package main

import (
	"encoding/json"
	"fmt"
	"slices"
	"sync"
	"time"

	"github.com/mattermost/mattermost/server/public/pluginapi"

	"git.nakama.town/fmartingr/mattermost-plugin-shelfmark/server/shelfmark"
)

const (
	// kvTaskPrefix is the KV store key prefix for download tasks.
	kvTaskPrefix = "dl_task_"
	// kvTaskIndexKey stores the list of active task IDs.
	kvTaskIndexKey = "dl_task_index"
	// taskMaxAge is the maximum time a task can be pending before it's considered stale.
	taskMaxAge = 30 * time.Minute
)

// DownloadTaskStatus represents the state of a download task.
type DownloadTaskStatus string

const (
	TaskStatusPending     DownloadTaskStatus = "pending"     // Waiting for releases lookup
	TaskStatusQueued      DownloadTaskStatus = "queued"      // Download queued on Shelfmark
	TaskStatusDownloading DownloadTaskStatus = "downloading" // Download in progress
	TaskStatusComplete    DownloadTaskStatus = "complete"    // Download complete, file ready
	TaskStatusUploaded    DownloadTaskStatus = "uploaded"    // File uploaded to Mattermost
	TaskStatusFailed      DownloadTaskStatus = "failed"      // Download or upload failed
)

// DownloadTask represents a pending book download task persisted in the KV store.
type DownloadTask struct {
	// ID is a unique identifier for this task (typically the Shelfmark source_id).
	ID string `json:"id"`

	// ShelfmarkTaskID is the task/source_id on the Shelfmark server.
	ShelfmarkTaskID string `json:"shelfmark_task_id"`

	// PostID is the Mattermost post ID of the book announcement post.
	// Empty until the post is created when the file is ready.
	PostID string `json:"post_id"`

	// ChannelID is the Mattermost channel where the post will be created.
	ChannelID string `json:"channel_id"`

	// BookTitle is the title of the book.
	BookTitle string `json:"book_title"`

	// BookProvider is the metadata provider name.
	BookProvider string `json:"book_provider"`

	// BookProviderID is the book's ID on the metadata provider.
	BookProviderID string `json:"book_provider_id"`

	// BookCoverURL is the Shelfmark-relative URL for the cover image.
	BookCoverURL string `json:"book_cover_url,omitempty"`

	// BookAuthors is the list of authors from the search result.
	BookAuthors []string `json:"book_authors,omitempty"`

	// Language is the ISO language code for filtering releases (e.g., "en", "es").
	Language string `json:"language,omitempty"`

	// LocalizedTitle is the book title in the requested language, set after releases
	// are fetched. Falls back to BookTitle if unavailable.
	LocalizedTitle string `json:"localized_title,omitempty"`

	// Status is the current state of the task.
	Status DownloadTaskStatus `json:"status"`

	// ErrorMessage stores any error message if the task failed.
	ErrorMessage string `json:"error_message,omitempty"`

	// CreatedAt is when the task was created.
	CreatedAt time.Time `json:"created_at"`

	// UpdatedAt is when the task was last updated.
	UpdatedAt time.Time `json:"updated_at"`

	// RequestedBy is the Mattermost user ID of the user who requested the book.
	RequestedBy string `json:"requested_by"`

	// RequesterLocale is the Mattermost locale of the requesting user, stored at task
	// creation so background job messages can be localized without extra API calls.
	RequesterLocale string `json:"requester_locale,omitempty"`

	// SelectedRelease is the release chosen by the user via the RHS panel.
	// When set, the background job uses this release instead of auto-selecting.
	SelectedRelease *shelfmark.Release `json:"selected_release,omitempty"`
}

// EffectiveTitle returns the localized title if available, otherwise the original book title.
func (t *DownloadTask) EffectiveTitle() string {
	if t.LocalizedTitle != "" {
		return t.LocalizedTitle
	}
	return t.BookTitle
}

// taskStore provides methods to persist and retrieve download tasks.
type taskStore struct {
	client  *pluginapi.Client
	mu      sync.Mutex
	logFunc func(msg string, keyvals ...string)
}

// newTaskStore creates a new task store.
func newTaskStore(client *pluginapi.Client, logFunc func(msg string, keyvals ...string)) *taskStore {
	return &taskStore{client: client, logFunc: logFunc}
}

// SaveTask persists a download task to the KV store and adds it to the index.
func (s *taskStore) SaveTask(task *DownloadTask) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	task.UpdatedAt = time.Now()

	data, err := json.Marshal(task)
	if err != nil {
		return fmt.Errorf("failed to marshal task: %w", err)
	}

	key := kvTaskPrefix + task.ID
	if _, setErr := s.client.KV.Set(key, data); setErr != nil {
		return fmt.Errorf("failed to save task: %w", setErr)
	}

	// Add to index if not already present.
	index, err := s.getIndex()
	if err != nil {
		return fmt.Errorf("failed to get task index: %w", err)
	}

	if !slices.Contains(index, task.ID) {
		index = append(index, task.ID)
		if err := s.saveIndex(index); err != nil {
			return fmt.Errorf("failed to save task index: %w", err)
		}
	}

	return nil
}

// GetTask retrieves a download task from the KV store.
func (s *taskStore) GetTask(id string) (*DownloadTask, error) {
	key := kvTaskPrefix + id

	var data []byte
	if err := s.client.KV.Get(key, &data); err != nil {
		return nil, fmt.Errorf("failed to get task: %w", err)
	}

	if data == nil {
		return nil, nil
	}

	var task DownloadTask
	if err := json.Unmarshal(data, &task); err != nil {
		return nil, fmt.Errorf("failed to unmarshal task: %w", err)
	}

	return &task, nil
}

// DeleteTask removes a download task from the KV store and the index.
func (s *taskStore) DeleteTask(id string) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	return s.deleteTaskLocked(id)
}

// deleteTaskLocked removes a task. Must be called with s.mu held.
func (s *taskStore) deleteTaskLocked(id string) error {
	key := kvTaskPrefix + id
	if err := s.client.KV.Delete(key); err != nil {
		return fmt.Errorf("failed to delete task: %w", err)
	}

	index, err := s.getIndex()
	if err != nil {
		return err
	}

	newIndex := make([]string, 0, len(index))
	for _, taskID := range index {
		if taskID != id {
			newIndex = append(newIndex, taskID)
		}
	}

	return s.saveIndex(newIndex)
}

// ListActiveTasks returns all tasks that are still being processed
// (pending, queued, downloading, or complete).
func (s *taskStore) ListActiveTasks() ([]*DownloadTask, error) {
	s.mu.Lock()
	defer s.mu.Unlock()

	index, err := s.getIndex()
	if err != nil {
		return nil, err
	}

	var tasks []*DownloadTask
	var toDelete []string

	for _, id := range index {
		task, err := s.GetTask(id)
		if err != nil {
			if s.logFunc != nil {
				s.logFunc("Failed to load task from KV store", "task_id", id, "error", err.Error())
			}
			continue
		}
		if task == nil {
			toDelete = append(toDelete, id)
			continue
		}

		switch task.Status {
		case TaskStatusUploaded, TaskStatusFailed:
			toDelete = append(toDelete, id)
		default:
			tasks = append(tasks, task)
		}
	}

	// Batch-remove terminal/orphaned tasks from index.
	if len(toDelete) > 0 {
		s.removeFromIndexLocked(toDelete)
	}

	return tasks, nil
}

// removeFromIndexLocked removes multiple IDs from the index in a single write.
// Must be called with s.mu held.
func (s *taskStore) removeFromIndexLocked(idsToRemove []string) {
	index, err := s.getIndex()
	if err != nil {
		return
	}

	removeSet := make(map[string]struct{}, len(idsToRemove))
	for _, id := range idsToRemove {
		removeSet[id] = struct{}{}
		// Also delete the individual task data.
		key := kvTaskPrefix + id
		_ = s.client.KV.Delete(key)
	}

	newIndex := make([]string, 0, len(index))
	for _, id := range index {
		if _, remove := removeSet[id]; !remove {
			newIndex = append(newIndex, id)
		}
	}

	_ = s.saveIndex(newIndex)
}

// getIndex retrieves the task index from the KV store.
func (s *taskStore) getIndex() ([]string, error) {
	var index []string
	if err := s.client.KV.Get(kvTaskIndexKey, &index); err != nil {
		return nil, fmt.Errorf("failed to get task index: %w", err)
	}

	if index == nil {
		return []string{}, nil
	}

	return index, nil
}

// saveIndex persists the task index to the KV store.
func (s *taskStore) saveIndex(index []string) error {
	if _, err := s.client.KV.Set(kvTaskIndexKey, index); err != nil {
		return fmt.Errorf("failed to save task index: %w", err)
	}
	return nil
}