package email

import (
	"bytes"
	"context"
	"crypto/tls"
	"crypto/x509"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"mime"
	"net"
	"net/mail"
	"net/smtp"
	"strconv"
	"strings"
	"time"
)

const delimiter = "**=myohmy689407924327"

const (
	DefaultConnectTimeout     = 10 * time.Second
	DefaultOperationTimeout   = 30 * time.Second
	maximumEnvelopeRecipients = 100
)

// TLSMode makes transport security an explicit configuration decision.
type TLSMode string

const (
	TLSModeNone     TLSMode = "none"
	TLSModeSTARTTLS TLSMode = "starttls"
	TLSModeImplicit TLSMode = "implicit"
)

// Config is the canonical transport configuration. RootCAs, when non-nil,
// deliberately replaces the operating-system roots; callers that want to add
// a private CA should start from x509.SystemCertPool and append to it.
type Config struct {
	Auth             smtp.Auth
	Host             string
	Port             string
	From             string
	TLSMode          TLSMode
	ConnectTimeout   time.Duration
	OperationTimeout time.Duration
	RootCAs          *x509.CertPool
}

// InsecureConfig is retained for source compatibility. NewInsecureNoAuth is
// an explicitly plaintext, no-authentication development transport.
type InsecureConfig struct {
	Host             string
	Port             string
	From             string
	ConnectTimeout   time.Duration
	OperationTimeout time.Duration
}

// SecureConfig is retained for the legacy secure constructors.
type SecureConfig struct {
	Auth             smtp.Auth
	Host             string
	Port             string
	From             string
	ConnectTimeout   time.Duration
	OperationTimeout time.Duration
	RootCAs          *x509.CertPool
}

type MessageWithAttachments struct {
	To          string
	Subject     string
	Body        string
	Attachments []EmailAttachment
}

// RawMessage keeps the SMTP envelope separate from the caller-built RFC/MIME
// bytes. To and Body are deprecated compatibility fields; new callers should
// use EnvelopeRecipients and Data. EnvelopeFrom may be empty to use Config.From.
type RawMessage struct {
	EnvelopeFrom       string
	EnvelopeRecipients []string
	Data               []byte

	// Deprecated: use EnvelopeRecipients.
	To string
	// Deprecated: use Data.
	Body string
}

// SMTPClientIface and SmtpDialFn remain exported for source compatibility with
// earlier test doubles. The canonical transport uses context-aware net.Conn
// dialing directly.
type SMTPClientIface interface {
	StartTLS(*tls.Config) error
	Auth(smtp.Auth) error
	Close() error
	Data() (io.WriteCloser, error)
	Mail(string) error
	Quit() error
	Rcpt(string) error
}

type SmtpDialFn func(hostPort string) (SMTPClientIface, error) //nolint:revive // Retained for source compatibility.

type dialContextFunc func(context.Context, string, string) (net.Conn, error)

type EmailService struct {
	auth             smtp.Auth
	host             string
	port             string
	from             string
	tlsMode          TLSMode
	connectTimeout   time.Duration
	operationTimeout time.Duration
	rootCAs          *x509.CertPool
	dialContext      dialContextFunc
	configurationErr error
	sendRawHook      func(context.Context, RawMessage) error
}

// New constructs the canonical validated SMTP transport.
func New(config Config) (*EmailService, error) {
	config.Host = strings.TrimSpace(config.Host)
	config.Port = strings.TrimSpace(config.Port)
	config.From = strings.TrimSpace(config.From)
	if config.ConnectTimeout == 0 {
		config.ConnectTimeout = DefaultConnectTimeout
	}
	if config.OperationTimeout == 0 {
		config.OperationTimeout = DefaultOperationTimeout
	}
	if err := validateConfig(config); err != nil {
		return nil, err
	}
	rootCAs := config.RootCAs
	if rootCAs != nil {
		rootCAs = rootCAs.Clone()
	}
	dialer := &net.Dialer{Timeout: config.ConnectTimeout}
	return &EmailService{
		auth:             config.Auth,
		host:             config.Host,
		port:             config.Port,
		from:             config.From,
		tlsMode:          config.TLSMode,
		connectTimeout:   config.ConnectTimeout,
		operationTimeout: config.OperationTimeout,
		rootCAs:          rootCAs,
		dialContext:      dialer.DialContext,
	}, nil
}

// NewInsecureNoAuth constructs explicit plaintext SMTP for trusted local
// development systems such as Mailpit. It never sends authentication data.
func NewInsecureNoAuth(config InsecureConfig) *EmailService {
	return legacyService(Config{
		Host:             config.Host,
		Port:             config.Port,
		From:             config.From,
		TLSMode:          TLSModeNone,
		ConnectTimeout:   config.ConnectTimeout,
		OperationTimeout: config.OperationTimeout,
	})
}

// NewInsecure is retained for compatibility but no longer disables TLS
// verification. It now requires verified STARTTLS exactly like NewSecure.
func NewInsecure(config SecureConfig) *EmailService { return NewSecure(config) }

// NewSecure constructs verified, required STARTTLS.
func NewSecure(config SecureConfig) *EmailService {
	return legacyService(Config{
		Auth:             config.Auth,
		Host:             config.Host,
		Port:             config.Port,
		From:             config.From,
		TLSMode:          TLSModeSTARTTLS,
		ConnectTimeout:   config.ConnectTimeout,
		OperationTimeout: config.OperationTimeout,
		RootCAs:          config.RootCAs,
	})
}

// NewSecure465 constructs verified implicit TLS. The name is retained for
// compatibility; TLS mode is explicit and does not depend on the port value.
func NewSecure465(config SecureConfig) *EmailService {
	return legacyService(Config{
		Auth:             config.Auth,
		Host:             config.Host,
		Port:             config.Port,
		From:             config.From,
		TLSMode:          TLSModeImplicit,
		ConnectTimeout:   config.ConnectTimeout,
		OperationTimeout: config.OperationTimeout,
		RootCAs:          config.RootCAs,
	})
}

func legacyService(config Config) *EmailService {
	service, err := New(config)
	if err == nil {
		return service
	}
	return &EmailService{
		connectTimeout:   defaultDuration(config.ConnectTimeout, DefaultConnectTimeout),
		operationTimeout: defaultDuration(config.OperationTimeout, DefaultOperationTimeout),
		configurationErr: err,
	}
}

func defaultDuration(value, fallback time.Duration) time.Duration {
	if value <= 0 {
		return fallback
	}
	return value
}

func validateConfig(config Config) error {
	if config.Host == "" || strings.ContainsAny(config.Host, "\r\n\t /") {
		return permanentError("validate configuration", errors.New("SMTP host is invalid"))
	}
	port, err := strconv.Atoi(config.Port)
	if err != nil || port < 1 || port > 65535 {
		return permanentError("validate configuration", errors.New("SMTP port is invalid"))
	}
	if err := validateEnvelopeAddress(config.From); err != nil {
		return permanentError("validate configuration", fmt.Errorf("envelope sender: %w", err))
	}
	switch config.TLSMode {
	case TLSModeNone, TLSModeSTARTTLS, TLSModeImplicit:
	default:
		return permanentError("validate configuration", errors.New("SMTP TLS mode is invalid"))
	}
	if config.ConnectTimeout <= 0 || config.OperationTimeout <= 0 {
		return permanentError("validate configuration", errors.New("SMTP timeouts must be positive"))
	}
	if config.TLSMode == TLSModeNone && config.Auth != nil {
		return permanentError("validate configuration", errors.New("SMTP authentication requires TLS"))
	}
	return nil
}

// SendEmailContext builds the legacy HTML/attachment message and sends it
// through the canonical context-aware raw path.
func (service *EmailService) SendEmailContext(ctx context.Context, emailData MessageWithAttachments) error {
	if strings.TrimSpace(emailData.Subject) == "" || containsHeaderBreak(emailData.Subject) {
		return permanentError("prepare message", errors.New("subject is invalid"))
	}
	message, err := newMessage(service.from, emailData.To, emailData.Subject).
		withAttachments(emailData.Body, emailData.Attachments)
	if err != nil {
		return permanentError("prepare message", err)
	}
	return service.SendRawContext(ctx, RawMessage{
		EnvelopeFrom:       service.from,
		EnvelopeRecipients: []string{emailData.To},
		Data:               message,
	})
}

// SendEmail is a compatibility wrapper with an explicit bounded lifetime.
func (service *EmailService) SendEmail(emailData MessageWithAttachments) error {
	ctx, cancel := service.legacyContext()
	defer cancel()
	return service.SendEmailContext(ctx, emailData)
}

// SendRaw is a compatibility wrapper with an explicit bounded lifetime.
func (service *EmailService) SendRaw(emailData RawMessage) error {
	ctx, cancel := service.legacyContext()
	defer cancel()
	return service.SendRawContext(ctx, emailData)
}

func (service *EmailService) legacyContext() (context.Context, context.CancelFunc) {
	connectTimeout := defaultDuration(service.connectTimeout, DefaultConnectTimeout)
	operationTimeout := defaultDuration(service.operationTimeout, DefaultOperationTimeout)
	return context.WithTimeout(context.Background(), connectTimeout+10*operationTimeout)
}

type message struct {
	from    string
	to      string
	subject string
}

func newMessage(from, to, subject string) message {
	return message{from: from, to: to, subject: subject}
}

func (m message) withAttachments(body string, attachments []EmailAttachment) ([]byte, error) {
	if err := validateEnvelopeAddress(m.from); err != nil {
		return nil, fmt.Errorf("from address: %w", err)
	}
	if err := validateEnvelopeAddress(m.to); err != nil {
		return nil, fmt.Errorf("recipient address: %w", err)
	}
	if strings.TrimSpace(m.subject) == "" || containsHeaderBreak(m.subject) {
		return nil, errors.New("subject is invalid")
	}

	var output bytes.Buffer
	output.WriteString("From: " + m.from + "\r\n")
	output.WriteString("To: " + m.to + "\r\n")
	output.WriteString("Subject: " + mime.QEncoding.Encode("UTF-8", m.subject) + "\r\n")
	output.WriteString("MIME-Version: 1.0\r\n")
	_, _ = fmt.Fprintf(&output, "Content-Type: multipart/mixed; boundary=%q\r\n\r\n", delimiter)
	output.WriteString("--" + delimiter + "\r\n")
	output.WriteString("Content-Type: text/html; charset=\"UTF-8\"\r\n\r\n")
	output.WriteString(body + "\r\n\r\n")

	for _, attachment := range attachments {
		if strings.TrimSpace(attachment.Title) == "" || containsHeaderBreak(attachment.Title) {
			return nil, errors.New("attachment title is invalid")
		}
		content, err := attachment.ReadContent()
		if err != nil {
			return nil, err
		}
		output.WriteString("--" + delimiter + "\r\n")
		output.WriteString("Content-Disposition: " + mime.FormatMediaType("attachment", map[string]string{"filename": attachment.Title}) + "\r\n")
		output.WriteString("Content-Type: application/octet-stream\r\n")
		output.WriteString("Content-Transfer-Encoding: base64\r\n\r\n")
		output.WriteString(base64.StdEncoding.EncodeToString(content) + "\r\n")
	}

	output.WriteString("--" + delimiter + "--\r\n")
	return output.Bytes(), nil
}

type EmailAttachment struct {
	File  io.Reader
	Title string
}

func (attachment EmailAttachment) ReadContent() ([]byte, error) {
	content, err := io.ReadAll(attachment.File)
	if err != nil {
		return nil, fmt.Errorf("load attachment: %w", err)
	}
	return content, nil
}

func validateEnvelopeAddress(value string) error {
	if value == "" || value != strings.TrimSpace(value) || containsHeaderBreak(value) {
		return errors.New("address is invalid")
	}
	parsed, err := mail.ParseAddress(value)
	if err != nil || parsed.Address != value {
		return errors.New("address must be one mailbox without a display name")
	}
	return nil
}

func containsHeaderBreak(value string) bool {
	return strings.ContainsAny(value, "\r\n")
}