// codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/internal/service/me/change_email_request.go
package me

import (
	"context"
	"crypto/rand"
	"errors"
	"fmt"
	"html"
	"net/mail"
	"strings"
	"time"

	"github.com/awnumar/memguard"
	"github.com/gocql/gocql"
	"go.uber.org/zap"

	"codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/config"
	"codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/config/constants"
	uc_user "codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/internal/usecase/user"
	"codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/pkg/auditlog"
	"codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/pkg/emailer/mailgun"
	"codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/pkg/httperror"
	"codeberg.org/mapleopentech/monorepo/cloud/maplefile-backend/pkg/validation"
)

type ChangeEmailRequestDTO struct {
	NewEmail string `json:"new_email"`
}

type ChangeEmailRequestResponseDTO struct {
	Message string `json:"message"`
	Success bool   `json:"success"`
}

type ChangeEmailRequestService interface {
	Execute(ctx context.Context, req *ChangeEmailRequestDTO) (*ChangeEmailRequestResponseDTO, error)
}

type changeEmailRequestServiceImpl struct {
	config                *config.Configuration
	logger                *zap.Logger
	auditLogger           auditlog.AuditLogger
	userGetByIDUseCase    uc_user.UserGetByIDUseCase
	userGetByEmailUseCase uc_user.UserGetByEmailUseCase
	userUpdateUseCase     uc_user.UserUpdateUseCase
	emailer               mailgun.Emailer
}

func NewChangeEmailRequestService(
	config *config.Configuration,
	logger *zap.Logger,
	auditLogger auditlog.AuditLogger,
	userGetByIDUseCase uc_user.UserGetByIDUseCase,
	userGetByEmailUseCase uc_user.UserGetByEmailUseCase,
	userUpdateUseCase uc_user.UserUpdateUseCase,
	emailer mailgun.Emailer,
) ChangeEmailRequestService {
	logger = logger.Named("ChangeEmailRequestService")

	return &changeEmailRequestServiceImpl{
		config:                config,
		logger:                logger,
		auditLogger:           auditLogger,
		userGetByIDUseCase:    userGetByIDUseCase,
		userGetByEmailUseCase: userGetByEmailUseCase,
		userUpdateUseCase:     userUpdateUseCase,
		emailer:               emailer,
	}
}

func (svc *changeEmailRequestServiceImpl) Execute(ctx context.Context, req *ChangeEmailRequestDTO) (*ChangeEmailRequestResponseDTO, error) {
	//
	// Get user from context
	//

	userID, ok := ctx.Value(constants.SessionUserID).(gocql.UUID)
	if !ok {
		svc.logger.Error("Failed getting user id from context")
		return nil, errors.New("user id not found in context")
	}

	//
	// Validation
	//

	if req == nil {
		svc.logger.Warn("Request is nil")
		return nil, httperror.NewForBadRequestWithSingleField("non_field_error", "Request is required")
	}

	// Sanitize and validate new email
	req.NewEmail = strings.ToLower(strings.TrimSpace(req.NewEmail))

	e := make(map[string]string)
	if req.NewEmail == "" {
		e["new_email"] = "New email address is required"
	} else {
		// Validate email format
		if _, err := mail.ParseAddress(req.NewEmail); err != nil {
			e["new_email"] = "Please enter a valid email address"
		}
	}

	if len(e) != 0 {
		svc.logger.Warn("Validation failed", zap.Any("errors", e))
		return nil, httperror.NewForBadRequest(&e)
	}

	//
	// Get current user
	//

	user, err := svc.userGetByIDUseCase.Execute(ctx, userID)
	if err != nil {
		svc.logger.Error("Failed getting user by ID", zap.Error(err))
		return nil, err
	}
	if user == nil {
		err := fmt.Errorf("user is nil after lookup for id: %v", userID.String())
		svc.logger.Error("User not found", zap.Error(err))
		return nil, err
	}

	//
	// Check if new email is same as current
	//

	if req.NewEmail == user.Email {
		e["new_email"] = "New email is the same as your current email"
		return nil, httperror.NewForBadRequest(&e)
	}

	//
	// Check if new email is already in use
	//

	existingUser, err := svc.userGetByEmailUseCase.Execute(ctx, req.NewEmail)
	if err != nil {
		svc.logger.Error("Failed checking if email exists",
			zap.String("email", validation.MaskEmail(req.NewEmail)),
			zap.Error(err))
		return nil, err
	}
	if existingUser != nil {
		svc.logger.Warn("Attempted to change to email already in use",
			zap.String("user_id", userID.String()),
			zap.String("existing_user_id", existingUser.ID.String()),
			zap.String("new_email", validation.MaskEmail(req.NewEmail)))
		e["new_email"] = "This email address is already in use"
		return nil, httperror.NewForBadRequest(&e)
	}

	//
	// Generate verification code
	//

	verificationCode := svc.generateVerificationCode()
	verificationExpiry := time.Now().Add(24 * time.Hour)

	svc.logger.Debug("Generated verification code",
		zap.String("user_id", user.ID.String()),
		zap.String("code", verificationCode),
		zap.Int("code_length", len(verificationCode)),
		zap.String("code_bytes", fmt.Sprintf("%v", []byte(verificationCode))))

	//
	// Store pending email change
	//

	user.SecurityData.PendingEmail = req.NewEmail
	user.SecurityData.PendingEmailCode = verificationCode
	user.SecurityData.PendingEmailExpiry = verificationExpiry
	user.ModifiedAt = time.Now()

	if err := svc.userUpdateUseCase.Execute(ctx, user); err != nil {
		svc.logger.Error("Failed to update user with pending email",
			zap.String("user_id", user.ID.String()),
			zap.Error(err))
		return nil, httperror.NewInternalServerError("Failed to initiate email change. Please try again.")
	}

	// Verify code was saved correctly by reading it back
	verifyUser, err := svc.userGetByIDUseCase.Execute(ctx, userID)
	if err == nil && verifyUser != nil {
		svc.logger.Debug("Verified stored code after save",
			zap.String("user_id", userID.String()),
			zap.String("stored_code", verifyUser.SecurityData.PendingEmailCode),
			zap.String("original_code", verificationCode),
			zap.Bool("codes_match", verifyUser.SecurityData.PendingEmailCode == verificationCode))
	}

	//
	// Send verification email to NEW address
	//

	if err := svc.sendVerificationEmail(ctx, req.NewEmail, user.FirstName, verificationCode); err != nil {
		svc.logger.Error("Failed to send verification email to new address",
			zap.String("email", validation.MaskEmail(req.NewEmail)),
			zap.Error(err))

		// Rollback: Clear pending email change
		user.SecurityData.PendingEmail = ""
		user.SecurityData.PendingEmailCode = ""
		user.SecurityData.PendingEmailExpiry = time.Time{}
		svc.userUpdateUseCase.Execute(ctx, user)

		return nil, httperror.NewInternalServerError("Failed to send verification email. Please try again.")
	}

	//
	// Send notification to OLD address
	//

	if err := svc.sendChangeNotificationEmail(ctx, user.Email, user.FirstName, req.NewEmail); err != nil {
		// Log error but don't fail the request - notification is informational
		svc.logger.Warn("Failed to send change notification to old email",
			zap.String("email", validation.MaskEmail(user.Email)),
			zap.Error(err))
	}

	//
	// Audit log
	//

	svc.auditLogger.LogAuth(ctx, "email_change_requested", auditlog.OutcomeSuccess,
		validation.MaskEmail(user.Email), "", map[string]string{
			"user_id":   user.ID.String(),
			"new_email": validation.MaskEmail(req.NewEmail),
		})

	svc.logger.Info("Email change requested successfully",
		zap.String("user_id", user.ID.String()),
		zap.String("old_email", validation.MaskEmail(user.Email)),
		zap.String("new_email", validation.MaskEmail(req.NewEmail)))

	return &ChangeEmailRequestResponseDTO{
		Message: fmt.Sprintf("Verification code sent to %s. Please check your email and verify within 24 hours.", req.NewEmail),
		Success: true,
	}, nil
}

func (svc *changeEmailRequestServiceImpl) generateVerificationCode() string {
	// Generate random 8-digit code for increased entropy
	// 8 digits = 90,000,000 combinations vs 6 digits = 900,000
	b := make([]byte, 4)
	rand.Read(b)
	defer memguard.WipeBytes(b) // SECURITY: Wipe random bytes after use
	code := int(b[0])<<24 | int(b[1])<<16 | int(b[2])<<8 | int(b[3])
	code = (code % 90000000) + 10000000
	return fmt.Sprintf("%d", code)
}

func (svc *changeEmailRequestServiceImpl) sendVerificationEmail(ctx context.Context, email, firstName, code string) error {
	subject := "Verify Your New Email Address"
	sender := svc.emailer.GetSenderEmail()

	// Escape user input to prevent HTML injection
	safeFirstName := html.EscapeString(firstName)

	htmlContent := fmt.Sprintf(`
		<html>
			<body>
				<h2>Hi %s,</h2>
				<p>You requested to change your email address on MapleFile. Please verify your new email address by entering this code:</p>
				<h1 style="color: #4CAF50; font-size: 32px; letter-spacing: 5px;">%s</h1>
				<p>This code will expire in 24 hours.</p>
				<p><strong>If you didn't request this change, please ignore this email and contact support immediately.</strong></p>
				<p>Your account email will not be changed until you verify this code.</p>
			</body>
		</html>
	`, safeFirstName, code)

	return svc.emailer.Send(ctx, sender, subject, email, htmlContent)
}

func (svc *changeEmailRequestServiceImpl) sendChangeNotificationEmail(ctx context.Context, oldEmail, firstName, newEmail string) error {
	subject := "Email Change Request - Action Required"
	sender := svc.emailer.GetSenderEmail()

	// Escape user input to prevent HTML injection
	safeFirstName := html.EscapeString(firstName)
	safeNewEmail := html.EscapeString(newEmail) // Show full new email so user knows what email is being requested

	htmlContent := fmt.Sprintf(`
		<html>
			<body>
				<h2>Hi %s,</h2>
				<p><strong>Important Security Notice:</strong> A request was made to change your MapleFile account email address.</p>
				<p><strong>New email address:</strong> %s</p>
				<p>If you made this request, you can safely ignore this email. The new email address must be verified before the change takes effect.</p>
				<p><strong>If you did NOT request this change:</strong></p>
				<ul>
					<li>Your account may be compromised</li>
					<li>Change your password immediately</li>
					<li>Contact support at support@maplefile.com</li>
				</ul>
				<p>This email change request will expire in 24 hours if not verified.</p>
			</body>
		</html>
	`, safeFirstName, safeNewEmail)

	return svc.emailer.Send(ctx, sender, subject, oldEmail, htmlContent)
}