package bookings

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

	"bookra/apps/backend/internal/db"
	"bookra/apps/backend/internal/domain"

	"github.com/jackc/pgx/v5"
)

var (
	ErrTenantNotFound   = errors.New("tenant not found")
	ErrInvalidBooking   = errors.New("invalid booking request")
	ErrBookingConflict  = errors.New("booking conflict")
	ErrTenantMembership = errors.New("tenant membership not found")
)

type Service struct {
	repo db.Repository
}

func NewService(repo db.Repository) *Service {
	return &Service{repo: repo}
}

func (s *Service) Availability(ctx context.Context, tenantSlug string) (domain.PublicAvailability, error) {
	tenant, err := s.repo.GetTenantBySlug(ctx, tenantSlug)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return domain.PublicAvailability{}, ErrTenantNotFound
		}
		return domain.PublicAvailability{}, err
	}

	services, err := s.repo.ListServicesByTenant(ctx, tenant.ID)
	if err != nil {
		return domain.PublicAvailability{}, err
	}
	rules, err := s.repo.ListAvailabilityRulesByTenant(ctx, tenant.ID)
	if err != nil {
		return domain.PublicAvailability{}, err
	}
	windowStart := time.Now().UTC()
	windowEnd := windowStart.AddDate(0, 0, 7)
	existingBookings, err := s.repo.ListBookingsByTenantBetween(ctx, tenant.ID, windowStart, windowEnd)
	if err != nil {
		return domain.PublicAvailability{}, err
	}
	classSessions, err := s.repo.ListClassSessionsByTenant(ctx, tenant.ID, windowStart, 8)
	if err != nil {
		return domain.PublicAvailability{}, err
	}

	slots := make([]domain.TimeSlot, 0, 8)
	slots = append(slots, generateAppointmentSlots(tenant, services, rules, existingBookings)...)
	slots = append(slots, generateClassSlots(classSessions, existingBookings)...)
	sort.Slice(slots, func(i, j int) bool { return slots[i].StartsAt < slots[j].StartsAt })
	if len(slots) > 8 {
		slots = slots[:8]
	}

	return domain.PublicAvailability{
		TenantSlug: tenant.Slug,
		Timezone:   tenant.Timezone,
		Locale:     tenant.Locale,
		Slots:      slots,
	}, nil
}

func (s *Service) Create(ctx context.Context, request domain.CreateBookingRequest) (domain.CreateBookingResponse, error) {
	tenant, err := s.repo.GetTenantBySlug(ctx, request.TenantSlug)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return domain.CreateBookingResponse{}, ErrTenantNotFound
		}
		return domain.CreateBookingResponse{}, err
	}

	startsAt, err := time.Parse(time.RFC3339, request.StartsAt)
	if err != nil {
		return domain.CreateBookingResponse{}, fmt.Errorf("%w: startsAt must be RFC3339", ErrInvalidBooking)
	}
	endsAt, err := time.Parse(time.RFC3339, request.EndsAt)
	if err != nil {
		return domain.CreateBookingResponse{}, fmt.Errorf("%w: endsAt must be RFC3339", ErrInvalidBooking)
	}
	if !endsAt.After(startsAt) {
		return domain.CreateBookingResponse{}, fmt.Errorf("%w: endsAt must be after startsAt", ErrInvalidBooking)
	}

	existing, err := s.repo.ListBookingsByTenantBetween(ctx, tenant.ID, startsAt, endsAt)
	if err != nil {
		return domain.CreateBookingResponse{}, err
	}

	status := "confirmed"
	if request.BookingMode == "class" && request.ClassSessionID != nil {
		classBookings := countClassBookings(existing, *request.ClassSessionID)
		classSessions, err := s.repo.ListClassSessionsByTenant(ctx, tenant.ID, startsAt.Add(-1*time.Minute), 16)
		if err != nil {
			return domain.CreateBookingResponse{}, err
		}
		sessionCapacity := int32(0)
		for _, session := range classSessions {
			if session.ID == *request.ClassSessionID {
				sessionCapacity = session.Capacity
				break
			}
		}
		if sessionCapacity > 0 && classBookings >= sessionCapacity {
			status = "waitlisted"
		}
	} else {
		for _, booking := range existing {
			if booking.Status == "cancelled" {
				continue
			}
			if sameResource(booking.StaffID, request.StaffID) || sameResource(booking.LocationID, request.LocationID) {
				return domain.CreateBookingResponse{}, ErrBookingConflict
			}
		}
	}

	created, err := s.repo.CreateBooking(ctx, db.CreateBookingParams{
		TenantID:       tenant.ID,
		ServiceID:      request.ServiceID,
		ClassSessionID: request.ClassSessionID,
		StaffID:        request.StaffID,
		LocationID:     request.LocationID,
		BookingMode:    request.BookingMode,
		CustomerName:   request.CustomerName,
		CustomerEmail:  request.CustomerEmail,
		StartsAt:       startsAt.UTC(),
		EndsAt:         endsAt.UTC(),
		Status:         status,
		Reference:      db.Reference("BK", time.Now()),
		Notes:          request.Notes,
	})
	if err != nil {
		return domain.CreateBookingResponse{}, err
	}

	if status == "waitlisted" && request.ClassSessionID != nil {
		waitlistPosition := int(countClassBookings(existing, *request.ClassSessionID)) + 1
		if err := s.repo.AppendWaitlistEntry(ctx, db.WaitlistEntryParams{
			TenantID:       tenant.ID,
			ClassSessionID: *request.ClassSessionID,
			CustomerName:   request.CustomerName,
			CustomerEmail:  request.CustomerEmail,
			Position:       waitlistPosition,
		}); err != nil {
			return domain.CreateBookingResponse{}, err
		}
	}

	if scheduledFor, ok := reminderSchedule(startsAt); ok {
		if err := s.repo.CreateReminderJob(ctx, db.ReminderJobParams{
			TenantID:     tenant.ID,
			BookingID:    created.ID,
			Channel:      "email",
			ScheduledFor: scheduledFor,
		}); err != nil {
			return domain.CreateBookingResponse{}, err
		}
	}

	return domain.CreateBookingResponse{
		BookingID: created.ID,
		Reference: created.Reference,
		Status:    created.Status,
	}, nil
}

func (s *Service) DashboardSummary(ctx context.Context, principal domain.Principal) (domain.DashboardSummary, error) {
	membership, err := s.repo.GetTenantMembershipByUserID(ctx, principal.Subject)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return domain.DashboardSummary{}, ErrTenantMembership
		}
		return domain.DashboardSummary{}, err
	}

	now := time.Now().UTC()
	weekEnd := now.AddDate(0, 0, 7)
	metrics, err := s.repo.GetDashboardMetrics(ctx, membership.Tenant.ID, now, weekEnd)
	if err != nil {
		return domain.DashboardSummary{}, err
	}

	return domain.DashboardSummary{
		TenantName: membership.Tenant.Name,
		Locale:     membership.Tenant.Locale,
		Timezone:   membership.Tenant.Timezone,
		PlanCode:   membership.Tenant.PlanCode,
		KPIs: []domain.DashboardKPI{
			{Code: "bookings_this_week", Label: "Bookings this week", Value: fmt.Sprintf("%d", metrics.BookingsCount)},
			{Code: "cancellations", Label: "Cancellations", Value: fmt.Sprintf("%d", metrics.CancellationsCount)},
			{Code: "utilization", Label: "Utilization", Value: fmt.Sprintf("%d%%", metrics.UtilizationPercent)},
		},
	}, nil
}

func generateAppointmentSlots(
	tenant db.TenantRecord,
	services []db.ServiceRecord,
	rules []db.AvailabilityRuleRecord,
	existing []db.BookingRecord,
) []domain.TimeSlot {
	if len(services) == 0 || len(rules) == 0 {
		return nil
	}

	location, err := time.LoadLocation(tenant.Timezone)
	if err != nil {
		location = time.UTC
	}

	service := services[0]
	now := time.Now().In(location)
	var slots []domain.TimeSlot

	for dayOffset := 0; dayOffset < 7 && len(slots) < 6; dayOffset++ {
		day := now.AddDate(0, 0, dayOffset)
		for _, rule := range rules {
			if int(day.Weekday()) != rule.DayOfWeek {
				continue
			}

			startsLocal, err := time.ParseInLocation("15:04:05", rule.StartsLocal, location)
			if err != nil {
				continue
			}
			endsLocal, err := time.ParseInLocation("15:04:05", rule.EndsLocal, location)
			if err != nil {
				continue
			}

			windowStart := time.Date(day.Year(), day.Month(), day.Day(), startsLocal.Hour(), startsLocal.Minute(), 0, 0, location)
			windowEnd := time.Date(day.Year(), day.Month(), day.Day(), endsLocal.Hour(), endsLocal.Minute(), 0, 0, location)

			step := time.Duration(service.DurationMinutes+service.BufferAfterMinutes) * time.Minute
			duration := time.Duration(service.DurationMinutes) * time.Minute
			for slotStart := windowStart; slotStart.Add(duration).Before(windowEnd) || slotStart.Add(duration).Equal(windowEnd); slotStart = slotStart.Add(step) {
				if slotStart.Before(now.Add(2 * time.Hour)) {
					continue
				}

				slotEnd := slotStart.Add(duration)
				if collides(existing, rule.StaffID, nil, slotStart.UTC(), slotEnd.UTC()) {
					continue
				}

				serviceID := service.ID
				slots = append(slots, domain.TimeSlot{
					ServiceID: &serviceID,
					StaffID:   rule.StaffID,
					StartsAt:  slotStart.UTC().Format(time.RFC3339),
					EndsAt:    slotEnd.UTC().Format(time.RFC3339),
					Mode:      "appointment",
					Label:     service.Name,
				})
				if len(slots) >= 6 {
					break
				}
			}
		}
	}

	return slots
}

func generateClassSlots(classSessions []db.ClassSessionRecord, existing []db.BookingRecord) []domain.TimeSlot {
	slots := make([]domain.TimeSlot, 0, len(classSessions))
	for _, session := range classSessions {
		remaining := session.Capacity - countClassBookings(existing, session.ID)
		if remaining < 0 {
			remaining = 0
		}
		classSessionID := session.ID
		locationID := session.LocationID
		slots = append(slots, domain.TimeSlot{
			ClassSessionID:    &classSessionID,
			LocationID:        locationID,
			StartsAt:          session.StartsAt.UTC().Format(time.RFC3339),
			EndsAt:            session.EndsAt.UTC().Format(time.RFC3339),
			Mode:              "class",
			Label:             session.Title,
			RemainingCapacity: &remaining,
		})
	}
	return slots
}

func collides(bookings []db.BookingRecord, staffID *string, locationID *string, startsAt time.Time, endsAt time.Time) bool {
	for _, booking := range bookings {
		if booking.Status == "cancelled" || booking.Status == "waitlisted" {
			continue
		}
		if !(booking.StartsAt.Before(endsAt) && booking.EndsAt.After(startsAt)) {
			continue
		}
		if sameResource(booking.StaffID, staffID) || sameResource(booking.LocationID, locationID) {
			return true
		}
	}
	return false
}

func sameResource(left *string, right *string) bool {
	if left == nil || right == nil {
		return false
	}
	return *left == *right
}

func countClassBookings(bookings []db.BookingRecord, classSessionID string) int32 {
	var total int32
	for _, booking := range bookings {
		if booking.ClassSessionID == nil {
			continue
		}
		if *booking.ClassSessionID == classSessionID && booking.Status == "confirmed" {
			total++
		}
	}
	return total
}

func reminderSchedule(startsAt time.Time) (time.Time, bool) {
	now := time.Now().UTC()
	switch {
	case startsAt.After(now.Add(25 * time.Hour)):
		return startsAt.Add(-24 * time.Hour), true
	case startsAt.After(now.Add(3 * time.Hour)):
		return startsAt.Add(-2 * time.Hour), true
	default:
		return time.Time{}, false
	}
}