package subscription

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/google/uuid"
	"go.uber.org/zap"
	"gorm.io/gorm"
)

// Service errors
var (
	ErrPlanNotFound          = errors.New("subscription plan not found")
	ErrSubscriptionNotFound  = errors.New("subscription not found")
	ErrAlreadySubscribed     = errors.New("user already has an active subscription to this plan")
	ErrCannotDowngradeDuring = errors.New("downgrade takes effect at end of current period")
	ErrNoActiveSubscription  = errors.New("no active subscription found")
	ErrInvalidBillingCycle   = errors.New("invalid billing cycle: must be 'monthly' or 'yearly'")
	ErrFreePlanNoBilling     = errors.New("free plan does not require billing")
)

// PaymentProvider defines the interface for subscription payments
type PaymentProvider interface {
	CreateSubscriptionPayment(ctx context.Context, amountCents int, currency, subscriptionID, returnURL string, metadata map[string]string) (paymentID, clientSecret string, err error)
	GetPayment(ctx context.Context, paymentID string) (status string, err error)
}

// ServiceOption is a functional option for configuring the Service
type ServiceOption func(*Service)

// WithPaymentProvider sets the payment provider for the subscription service
func WithPaymentProvider(p PaymentProvider) ServiceOption {
	return func(s *Service) {
		s.paymentProvider = p
	}
}

// Service handles subscription business logic
type Service struct {
	db              *gorm.DB
	logger          *zap.Logger
	paymentProvider PaymentProvider
}

// NewService creates a new subscription service
func NewService(db *gorm.DB, logger *zap.Logger, opts ...ServiceOption) *Service {
	s := &Service{
		db:     db,
		logger: logger,
	}
	for _, opt := range opts {
		opt(s)
	}
	return s
}

// ListPlans returns all active subscription plans ordered by sort_order
func (s *Service) ListPlans(ctx context.Context) ([]Plan, error) {
	var plans []Plan
	if err := s.db.WithContext(ctx).
		Where("is_active = ?", true).
		Order("sort_order ASC").
		Find(&plans).Error; err != nil {
		return nil, fmt.Errorf("failed to list plans: %w", err)
	}
	return plans, nil
}

// GetPlan returns a plan by ID
func (s *Service) GetPlan(ctx context.Context, planID uuid.UUID) (*Plan, error) {
	var plan Plan
	if err := s.db.WithContext(ctx).First(&plan, "id = ?", planID).Error; err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrPlanNotFound
		}
		return nil, fmt.Errorf("failed to get plan: %w", err)
	}
	return &plan, nil
}

// GetPlanByName returns a plan by its name
func (s *Service) GetPlanByName(ctx context.Context, name PlanName) (*Plan, error) {
	var plan Plan
	if err := s.db.WithContext(ctx).First(&plan, "name = ?", string(name)).Error; err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrPlanNotFound
		}
		return nil, fmt.Errorf("failed to get plan by name: %w", err)
	}
	return &plan, nil
}

// GetUserSubscription returns the user's current active/trialing subscription
func (s *Service) GetUserSubscription(ctx context.Context, userID uuid.UUID) (*UserSubscription, error) {
	var sub UserSubscription
	err := s.db.WithContext(ctx).
		Preload("Plan").
		Where("user_id = ? AND status IN ?", userID, []string{string(StatusActive), string(StatusTrialing)}).
		First(&sub).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrNoActiveSubscription
		}
		return nil, fmt.Errorf("failed to get user subscription: %w", err)
	}
	return &sub, nil
}

// GetUserSubscriptionHistory returns all subscriptions for a user (including canceled/expired)
func (s *Service) GetUserSubscriptionHistory(ctx context.Context, userID uuid.UUID, limit, offset int) ([]UserSubscription, error) {
	if limit <= 0 || limit > 100 {
		limit = 20
	}
	if offset < 0 {
		offset = 0
	}

	var subs []UserSubscription
	err := s.db.WithContext(ctx).
		Preload("Plan").
		Where("user_id = ?", userID).
		Order("created_at DESC").
		Limit(limit).
		Offset(offset).
		Find(&subs).Error
	if err != nil {
		return nil, fmt.Errorf("failed to get subscription history: %w", err)
	}
	return subs, nil
}

// SubscribeRequest holds the parameters for subscribing to a plan
type SubscribeRequest struct {
	PlanID       uuid.UUID    `json:"plan_id" binding:"required"`
	BillingCycle BillingCycle `json:"billing_cycle" binding:"required"`
}

// SubscribeResponse holds the result of a subscription creation
type SubscribeResponse struct {
	Subscription *UserSubscription `json:"subscription"`
	ClientSecret string            `json:"client_secret,omitempty"` // For Hyperswitch payment
	PaymentID    string            `json:"payment_id,omitempty"`
}

// Subscribe creates a new subscription for a user
func (s *Service) Subscribe(ctx context.Context, userID uuid.UUID, req SubscribeRequest) (*SubscribeResponse, error) {
	if req.BillingCycle != BillingMonthly && req.BillingCycle != BillingYearly {
		return nil, ErrInvalidBillingCycle
	}

	plan, err := s.GetPlan(ctx, req.PlanID)
	if err != nil {
		return nil, err
	}

	if plan.Name == PlanFree {
		return s.subscribeToFreePlan(ctx, userID, plan)
	}

	// Check for existing active subscription
	existing, err := s.GetUserSubscription(ctx, userID)
	if err != nil && !errors.Is(err, ErrNoActiveSubscription) {
		return nil, err
	}

	if existing != nil && existing.PlanID == req.PlanID {
		return nil, ErrAlreadySubscribed
	}

	// If upgrading from a lower plan, handle the transition
	if existing != nil {
		return s.changePlan(ctx, userID, existing, plan, req.BillingCycle)
	}

	return s.createNewSubscription(ctx, userID, plan, req.BillingCycle)
}

// subscribeToFreePlan assigns the free plan without payment
func (s *Service) subscribeToFreePlan(ctx context.Context, userID uuid.UUID, plan *Plan) (*SubscribeResponse, error) {
	// Cancel any existing subscription first
	existing, err := s.GetUserSubscription(ctx, userID)
	if err != nil && !errors.Is(err, ErrNoActiveSubscription) {
		return nil, err
	}
	if existing != nil {
		if err := s.cancelImmediately(ctx, existing); err != nil {
			return nil, err
		}
	}

	now := time.Now()
	sub := &UserSubscription{
		UserID:             userID,
		PlanID:             plan.ID,
		Status:             StatusActive,
		BillingCycle:       BillingMonthly,
		CurrentPeriodStart: now,
		CurrentPeriodEnd:   now.AddDate(100, 0, 0), // effectively never expires
	}

	if err := s.db.WithContext(ctx).Create(sub).Error; err != nil {
		return nil, fmt.Errorf("failed to create free subscription: %w", err)
	}

	sub.Plan = *plan
	return &SubscribeResponse{Subscription: sub}, nil
}

// createNewSubscription creates a subscription for a paid plan
func (s *Service) createNewSubscription(ctx context.Context, userID uuid.UUID, plan *Plan, cycle BillingCycle) (*SubscribeResponse, error) {
	now := time.Now()
	var periodEnd time.Time
	var amountCents int

	switch cycle {
	case BillingYearly:
		periodEnd = now.AddDate(1, 0, 0)
		amountCents = plan.PriceYearly
	default:
		periodEnd = now.AddDate(0, 1, 0)
		amountCents = plan.PriceMonthly
	}

	sub := &UserSubscription{
		UserID:             userID,
		PlanID:             plan.ID,
		BillingCycle:       cycle,
		CurrentPeriodStart: now,
		CurrentPeriodEnd:   periodEnd,
	}

	var clientSecret, paymentID string

	// SECURITY(REM-015): Trial check + subscription creation in single transaction to prevent
	// race condition where two concurrent requests both see previousTrialCount=0.
	err := s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		// Apply trial if available — checked INSIDE transaction for atomicity
		if plan.TrialDays > 0 {
			var previousTrialCount int64
			tx.Model(&UserSubscription{}).
				Where("user_id = ? AND trial_start IS NOT NULL", userID).
				Count(&previousTrialCount)
			if previousTrialCount > 0 {
				sub.Status = StatusActive
			} else {
				trialEnd := now.AddDate(0, 0, plan.TrialDays)
				sub.Status = StatusTrialing
				sub.TrialStart = &now
				sub.TrialEnd = &trialEnd
				sub.CurrentPeriodEnd = trialEnd
			}
		} else {
			sub.Status = StatusActive
		}

		if err := tx.Create(sub).Error; err != nil {
			return fmt.Errorf("failed to create subscription: %w", err)
		}

		// Create invoice (for paid plans, not during trial)
		if !sub.IsTrialing() && amountCents > 0 {
			invoice := &Invoice{
				SubscriptionID:     sub.ID,
				UserID:             userID,
				AmountCents:        amountCents,
				Currency:           plan.Currency,
				Status:             InvoicePending,
				BillingPeriodStart: now,
				BillingPeriodEnd:   periodEnd,
			}
			if err := tx.Create(invoice).Error; err != nil {
				return fmt.Errorf("failed to create invoice: %w", err)
			}

			// Initiate payment if provider is configured
			if s.paymentProvider != nil {
				var err error
				paymentID, clientSecret, err = s.paymentProvider.CreateSubscriptionPayment(
					ctx, amountCents, plan.Currency, sub.ID.String(),
					"", // returnURL to be set by frontend
					map[string]string{
						"user_id":         userID.String(),
						"subscription_id": sub.ID.String(),
						"plan":            string(plan.Name),
						"billing_cycle":   string(cycle),
					},
				)
				if err != nil {
					return fmt.Errorf("failed to create payment: %w", err)
				}
				invoice.HyperswitchPaymentID = paymentID
				if err := tx.Save(invoice).Error; err != nil {
					return fmt.Errorf("failed to update invoice with payment ID: %w", err)
				}
			}
		}

		return nil
	})

	if err != nil {
		return nil, err
	}

	sub.Plan = *plan
	return &SubscribeResponse{
		Subscription: sub,
		ClientSecret: clientSecret,
		PaymentID:    paymentID,
	}, nil
}

// changePlan handles upgrade or downgrade between plans
func (s *Service) changePlan(ctx context.Context, userID uuid.UUID, current *UserSubscription, newPlan *Plan, cycle BillingCycle) (*SubscribeResponse, error) {
	currentPlan := &current.Plan
	if currentPlan.ID == uuid.Nil {
		var err error
		currentPlan, err = s.GetPlan(ctx, current.PlanID)
		if err != nil {
			return nil, err
		}
	}

	isUpgrade := newPlan.SortOrder > currentPlan.SortOrder

	if isUpgrade {
		// Upgrade: takes effect immediately
		return s.upgradeSubscription(ctx, userID, current, newPlan, cycle)
	}

	// Downgrade: takes effect at end of current period
	return s.scheduleDowngrade(ctx, userID, current, newPlan, cycle)
}

// upgradeSubscription applies an immediate upgrade
func (s *Service) upgradeSubscription(ctx context.Context, userID uuid.UUID, current *UserSubscription, newPlan *Plan, cycle BillingCycle) (*SubscribeResponse, error) {
	now := time.Now()

	err := s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		// Expire the current subscription
		current.Status = StatusExpired
		if err := tx.Save(current).Error; err != nil {
			return fmt.Errorf("failed to expire current subscription: %w", err)
		}
		return nil
	})
	if err != nil {
		return nil, err
	}

	s.logger.Info("User upgraded subscription",
		zap.String("user_id", userID.String()),
		zap.String("from_plan", string(current.Plan.Name)),
		zap.String("to_plan", string(newPlan.Name)),
		zap.Time("upgraded_at", now),
	)

	return s.createNewSubscription(ctx, userID, newPlan, cycle)
}

// scheduleDowngrade schedules a downgrade at end of current period
func (s *Service) scheduleDowngrade(ctx context.Context, userID uuid.UUID, current *UserSubscription, newPlan *Plan, cycle BillingCycle) (*SubscribeResponse, error) {
	s.logger.Info("User scheduled downgrade",
		zap.String("user_id", userID.String()),
		zap.String("from_plan", string(current.Plan.Name)),
		zap.String("to_plan", string(newPlan.Name)),
		zap.Time("effective_at", current.CurrentPeriodEnd),
	)

	// For now, we mark the current as cancel_at_period_end and return info
	// The actual downgrade will happen when the period ends (via ProcessExpiredSubscriptions)
	current.CancelAtPeriodEnd = true
	now := time.Now()
	current.CanceledAt = &now

	if err := s.db.WithContext(ctx).Save(current).Error; err != nil {
		return nil, fmt.Errorf("failed to schedule downgrade: %w", err)
	}

	current.Plan = *newPlan // indicate the target plan in response
	return &SubscribeResponse{
		Subscription: current,
	}, nil
}

// CancelSubscription cancels a user's subscription at the end of the current period
func (s *Service) CancelSubscription(ctx context.Context, userID uuid.UUID) (*UserSubscription, error) {
	sub, err := s.GetUserSubscription(ctx, userID)
	if err != nil {
		return nil, err
	}

	// Free plan: cancel immediately
	if sub.Plan.Name == PlanFree {
		return nil, ErrFreePlanNoBilling
	}

	now := time.Now()
	sub.CancelAtPeriodEnd = true
	sub.CanceledAt = &now

	if err := s.db.WithContext(ctx).Save(sub).Error; err != nil {
		return nil, fmt.Errorf("failed to cancel subscription: %w", err)
	}

	s.logger.Info("User canceled subscription",
		zap.String("user_id", userID.String()),
		zap.String("plan", string(sub.Plan.Name)),
		zap.Time("access_until", sub.CurrentPeriodEnd),
	)

	return sub, nil
}

// ReactivateSubscription removes the cancellation flag if still within the period
func (s *Service) ReactivateSubscription(ctx context.Context, userID uuid.UUID) (*UserSubscription, error) {
	sub, err := s.GetUserSubscription(ctx, userID)
	if err != nil {
		return nil, err
	}

	if !sub.CancelAtPeriodEnd {
		return sub, nil // not canceled, nothing to do
	}

	sub.CancelAtPeriodEnd = false
	sub.CanceledAt = nil

	if err := s.db.WithContext(ctx).Save(sub).Error; err != nil {
		return nil, fmt.Errorf("failed to reactivate subscription: %w", err)
	}

	s.logger.Info("User reactivated subscription",
		zap.String("user_id", userID.String()),
		zap.String("plan", string(sub.Plan.Name)),
	)

	return sub, nil
}

// cancelImmediately expires a subscription right away (used internally for plan switches)
func (s *Service) cancelImmediately(ctx context.Context, sub *UserSubscription) error {
	now := time.Now()
	sub.Status = StatusExpired
	sub.CanceledAt = &now
	return s.db.WithContext(ctx).Save(sub).Error
}

// GetUserInvoices returns invoices for a user
func (s *Service) GetUserInvoices(ctx context.Context, userID uuid.UUID, limit, offset int) ([]Invoice, error) {
	if limit <= 0 || limit > 100 {
		limit = 20
	}
	if offset < 0 {
		offset = 0
	}

	var invoices []Invoice
	err := s.db.WithContext(ctx).
		Where("user_id = ?", userID).
		Order("created_at DESC").
		Limit(limit).
		Offset(offset).
		Find(&invoices).Error
	if err != nil {
		return nil, fmt.Errorf("failed to get invoices: %w", err)
	}
	return invoices, nil
}

// ProcessExpiredSubscriptions checks for subscriptions past their period end and expires them
// This should be called periodically (e.g., daily cron job)
func (s *Service) ProcessExpiredSubscriptions(ctx context.Context) (int, error) {
	now := time.Now()
	var count int64

	// Expire subscriptions that have cancel_at_period_end and period has ended
	result := s.db.WithContext(ctx).
		Model(&UserSubscription{}).
		Where("cancel_at_period_end = ? AND current_period_end < ? AND status IN ?",
			true, now, []string{string(StatusActive), string(StatusTrialing)}).
		Updates(map[string]interface{}{
			"status":     StatusExpired,
			"updated_at": now,
		})

	if result.Error != nil {
		return 0, fmt.Errorf("failed to expire subscriptions: %w", result.Error)
	}
	count = result.RowsAffected

	// Expire trials that have ended without payment
	trialResult := s.db.WithContext(ctx).
		Model(&UserSubscription{}).
		Where("status = ? AND trial_end < ?", StatusTrialing, now).
		Updates(map[string]interface{}{
			"status":     StatusExpired,
			"updated_at": now,
		})

	if trialResult.Error != nil {
		return int(count), fmt.Errorf("failed to expire trials: %w", trialResult.Error)
	}
	count += trialResult.RowsAffected

	if count > 0 {
		s.logger.Info("Processed expired subscriptions", zap.Int64("expired_count", count))
	}

	return int(count), nil
}

// ChangeBillingCycle switches between monthly and yearly billing
func (s *Service) ChangeBillingCycle(ctx context.Context, userID uuid.UUID, newCycle BillingCycle) (*UserSubscription, error) {
	if newCycle != BillingMonthly && newCycle != BillingYearly {
		return nil, ErrInvalidBillingCycle
	}

	sub, err := s.GetUserSubscription(ctx, userID)
	if err != nil {
		return nil, err
	}

	if sub.BillingCycle == newCycle {
		return sub, nil // already on this cycle
	}

	sub.BillingCycle = newCycle
	// The new cycle takes effect at the next renewal
	if err := s.db.WithContext(ctx).Save(sub).Error; err != nil {
		return nil, fmt.Errorf("failed to change billing cycle: %w", err)
	}

	s.logger.Info("User changed billing cycle",
		zap.String("user_id", userID.String()),
		zap.String("new_cycle", string(newCycle)),
	)

	return sub, nil
}