package api

import (
	"context"
	"fmt"
	"log"
	"strings"
	"time"

	"task-dashboard/internal/models"

	"google.golang.org/api/option"
	"google.golang.org/api/tasks/v1"
)

// GoogleTasksClient handles interactions with Google Tasks API
type GoogleTasksClient struct {
	srv        *tasks.Service
	tasklistID string
	displayTZ  *time.Location
}

// NewGoogleTasksClient creates a client for Google Tasks authenticated via
// the given option (e.g. option.WithHTTPClient for OAuth -- see
// NewGoogleTasksOAuthClient in google_tasks_oauth.go). Service-account auth
// (option.WithCredentialsFile) does NOT work for Tasks: a service account
// has no path to a regular user's personal task lists, unlike Calendar,
// which supports sharing a calendar with any email including a service
// account's. Confirmed 2026-07-14 (see project_doot_google_tasks_oauth_limitation
// memory) -- always returns exactly one list ("My Tasks") with zero items,
// the service account's own empty default list, never the real user's.
// tasklistID can be "@default" for the primary list, or a specific list ID.
// Multiple lists can be comma-separated.
func NewGoogleTasksClient(ctx context.Context, clientOpt option.ClientOption, tasklistID, timezone string) (*GoogleTasksClient, error) {
	srv, err := tasks.NewService(ctx, clientOpt)
	if err != nil {
		return nil, fmt.Errorf("unable to create Tasks client: %v", err)
	}

	if tasklistID == "" {
		tasklistID = "@default"
	}

	// Load display timezone
	displayTZ, err := time.LoadLocation(timezone)
	if err != nil {
		log.Printf("Warning: invalid timezone %q, using UTC: %v", timezone, err)
		displayTZ = time.UTC
	}

	return &GoogleTasksClient{
		srv:        srv,
		tasklistID: tasklistID,
		displayTZ:  displayTZ,
	}, nil
}

// GetTasks fetches all incomplete tasks from the configured task list(s)
func (c *GoogleTasksClient) GetTasks(ctx context.Context) ([]models.GoogleTask, error) {
	var allTasks []models.GoogleTask

	// Support comma-separated list IDs
	listIDs := strings.Split(c.tasklistID, ",")
	for _, listID := range listIDs {
		listID = strings.TrimSpace(listID)
		if listID == "" {
			continue
		}

		tasks, err := c.getTasksFromList(ctx, listID)
		if err != nil {
			log.Printf("Warning: failed to fetch tasks from list %s: %v", listID, err)
			continue
		}
		allTasks = append(allTasks, tasks...)
	}

	return allTasks, nil
}

// getTasksFromList fetches tasks from a specific list
func (c *GoogleTasksClient) getTasksFromList(ctx context.Context, listID string) ([]models.GoogleTask, error) {
	call := c.srv.Tasks.List(listID).
		ShowCompleted(false).
		ShowHidden(false).
		MaxResults(100)

	taskList, err := call.Context(ctx).Do()
	if err != nil {
		return nil, fmt.Errorf("failed to list tasks: %v", err)
	}

	var result []models.GoogleTask
	for _, item := range taskList.Items {
		task := models.GoogleTask{
			ID:        item.Id,
			Title:     item.Title,
			Notes:     item.Notes,
			Status:    item.Status,
			Completed: item.Status == "completed",
			ListID:    listID,
		}

		// Parse due date if present (RFC3339 format, but date-only)
		if item.Due != "" {
			// Google Tasks due dates are date-only, but the API always
			// serializes them as an RFC3339 timestamp fixed at midnight UTC
			// regardless of the user's timezone. Converting that instant
			// into displayTZ (dueDate.In(c.displayTZ)) reinterprets it as a
			// real moment in time and shifts the calendar date backward for
			// any timezone behind UTC -- midnight UTC becomes 2pm the
			// PREVIOUS day in UTC-10, silently moving every dated Google
			// Task one day earlier than its actual due date. The Y/M/D of
			// the UTC-anchored timestamp IS the intended calendar date, so
			// re-anchor those same components to displayTZ midnight instead
			// of converting the instant.
			dueDate, err := time.Parse(time.RFC3339, item.Due)
			if err != nil {
				// Try date-only format
				dueDate, err = time.ParseInLocation("2006-01-02", item.Due[:10], c.displayTZ)
			}
			if err == nil {
				y, m, d := dueDate.Date()
				dueInTZ := time.Date(y, m, d, 0, 0, 0, 0, c.displayTZ)
				task.DueDate = &dueInTZ
			}
		}

		// Parse updated time
		if item.Updated != "" {
			if updated, err := time.Parse(time.RFC3339, item.Updated); err == nil {
				task.UpdatedAt = updated.In(c.displayTZ)
			}
		}

		// Build URL to Google Tasks
		task.URL = "https://tasks.google.com/embed/?origin=https://mail.google.com&fullWidth=1"

		result = append(result, task)
	}

	return result, nil
}

// GetTasksByDateRange fetches tasks with due dates in the specified range.
// Tasks without due dates are included and treated as "today" tasks.
func (c *GoogleTasksClient) GetTasksByDateRange(ctx context.Context, start, end time.Time) ([]models.GoogleTask, error) {
	allTasks, err := c.GetTasks(ctx)
	if err != nil {
		return nil, err
	}

	startDay := time.Date(start.Year(), start.Month(), start.Day(), 0, 0, 0, 0, c.displayTZ)
	endDay := time.Date(end.Year(), end.Month(), end.Day(), 0, 0, 0, 0, c.displayTZ)

	// Filter by date range, include tasks without due dates
	var filtered []models.GoogleTask
	for _, task := range allTasks {
		if task.DueDate == nil {
			// Include tasks without due dates (they'll be shown in "today" section)
			filtered = append(filtered, task)
			continue
		}
		dueDay := time.Date(task.DueDate.Year(), task.DueDate.Month(), task.DueDate.Day(), 0, 0, 0, 0, c.displayTZ)
		if !dueDay.Before(startDay) && dueDay.Before(endDay) {
			filtered = append(filtered, task)
		}
	}

	return filtered, nil
}

// CompleteTask marks a task as completed
func (c *GoogleTasksClient) CompleteTask(ctx context.Context, listID, taskID string) error {
	task := &tasks.Task{
		Status: "completed",
	}
	_, err := c.srv.Tasks.Patch(listID, taskID, task).Context(ctx).Do()
	if err != nil {
		return fmt.Errorf("failed to complete task: %v", err)
	}
	return nil
}

// UpdateTask updates a task's title and notes (description). An empty
// title is omitted from the PATCH body by the underlying client library
// (omitempty), not sent as a blank -- so callers that don't support editing
// title (e.g. the widget's description-only edit popup) can pass the
// task's existing title back unchanged, or "" to leave it untouched.
func (c *GoogleTasksClient) UpdateTask(ctx context.Context, listID, taskID, title, notes string) error {
	task := &tasks.Task{
		Title: title,
		Notes: notes,
	}
	_, err := c.srv.Tasks.Patch(listID, taskID, task).Context(ctx).Do()
	if err != nil {
		return fmt.Errorf("failed to update task: %v", err)
	}
	return nil
}

// UncompleteTask marks a task as not completed
func (c *GoogleTasksClient) UncompleteTask(ctx context.Context, listID, taskID string) error {
	task := &tasks.Task{
		Status: "needsAction",
	}
	_, err := c.srv.Tasks.Patch(listID, taskID, task).Context(ctx).Do()
	if err != nil {
		return fmt.Errorf("failed to uncomplete task: %v", err)
	}
	return nil
}

// SetTaskListID updates the task list ID(s) used by the client
func (c *GoogleTasksClient) SetTaskListID(id string) {
	c.tasklistID = id
}

// GetTaskLists returns all task lists accessible to the user
func (c *GoogleTasksClient) GetTaskLists(ctx context.Context) ([]models.TaskListInfo, error) {
	list, err := c.srv.Tasklists.List().Context(ctx).Do()
	if err != nil {
		return nil, fmt.Errorf("failed to fetch task lists: %v", err)
	}

	var lists []models.TaskListInfo
	for _, item := range list.Items {
		lists = append(lists, models.TaskListInfo{
			ID:   item.Id,
			Name: item.Title,
		})
	}
	return lists, nil
}