil
}