package usecase

import (
	"context"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"io"
	"strings"
	"time"

	"cmroubao/backend-api/internal/domain"
)

const (
	createTaskOperation  = "CREATE_PURCHASE_TASK"
	defaultTaskListLimit = 20
	maxTaskListLimit     = 100
	maxTaskQueryBytes    = 256
)

type TaskService struct {
	repository TaskRepository
	clock      Clock
	ids        IDGenerator
}

type CreateTaskCommand struct {
	CreatorSubject string
	ActorUserID    string
	IdempotencyKey string
	SourceRef      *string
	Title          string
	Description    string
	SKU            string
	ImageAssetID   string
	Quantity       int
	MaxBudget      *string
}

type CreateTaskResult struct {
	Task     domain.PurchaseTask
	Replayed bool
}

type ListTasksQuery struct {
	CreatorSubject string
	Status         *string
	Query          string
	CreatedFrom    *time.Time
	CreatedTo      *time.Time
	Limit          int
	Cursor         string
}

type TaskPage struct {
	Items      []domain.PurchaseTask
	NextCursor string
}

type CancelTaskCommand struct {
	CreatorSubject string
	ActorUserID    string
	TaskID         string
	Reason         string
}

func NewTaskService(
	repository TaskRepository,
	clock Clock,
	ids IDGenerator,
) (*TaskService, error) {
	if repository == nil || clock == nil || ids == nil {
		return nil, errors.New("task service dependencies are required")
	}
	return &TaskService{
		repository: repository,
		clock:      clock,
		ids:        ids,
	}, nil
}

func (s *TaskService) Create(
	ctx context.Context,
	command CreateTaskCommand,
) (CreateTaskResult, error) {
	if err := validateWriteIdentity(
		command.CreatorSubject,
		command.IdempotencyKey,
	); err != nil {
		return CreateTaskResult{}, err
	}
	command.CreatorSubject = strings.TrimSpace(command.CreatorSubject)
	command.ActorUserID = strings.TrimSpace(command.ActorUserID)
	command.Title = strings.TrimSpace(command.Title)
	command.SKU = strings.TrimSpace(command.SKU)
	command.ImageAssetID = strings.TrimSpace(command.ImageAssetID)
	if command.SourceRef != nil {
		value := strings.TrimSpace(*command.SourceRef)
		command.SourceRef = &value
	}
	if !isUUID(command.ImageAssetID) {
		return CreateTaskResult{}, invalidError(
			"TASK_VALIDATION_FAILED",
			"task validation failed",
			map[string]string{"image_asset_id": "must be a UUID"},
		)
	}
	if !isUUID(command.ActorUserID) {
		return CreateTaskResult{}, invalidError(
			"TASK_VALIDATION_FAILED",
			"task validation failed",
			map[string]string{"actor_user_id": "must be a UUID"},
		)
	}
	if err := domain.ValidateTaskInput(
		command.CreatorSubject,
		command.SourceRef,
		command.Title,
		command.Description,
		command.SKU,
		command.ImageAssetID,
		command.Quantity,
	); err != nil {
		var validationError *domain.TaskValidationError
		if errors.As(err, &validationError) {
			return CreateTaskResult{}, invalidError(
				"TASK_VALIDATION_FAILED",
				"task validation failed",
				validationError.Fields,
			)
		}
		return CreateTaskResult{}, newError(
			ErrorKindInternal,
			"INTERNAL_ERROR",
			"internal server error",
			err,
		)
	}
	budget, err := domain.ParseOptionalCNY(command.MaxBudget)
	if err != nil {
		return CreateTaskResult{}, invalidError(
			"TASK_VALIDATION_FAILED",
			"task validation failed",
			map[string]string{"max_budget": "must be a positive CNY amount with at most two decimals"},
		)
	}
	taskID, err := s.ids.NewID()
	if err != nil {
		return CreateTaskResult{}, newError(
			ErrorKindInternal,
			"INTERNAL_ERROR",
			"internal server error",
			err,
		)
	}
	eventID, err := s.ids.NewID()
	if err != nil {
		return CreateTaskResult{}, newError(
			ErrorKindInternal,
			"INTERNAL_ERROR",
			"internal server error",
			err,
		)
	}
	now := s.clock.Now().UTC()
	actorUserID := command.ActorUserID
	task := domain.PurchaseTask{
		ID:              taskID,
		CreatorSubject:  command.CreatorSubject,
		CreatedByUserID: &actorUserID,
		SourceRef:       command.SourceRef,
		Title:           command.Title,
		Description:     command.Description,
		SKU:             command.SKU,
		ImageAssetID:    command.ImageAssetID,
		Quantity:        command.Quantity,
		MaxBudgetCents:  budget,
		Currency:        domain.CurrencyCNY,
		Status:          domain.TaskStatusPending,
		Version:         1,
		CreatedAt:       now,
		UpdatedAt:       now,
	}
	event := domain.TaskEvent{
		ID:          eventID,
		TaskID:      taskID,
		ActorUserID: &actorUserID,
		Type:        "TASK_CREATED",
		Message:     "task created",
		OccurredAt:  now,
	}
	requestHash, err := hashCreateTaskCommand(command, budget)
	if err != nil {
		return CreateTaskResult{}, newError(
			ErrorKindInternal,
			"INTERNAL_ERROR",
			"internal server error",
			err,
		)
	}
	createdTask, created, err := s.repository.CreateTaskIdempotent(
		ctx,
		task,
		event,
		strings.TrimSpace(command.IdempotencyKey),
		requestHash,
	)
	if err != nil {
		return CreateTaskResult{}, wrapRepositoryError(err)
	}
	return CreateTaskResult{
		Task:     createdTask,
		Replayed: !created,
	}, nil
}

func (s *TaskService) List(
	ctx context.Context,
	query ListTasksQuery,
) (TaskPage, error) {
	query.CreatorSubject = strings.TrimSpace(query.CreatorSubject)
	if query.CreatorSubject == "" {
		return TaskPage{}, invalidError(
			"REQUEST_VALIDATION_FAILED",
			"request validation failed",
			map[string]string{"creator_subject": "required"},
		)
	}
	filter := TaskListFilter{
		CreatorSubject: query.CreatorSubject,
		Query:          strings.TrimSpace(query.Query),
		CreatedFrom:    query.CreatedFrom,
		CreatedTo:      query.CreatedTo,
		Limit:          query.Limit,
	}
	if len([]byte(filter.Query)) > maxTaskQueryBytes {
		return TaskPage{}, invalidError(
			"TASK_LIST_FILTER_INVALID",
			"task list filter is invalid",
			map[string]string{"q": "too long"},
		)
	}
	if filter.Limit == 0 {
		filter.Limit = defaultTaskListLimit
	}
	if filter.Limit < 1 || filter.Limit > maxTaskListLimit {
		return TaskPage{}, invalidError(
			"TASK_LIST_FILTER_INVALID",
			"task list filter is invalid",
			map[string]string{"limit": "must be between 1 and 100"},
		)
	}
	if query.Status != nil {
		status := domain.TaskStatus(strings.TrimSpace(*query.Status))
		if !domain.IsValidTaskStatus(status) {
			return TaskPage{}, invalidError(
				"TASK_LIST_FILTER_INVALID",
				"task list filter is invalid",
				map[string]string{"status": "unknown status"},
			)
		}
		filter.Status = &status
	}
	if query.CreatedFrom != nil && query.CreatedTo != nil &&
		query.CreatedFrom.After(*query.CreatedTo) {
		return TaskPage{}, invalidError(
			"TASK_LIST_FILTER_INVALID",
			"task list filter is invalid",
			map[string]string{"created_from": "must not be after created_to"},
		)
	}
	if query.Cursor != "" {
		cursor, err := decodeTaskCursor(query.Cursor)
		if err != nil {
			return TaskPage{}, invalidError(
				"TASK_CURSOR_INVALID",
				"task cursor is invalid",
				map[string]string{"cursor": "invalid"},
			)
		}
		filter.After = &cursor
	}
	filter.Limit++
	items, err := s.repository.ListTasks(ctx, filter)
	if err != nil {
		return TaskPage{}, wrapRepositoryError(err)
	}
	page := TaskPage{Items: items}
	if len(items) >= filter.Limit {
		page.Items = items[:filter.Limit-1]
		last := page.Items[len(page.Items)-1]
		page.NextCursor = encodeTaskCursor(TaskCursor{
			CreatedAt: last.CreatedAt,
			ID:        last.ID,
		})
	}
	return page, nil
}

func (s *TaskService) Get(
	ctx context.Context,
	creatorSubject string,
	taskID string,
) (domain.TaskDetail, error) {
	creatorSubject = strings.TrimSpace(creatorSubject)
	if creatorSubject == "" || !isUUID(taskID) {
		return domain.TaskDetail{}, newError(
			ErrorKindNotFound,
			"TASK_NOT_FOUND",
			"task not found",
			nil,
		)
	}
	detail, err := s.repository.GetTaskDetail(
		ctx,
		creatorSubject,
		taskID,
	)
	if err != nil {
		result := wrapRepositoryError(err)
		if typed, ok := result.(*Error); ok &&
			typed.Kind == ErrorKindNotFound {
			typed.Code = "TASK_NOT_FOUND"
			typed.Message = "task not found"
		}
		return domain.TaskDetail{}, result
	}
	return detail, nil
}

func (s *TaskService) Cancel(
	ctx context.Context,
	command CancelTaskCommand,
) (domain.PurchaseTask, error) {
	command.CreatorSubject = strings.TrimSpace(command.CreatorSubject)
	command.ActorUserID = strings.TrimSpace(command.ActorUserID)
	command.TaskID = strings.TrimSpace(command.TaskID)
	command.Reason = strings.TrimSpace(command.Reason)
	fields := make(map[string]string)
	if command.CreatorSubject == "" {
		fields["creator_subject"] = "required"
	}
	if !isUUID(command.TaskID) {
		fields["task_id"] = "must be a UUID"
	}
	if !isUUID(command.ActorUserID) {
		fields["actor_user_id"] = "must be a UUID"
	}
	if len([]byte(command.Reason)) > domain.MaxCancelReasonBytes {
		fields["reason"] = "too long"
	}
	if len(fields) > 0 {
		return domain.PurchaseTask{}, invalidError(
			"TASK_CANCEL_INVALID",
			"task cancellation is invalid",
			fields,
		)
	}
	eventID, err := s.ids.NewID()
	if err != nil {
		return domain.PurchaseTask{}, newError(
			ErrorKindInternal,
			"INTERNAL_ERROR",
			"internal server error",
			err,
		)
	}
	now := s.clock.Now().UTC()
	actorUserID := command.ActorUserID
	event := domain.TaskEvent{
		ID:          eventID,
		TaskID:      command.TaskID,
		ActorUserID: &actorUserID,
		Type:        "TASK_CANCELED",
		Message:     "task canceled",
		OccurredAt:  now,
	}
	task, err := s.repository.CancelTask(
		ctx,
		command.CreatorSubject,
		command.TaskID,
		command.Reason,
		now,
		event,
	)
	if err != nil {
		return domain.PurchaseTask{}, wrapRepositoryError(err)
	}
	return task, nil
}

func hashCreateTaskCommand(
	command CreateTaskCommand,
	budget *int64,
) (string, error) {
	payload := struct {
		ActorUserID    string  `json:"actor_user_id"`
		SourceRef      *string `json:"source_ref"`
		Title          string  `json:"title"`
		Description    string  `json:"description"`
		SKU            string  `json:"sku"`
		ImageAssetID   string  `json:"image_asset_id"`
		Quantity       int     `json:"quantity"`
		MaxBudgetCents *int64  `json:"max_budget_cents"`
	}{
		ActorUserID:    command.ActorUserID,
		SourceRef:      command.SourceRef,
		Title:          command.Title,
		Description:    command.Description,
		SKU:            command.SKU,
		ImageAssetID:   command.ImageAssetID,
		Quantity:       command.Quantity,
		MaxBudgetCents: budget,
	}
	encoded, err := json.Marshal(payload)
	if err != nil {
		return "", err
	}
	hash := sha256.Sum256(encoded)
	return hex.EncodeToString(hash[:]), nil
}

func encodeTaskCursor(cursor TaskCursor) string {
	payload := struct {
		CreatedAt string `json:"created_at"`
		ID        string `json:"id"`
	}{
		CreatedAt: cursor.CreatedAt.UTC().Format(time.RFC3339Nano),
		ID:        cursor.ID,
	}
	encoded, _ := json.Marshal(payload)
	return base64.RawURLEncoding.EncodeToString(encoded)
}

func decodeTaskCursor(value string) (TaskCursor, error) {
	encoded, err := base64.RawURLEncoding.DecodeString(value)
	if err != nil {
		return TaskCursor{}, err
	}
	var payload struct {
		CreatedAt string `json:"created_at"`
		ID        string `json:"id"`
	}
	decoder := json.NewDecoder(strings.NewReader(string(encoded)))
	decoder.DisallowUnknownFields()
	if err := decoder.Decode(&payload); err != nil {
		return TaskCursor{}, err
	}
	if err := decoder.Decode(&struct{}{}); !errors.Is(err, io.EOF) {
		return TaskCursor{}, errors.New("task cursor has trailing data")
	}
	createdAt, err := time.Parse(time.RFC3339Nano, payload.CreatedAt)
	if err != nil || !isUUID(payload.ID) {
		return TaskCursor{}, errors.New("invalid task cursor")
	}
	return TaskCursor{CreatedAt: createdAt.UTC(), ID: payload.ID}, nil
}

func isUUID(value string) bool {
	if len(value) != 36 {
		return false
	}
	for index, char := range value {
		if index == 8 || index == 13 || index == 18 || index == 23 {
			if char != '-' {
				return false
			}
			continue
		}
		if !((char >= '0' && char <= '9') ||
			(char >= 'a' && char <= 'f') ||
			(char >= 'A' && char <= 'F')) {
			return false
		}
	}
	return true
}