package service

import (
	"encoding/json"
	"errors"
	"regexp"
	"sort"
	"strings"

	"carrot_bbs/internal/dto"
	"carrot_bbs/internal/model"
	"carrot_bbs/internal/repository"

	"gorm.io/gorm"
)

var (
	ErrInvalidSchedulePayload = &ServiceError{Code: 400, Message: "invalid schedule payload"}
	ErrScheduleCourseNotFound = &ServiceError{Code: 404, Message: "schedule course not found"}
	ErrScheduleForbidden      = &ServiceError{Code: 403, Message: "forbidden schedule operation"}
	ErrScheduleColorDuplicated = &ServiceError{Code: 400, Message: "course color already used"}
)

var hexColorRegex = regexp.MustCompile(`^#[0-9A-F]{6}$`)

type CreateScheduleCourseInput struct {
	Name         string
	Teacher      string
	Location     string
	DayOfWeek    int
	StartSection int
	EndSection   int
	Weeks        []int
	Color        string
}

type ScheduleService interface {
	ListCourses(userID string, week int) ([]*dto.ScheduleCourseResponse, error)
	CreateCourse(userID string, input CreateScheduleCourseInput) (*dto.ScheduleCourseResponse, error)
	UpdateCourse(userID, courseID string, input CreateScheduleCourseInput) (*dto.ScheduleCourseResponse, error)
	DeleteCourse(userID, courseID string) error
}

type scheduleService struct {
	repo repository.ScheduleRepository
}

func NewScheduleService(repo repository.ScheduleRepository) ScheduleService {
	return &scheduleService{repo: repo}
}

func (s *scheduleService) ListCourses(userID string, week int) ([]*dto.ScheduleCourseResponse, error) {
	courses, err := s.repo.ListByUserID(userID)
	if err != nil {
		return nil, err
	}

	result := make([]*dto.ScheduleCourseResponse, 0, len(courses))
	for _, item := range courses {
		weeks := dto.ParseWeeksJSON(item.Weeks)
		if week > 0 && !containsWeek(weeks, week) {
			continue
		}
		result = append(result, dto.ConvertScheduleCourseToResponse(item, weeks))
	}
	return result, nil
}

func (s *scheduleService) CreateCourse(userID string, input CreateScheduleCourseInput) (*dto.ScheduleCourseResponse, error) {
	entity, weeks, err := buildScheduleEntity(userID, input, nil)
	if err != nil {
		return nil, err
	}
	if err := s.ensureUniqueColor(userID, entity.Color, ""); err != nil {
		return nil, err
	}
	if err := s.repo.Create(entity); err != nil {
		return nil, err
	}
	return dto.ConvertScheduleCourseToResponse(entity, weeks), nil
}

func (s *scheduleService) UpdateCourse(userID, courseID string, input CreateScheduleCourseInput) (*dto.ScheduleCourseResponse, error) {
	existing, err := s.repo.GetByID(courseID)
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrScheduleCourseNotFound
		}
		return nil, err
	}
	if existing.UserID != userID {
		return nil, ErrScheduleForbidden
	}

	entity, weeks, err := buildScheduleEntity(userID, input, existing)
	if err != nil {
		return nil, err
	}
	if err := s.ensureUniqueColor(userID, entity.Color, entity.ID); err != nil {
		return nil, err
	}
	if err := s.repo.Update(entity); err != nil {
		return nil, err
	}
	return dto.ConvertScheduleCourseToResponse(entity, weeks), nil
}

func (s *scheduleService) DeleteCourse(userID, courseID string) error {
	existing, err := s.repo.GetByID(courseID)
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return ErrScheduleCourseNotFound
		}
		return err
	}
	if existing.UserID != userID {
		return ErrScheduleForbidden
	}
	return s.repo.DeleteByID(courseID)
}

func buildScheduleEntity(userID string, input CreateScheduleCourseInput, target *model.ScheduleCourse) (*model.ScheduleCourse, []int, error) {
	name := strings.TrimSpace(input.Name)
	if name == "" || input.DayOfWeek < 0 || input.DayOfWeek > 6 || input.StartSection < 1 || input.EndSection < input.StartSection {
		return nil, nil, ErrInvalidSchedulePayload
	}

	weeks := normalizeWeeks(input.Weeks)
	if len(weeks) == 0 {
		return nil, nil, ErrInvalidSchedulePayload
	}
	weeksJSON, err := json.Marshal(weeks)
	if err != nil {
		return nil, nil, err
	}

	entity := target
	if entity == nil {
		entity = &model.ScheduleCourse{
			UserID: userID,
		}
	}

	normalizedColor := normalizeHexColor(input.Color)
	if normalizedColor == "" || !hexColorRegex.MatchString(normalizedColor) {
		return nil, nil, ErrInvalidSchedulePayload
	}

	entity.Name = name
	entity.Teacher = strings.TrimSpace(input.Teacher)
	entity.Location = strings.TrimSpace(input.Location)
	entity.DayOfWeek = input.DayOfWeek
	entity.StartSection = input.StartSection
	entity.EndSection = input.EndSection
	entity.Weeks = string(weeksJSON)
	entity.Color = normalizedColor

	return entity, weeks, nil
}

func (s *scheduleService) ensureUniqueColor(userID, color, excludeID string) error {
	exists, err := s.repo.ExistsColorByUser(userID, color, excludeID)
	if err != nil {
		return err
	}
	if exists {
		return ErrScheduleColorDuplicated
	}
	return nil
}

func normalizeWeeks(source []int) []int {
	unique := make(map[int]struct{}, len(source))
	result := make([]int, 0, len(source))
	for _, w := range source {
		if w < 1 || w > 30 {
			continue
		}
		if _, exists := unique[w]; exists {
			continue
		}
		unique[w] = struct{}{}
		result = append(result, w)
	}
	sort.Ints(result)
	return result
}

func containsWeek(weeks []int, target int) bool {
	for _, week := range weeks {
		if week == target {
			return true
		}
	}
	return false
}

func normalizeHexColor(color string) string {
	trimmed := strings.TrimSpace(color)
	if trimmed == "" {
		return ""
	}
	if strings.HasPrefix(trimmed, "#") {
		return strings.ToUpper(trimmed)
	}
	return "#" + strings.ToUpper(trimmed)
}