using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using CommunityToolkit.Diagnostics;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
using CommunityToolkit.Mvvm.Messaging;
using MoreLinq;
using Serilog;
using Wino.Calendar.ViewModels.Data;
using Wino.Calendar.ViewModels.Interfaces;
using Wino.Calendar.ViewModels.Messages;
using Wino.Core.Domain.Collections;
using Wino.Core.Domain.Entities.Calendar;
using Wino.Core.Domain.Enums;
using Wino.Core.Domain.Interfaces;
using Wino.Core.Domain.Models.Calendar;
using Wino.Core.Domain.Models.Calendar.CalendarTypeStrategies;
using Wino.Core.Domain.Models.Navigation;
using Wino.Core.ViewModels;
using Wino.Messaging.Client.Calendar;

namespace Wino.Calendar.ViewModels
{
    public partial class CalendarPageViewModel : CalendarBaseViewModel,
        IRecipient<LoadCalendarMessage>,
        IRecipient<CalendarSettingsUpdatedMessage>,
        IRecipient<CalendarItemTappedMessage>,
        IRecipient<CalendarItemDoubleTappedMessage>,
        IRecipient<CalendarItemRightTappedMessage>
    {
        #region Quick Event Creation

        [ObservableProperty]
        private bool _isQuickEventDialogOpen;

        [ObservableProperty]
        [NotifyPropertyChangedFor(nameof(SelectedQuickEventAccountCalendarName))]
        [NotifyCanExecuteChangedFor(nameof(SaveQuickEventCommand))]
        private AccountCalendarViewModel _selectedQuickEventAccountCalendar;

        public string SelectedQuickEventAccountCalendarName
        {
            get
            {
                return SelectedQuickEventAccountCalendar == null ? "Pick a calendar" : SelectedQuickEventAccountCalendar.Name;
            }
        }

        [ObservableProperty]
        private List<string> _hourSelectionStrings;

        // To be able to revert the values when the user enters an invalid time.
        private string _previousSelectedStartTimeString;
        private string _previousSelectedEndTimeString;

        [ObservableProperty]
        private DateTime? _selectedQuickEventDate;

        [ObservableProperty]
        private bool _isAllDay;

        [ObservableProperty]
        [NotifyCanExecuteChangedFor(nameof(SaveQuickEventCommand))]
        private string _selectedStartTimeString;

        [ObservableProperty]
        [NotifyCanExecuteChangedFor(nameof(SaveQuickEventCommand))]
        private string _selectedEndTimeString;

        [ObservableProperty]
        private string _location;

        [ObservableProperty]
        [NotifyCanExecuteChangedFor(nameof(SaveQuickEventCommand))]
        private string _eventName;

        public DateTime QuickEventStartTime => SelectedQuickEventDate.Value.Date.Add(_currentSettings.GetTimeSpan(SelectedStartTimeString).Value);
        public DateTime QuickEventEndTime => SelectedQuickEventDate.Value.Date.Add(_currentSettings.GetTimeSpan(SelectedEndTimeString).Value);

        public bool CanSaveQuickEvent => SelectedQuickEventAccountCalendar != null &&
                                        !string.IsNullOrWhiteSpace(EventName) &&
                                        !string.IsNullOrWhiteSpace(SelectedStartTimeString) &&
                                        !string.IsNullOrWhiteSpace(SelectedEndTimeString) &&
                                        QuickEventEndTime > QuickEventStartTime;

        #endregion

        [ObservableProperty]
        private ObservableRangeCollection<DayRangeRenderModel> _dayRanges = [];

        [ObservableProperty]
        private int _selectedDateRangeIndex;

        [ObservableProperty]
        private DayRangeRenderModel _selectedDayRange;

        [ObservableProperty]
        private bool _isCalendarEnabled = true;

        // TODO: Get rid of some of the items if we have too many.
        private const int maxDayRangeSize = 10;

        private readonly ICalendarService _calendarService;
        private readonly IKeyPressService _keyPressService;
        private readonly IPreferencesService _preferencesService;

        // Store latest rendered options.
        private CalendarDisplayType _currentDisplayType;
        private int _displayDayCount;

        private SemaphoreSlim _calendarLoadingSemaphore = new(1);
        private bool isLoadMoreBlocked = false;
        private CalendarSettings _currentSettings = null;

        public IStatePersistanceService StatePersistanceService { get; }
        public IAccountCalendarStateService AccountCalendarStateService { get; }

        public CalendarPageViewModel(IStatePersistanceService statePersistanceService,
                                     ICalendarService calendarService,
                                     IKeyPressService keyPressService,
                                     IAccountCalendarStateService accountCalendarStateService,
                                     IPreferencesService preferencesService)
        {
            StatePersistanceService = statePersistanceService;
            AccountCalendarStateService = accountCalendarStateService;

            _calendarService = calendarService;
            _keyPressService = keyPressService;
            _preferencesService = preferencesService;

            AccountCalendarStateService.AccountCalendarSelectionStateChanged += UpdateAccountCalendarRequested;
            AccountCalendarStateService.CollectiveAccountGroupSelectionStateChanged += AccountCalendarStateCollectivelyChanged;
        }

        private void AccountCalendarStateCollectivelyChanged(object sender, GroupedAccountCalendarViewModel e)
            => FilterActiveCalendars(DayRanges);

        private void UpdateAccountCalendarRequested(object sender, AccountCalendarViewModel e)
            => FilterActiveCalendars(DayRanges);

        private void FilterActiveCalendars(IEnumerable<DayRangeRenderModel> dayRangeRenderModels)
        {
            var days = dayRangeRenderModels.SelectMany(a => a.CalendarDays);

            days.ForEach(a => a.EventsCollection.FilterByCalendars(AccountCalendarStateService.ActiveCalendars.Select(a => a.Id)));
        }



        // TODO: Replace when calendar settings are updated.
        // Should be a field ideally.
        private BaseCalendarTypeDrawingStrategy GetDrawingStrategy(CalendarDisplayType displayType)
        {
            return displayType switch
            {
                CalendarDisplayType.Day => new DayCalendarDrawingStrategy(_currentSettings),
                CalendarDisplayType.Week => new WeekCalendarDrawingStrategy(_currentSettings),
                _ => throw new NotImplementedException(),
            };
        }

        public override void OnNavigatedFrom(NavigationMode mode, object parameters)
        {
            // Do not call base method because that will unregister messenger recipient.
            // This is a singleton view model and should not be unregistered.
        }

        public override void OnNavigatedTo(NavigationMode mode, object parameters)
        {
            base.OnNavigatedTo(mode, parameters);

            RefreshSettings();

            // Automatically select the first primary calendar for quick event dialog.
            SelectedQuickEventAccountCalendar = AccountCalendarStateService.ActiveCalendars.FirstOrDefault(a => a.IsPrimary);
        }

        [RelayCommand(AllowConcurrentExecutions = false, CanExecute = nameof(CanSaveQuickEvent))]
        private async Task SaveQuickEventAsync()
        {
            var durationSeconds = (QuickEventEndTime - QuickEventStartTime).TotalSeconds;

            var testCalendarItem = new CalendarItem
            {
                CalendarId = SelectedQuickEventAccountCalendar.Id,
                StartDate = QuickEventStartTime,
                DurationInSeconds = durationSeconds,
                CreatedAt = DateTime.UtcNow,
                Description = string.Empty,
                Location = Location,
                Title = EventName,
                Id = Guid.NewGuid()
            };

            IsQuickEventDialogOpen = false;
            await _calendarService.CreateNewCalendarItemAsync(testCalendarItem, null);

            // TODO: Create the request with the synchronizer.
        }

        [RelayCommand]
        private void MoreDetails()
        {
            // TODO: Navigate to advanced event creation  page with existing parameters.
        }

        public void SelectQuickEventTimeRange(TimeSpan startTime, TimeSpan endTime)
        {
            IsAllDay = false;

            SelectedStartTimeString = _currentSettings.GetTimeString(startTime);
            SelectedEndTimeString = _currentSettings.GetTimeString(endTime);
        }

        private void RefreshSettings()
        {
            _currentSettings = _preferencesService.GetCurrentCalendarSettings();

            // Populate the hour selection strings.
            var timeStrings = new List<string>();

            for (int hour = 0; hour < 24; hour++)
            {
                for (int minute = 0; minute < 60; minute += 30)
                {
                    var time = new DateTime(1, 1, 1, hour, minute, 0);

                    if (_currentSettings.DayHeaderDisplayType == DayHeaderDisplayType.TwentyFourHour)
                    {
                        timeStrings.Add(time.ToString("HH:mm"));
                    }
                    else
                    {
                        timeStrings.Add(time.ToString("h:mm tt"));
                    }
                }
            }

            HourSelectionStrings = timeStrings;
        }

        partial void OnIsCalendarEnabledChanging(bool oldValue, bool newValue) => Messenger.Send(new CalendarEnableStatusChangedMessage(newValue));

        private bool ShouldResetDayRanges(LoadCalendarMessage message)
        {
            if (message.ForceRedraw) return true;

            // Never reset if the initiative is from the app.
            if (message.CalendarInitInitiative == CalendarInitInitiative.App) return false;

            // 1. Display type is different.
            // 2. Day display count is different.
            // 3. Display date is not in the visible range.

            var loadedRange = GetLoadedDateRange();

            if (loadedRange == null) return false;

            return
                (_currentDisplayType != StatePersistanceService.CalendarDisplayType ||
                _displayDayCount != StatePersistanceService.DayDisplayCount ||
                !(message.DisplayDate >= loadedRange.StartDate && message.DisplayDate <= loadedRange.EndDate));
        }

        public async void Receive(LoadCalendarMessage message)
        {
            await _calendarLoadingSemaphore.WaitAsync();

            try
            {
                await ExecuteUIThread(() => IsCalendarEnabled = false);

                if (ShouldResetDayRanges(message))
                {
                    Debug.WriteLine("Will reset day ranges.");
                    await ClearDayRangeModelsAsync();
                }
                else if (ShouldScrollToItem(message))
                {
                    // Scroll to the selected date.
                    Messenger.Send(new ScrollToDateMessage(message.DisplayDate));
                    Debug.WriteLine("Scrolling to selected date.");
                    return;
                }

                // This will replace the whole collection because the user initiated a new render.
                await RenderDatesAsync(message.CalendarInitInitiative,
                                       message.DisplayDate,
                                       CalendarLoadDirection.Replace);
            }
            catch (Exception ex)
            {
                Log.Error(ex, "Error while loading calendar.");
                Debugger.Break();
            }
            finally
            {
                _calendarLoadingSemaphore.Release();

                await ExecuteUIThread(() => IsCalendarEnabled = true);
            }
        }

        private DateRange GetLoadedDateRange()
        {
            if (DayRanges.Count == 0) return null;

            var minimumLoadedDate = DayRanges[0].CalendarRenderOptions.DateRange.StartDate;
            var maximumLoadedDate = DayRanges[DayRanges.Count - 1].CalendarRenderOptions.DateRange.EndDate;

            return new DateRange(minimumLoadedDate, maximumLoadedDate);
        }

        private async Task AddDayRangeModelAsync(DayRangeRenderModel dayRangeRenderModel)
        {
            dayRangeRenderModel.CalendarDayEventCollectionUpdated -= EventsUpdatedInDayHeader;
            dayRangeRenderModel.CalendarDayEventCollectionUpdated += EventsUpdatedInDayHeader;

            await ExecuteUIThread(() =>
            {
                DayRanges.Add(dayRangeRenderModel);
            });
        }

        private async Task InsertDayRangeModelAsync(DayRangeRenderModel dayRangeRenderModel, int index)
        {
            if (dayRangeRenderModel == null) return;

            dayRangeRenderModel.CalendarDayEventCollectionUpdated -= EventsUpdatedInDayHeader;
            dayRangeRenderModel.CalendarDayEventCollectionUpdated += EventsUpdatedInDayHeader;

            await ExecuteUIThread(() =>
            {
                DayRanges.Insert(index, dayRangeRenderModel);
            });
        }

        private async Task RemoveDayRangeModelAsync(DayRangeRenderModel dayRangeRenderModel)
        {
            if (dayRangeRenderModel == null) return;

            dayRangeRenderModel.CalendarDayEventCollectionUpdated -= EventsUpdatedInDayHeader;
            dayRangeRenderModel.UnregisterAll();

            await ExecuteUIThread(() =>
            {
                DayRanges.Remove(dayRangeRenderModel);
            });
        }

        private async Task ClearDayRangeModelsAsync()
        {
            // Unregister all events and clear the list directly.

            foreach (var dayRangeModel in DayRanges)
            {
                dayRangeModel.CalendarDayEventCollectionUpdated -= EventsUpdatedInDayHeader;
                dayRangeModel.UnregisterAll();
            }

            await ExecuteUIThread(() =>
            {
                DayRanges.Clear();
            });
        }

        private async Task RenderDatesAsync(CalendarInitInitiative calendarInitInitiative,
                                            DateTime? loadingDisplayDate = null,
                                            CalendarLoadDirection calendarLoadDirection = CalendarLoadDirection.Replace)
        {
            isLoadMoreBlocked = calendarLoadDirection == CalendarLoadDirection.Replace;

            // This is the part we arrange the flip view calendar logic.

            /* Loading for a month of the selected date is fine.
             * If the selected date is in the loaded range, we'll just change the selected flip index to scroll.
             * If the selected date is not in the loaded range:
             * 1. Detect the direction of the scroll.
             * 2. Load the next month.
             * 3. Replace existing month with the new month.
             */

            // 2 things are important: How many items should 1 flip have, and, where we should start loading?

            // User initiated renders must always have a date to start with.
            if (calendarInitInitiative == CalendarInitInitiative.User) Guard.IsNotNull(loadingDisplayDate, nameof(loadingDisplayDate));

            var strategy = GetDrawingStrategy(StatePersistanceService.CalendarDisplayType);
            var displayDate = loadingDisplayDate.GetValueOrDefault();

            // How many days should be placed in 1 flip view item?
            int eachFlipItemCount = strategy.GetRenderDayCount(displayDate, StatePersistanceService.DayDisplayCount);

            DateRange flipLoadRange = null;

            var initializedDateRange = GetLoadedDateRange();

            if (calendarInitInitiative == CalendarInitInitiative.User || initializedDateRange == null)
            {
                flipLoadRange = strategy.GetRenderDateRange(displayDate, StatePersistanceService.DayDisplayCount);
            }
            else
            {
                // App is trying to load.
                // This should be based on direction. We'll load the next or previous range.
                // DisplayDate is either the start or end date of the current visible range.

                if (calendarLoadDirection == CalendarLoadDirection.Previous)
                {
                    flipLoadRange = strategy.GetPreviousDateRange(initializedDateRange, StatePersistanceService.DayDisplayCount);
                }
                else
                {
                    flipLoadRange = strategy.GetNextDateRange(initializedDateRange, StatePersistanceService.DayDisplayCount);
                }
            }

            // Create day ranges for each flip item until we reach the total days to load.
            int totalFlipItemCount = (int)Math.Ceiling((double)flipLoadRange.TotalDays / eachFlipItemCount);

            List<DayRangeRenderModel> renderModels = new();

            for (int i = 0; i < totalFlipItemCount; i++)
            {
                var startDate = flipLoadRange.StartDate.AddDays(i * eachFlipItemCount);
                var endDate = startDate.AddDays(eachFlipItemCount);

                var range = new DateRange(startDate, endDate);
                var renderOptions = new CalendarRenderOptions(range, _currentSettings);

                var dayRangeHeaderModel = new DayRangeRenderModel(renderOptions);
                renderModels.Add(dayRangeHeaderModel);
            }

            // Dates are loaded. Now load the events for them.
            foreach (var renderModel in renderModels)
            {
                await InitializeCalendarEventsForDayRangeAsync(renderModel).ConfigureAwait(false);
            }

            // Filter by active calendars. This is a quick operation, and things are not on the UI yet.
            FilterActiveCalendars(renderModels);

            CalendarLoadDirection animationDirection = calendarLoadDirection;

            bool removeCurrent = calendarLoadDirection == CalendarLoadDirection.Replace;

            if (calendarLoadDirection == CalendarLoadDirection.Replace)
            {
                // New date ranges are being replaced.
                // We must preserve existing selection if any, add the items before/after the current one, remove the current one.
                // This will make sure the new dates are animated in the correct direction.

                isLoadMoreBlocked = true;

                // Remove all other dates except this one.
                var rangesToRemove = DayRanges.Where(a => a != SelectedDayRange).ToList();

                foreach (var range in rangesToRemove)
                {
                    await RemoveDayRangeModelAsync(range);
                }

                animationDirection = displayDate <= SelectedDayRange?.CalendarRenderOptions.DateRange.StartDate ?
                    CalendarLoadDirection.Previous : CalendarLoadDirection.Next;
            }

            if (animationDirection == CalendarLoadDirection.Next)
            {
                foreach (var item in renderModels)
                {
                    await AddDayRangeModelAsync(item);
                }
            }
            else if (animationDirection == CalendarLoadDirection.Previous)
            {
                // Wait for the animation to finish.
                // Otherwise it somehow shutters a little, which is annoying.

                if (!removeCurrent) await Task.Delay(350);

                // Insert each render model in reverse order.
                for (int i = renderModels.Count - 1; i >= 0; i--)
                {
                    await InsertDayRangeModelAsync(renderModels[i], 0);
                }
            }

            Debug.WriteLine($"Flip count: ({DayRanges.Count})");

            foreach (var item in DayRanges)
            {
                Debug.WriteLine($"- {item.CalendarRenderOptions.DateRange.ToString()}");
            }

            if (removeCurrent)
            {
                await RemoveDayRangeModelAsync(SelectedDayRange);
            }

            // TODO...
            // await TryConsolidateItemsAsync();

            isLoadMoreBlocked = false;

            // Only scroll if the render is initiated by user.
            // Otherwise we'll scroll to the app rendered invisible date range.
            if (calendarInitInitiative == CalendarInitInitiative.User)
            {
                // Save the current settings for the page for later comparison.
                _currentDisplayType = StatePersistanceService.CalendarDisplayType;
                _displayDayCount = StatePersistanceService.DayDisplayCount;

                Messenger.Send(new ScrollToDateMessage(displayDate));
            }
        }

        // TODO...
        private void EventsUpdatedInDayHeader(object sender, CalendarDayModel e)
        {

        }

        protected override async void OnCalendarItemAdded(CalendarItem calendarItem)
        {
            base.OnCalendarItemAdded(calendarItem);

            // Check if event falls into the current date range.

            var loadedDateRange = GetLoadedDateRange();

            if (loadedDateRange == null) return;

            // Check whether this event falls into any of the loaded date ranges.
            var allDaysForEvent = DayRanges.SelectMany(a => a.CalendarDays).Where(a => a.Period.OverlapsWith(calendarItem.Period));

            foreach (var calendarDay in allDaysForEvent)
            {
                var calendarItemViewModel = new CalendarItemViewModel(calendarItem);

                await ExecuteUIThread(() =>
                {
                    calendarDay.EventsCollection.AddCalendarItem(calendarItemViewModel);
                });
            }

            FilterActiveCalendars(DayRanges);
        }

        private async Task InitializeCalendarEventsForDayRangeAsync(DayRangeRenderModel dayRangeRenderModel)
        {
            // Clear all events first for all days.
            foreach (var day in dayRangeRenderModel.CalendarDays)
            {
                await ExecuteUIThread(() =>
                {
                    day.EventsCollection.Clear();
                });
            }

            // Initialization is done for all calendars, regardless whether they are actively selected or not.
            // This is because the filtering is cached internally of the calendar items in CalendarEventCollection.
            var allCalendars = AccountCalendarStateService.GroupedAccountCalendars.SelectMany(a => a.AccountCalendars);

            foreach (var calendarViewModel in allCalendars)
            {
                // Check all the events for the given date range and calendar.
                // Then find the day representation for all the events returned, and add to the collection.

                var events = await _calendarService.GetCalendarEventsAsync(calendarViewModel, dayRangeRenderModel).ConfigureAwait(false);

                foreach (var @event in events)
                {
                    // Find the days that the event falls into.
                    // TODO: Multi-day events are not fully supported yet.

                    var allDaysForEvent = dayRangeRenderModel.CalendarDays.Where(a => a.Period.OverlapsWith(@event.Period));

                    foreach (var calendarDay in allDaysForEvent)
                    {
                        var calendarItemViewModel = new CalendarItemViewModel(@event);
                        await ExecuteUIThread(() =>
                        {
                            calendarDay.EventsCollection.AddCalendarItem(calendarItemViewModel);
                        });
                    }
                }
            }
        }

        private async Task TryConsolidateItemsAsync()
        {
            // Check if trimming is necessary
            if (DayRanges.Count > maxDayRangeSize)
            {
                Debug.WriteLine("Trimming items.");

                isLoadMoreBlocked = true;

                var removeCount = DayRanges.Count - maxDayRangeSize;

                await Task.Delay(500);

                // Right shifted, remove from the start.
                if (SelectedDateRangeIndex > DayRanges.Count / 2)
                {
                    DayRanges.RemoveRange(DayRanges.Take(removeCount).ToList());
                }
                else
                {
                    // Left shifted, remove from the end.
                    DayRanges.RemoveRange(DayRanges.Skip(DayRanges.Count - removeCount).Take(removeCount));
                }

                SelectedDateRangeIndex = DayRanges.IndexOf(SelectedDayRange);
            }
        }

        private bool ShouldScrollToItem(LoadCalendarMessage message)
        {
            // Never scroll if the initiative is from the app.
            if (message.CalendarInitInitiative == CalendarInitInitiative.App) return false;

            // Nothing to scroll.
            if (DayRanges.Count == 0) return false;

            var initializedDateRange = GetLoadedDateRange();

            if (initializedDateRange == null) return false;

            var selectedDate = message.DisplayDate;

            return selectedDate >= initializedDateRange.StartDate && selectedDate <= initializedDateRange.EndDate;
        }

        partial void OnIsAllDayChanged(bool value)
        {
            if (value)
            {
                SelectedStartTimeString = HourSelectionStrings.FirstOrDefault();
                SelectedEndTimeString = HourSelectionStrings.FirstOrDefault();
            }
            else
            {
                SelectedStartTimeString = _previousSelectedStartTimeString;
                SelectedEndTimeString = _previousSelectedEndTimeString;
            }
        }

        partial void OnSelectedQuickEventDateChanged(DateTime? value)
        {

        }

        partial void OnSelectedStartTimeStringChanged(string newValue)
        {
            var parsedTime = _currentSettings.GetTimeSpan(newValue);

            if (parsedTime == null)
            {
                SelectedStartTimeString = _previousSelectedStartTimeString;
            }
            else if (IsAllDay)
            {
                _previousSelectedStartTimeString = newValue;
            }
        }

        partial void OnSelectedEndTimeStringChanged(string newValue)
        {
            var parsedTime = _currentSettings.GetTimeSpan(newValue);

            if (parsedTime == null)
            {
                SelectedEndTimeString = _previousSelectedStartTimeString;
            }
            else if (IsAllDay)
            {
                _previousSelectedEndTimeString = newValue;
            }
        }

        partial void OnSelectedDayRangeChanged(DayRangeRenderModel value)
        {
            if (DayRanges.Count == 0 || SelectedDateRangeIndex < 0) return;

            var selectedRange = DayRanges[SelectedDateRangeIndex];

            Messenger.Send(new VisibleDateRangeChangedMessage(new DateRange(selectedRange.Period.Start, selectedRange.Period.End)));

            if (isLoadMoreBlocked) return;

            _ = LoadMoreAsync();
        }

        private async Task LoadMoreAsync()
        {
            try
            {
                await _calendarLoadingSemaphore.WaitAsync();

                // Depending on the selected index, we'll load more dates.
                // Day ranges may change while the async update is in progress.
                // Therefore we wait for semaphore to be released before we continue.
                // There is no need to load more if the current index is not in ideal position.

                if (SelectedDateRangeIndex == DayRanges.Count - 1)
                {
                    await RenderDatesAsync(CalendarInitInitiative.App, calendarLoadDirection: CalendarLoadDirection.Next);
                }
                else if (SelectedDateRangeIndex == 0)
                {
                    await RenderDatesAsync(CalendarInitInitiative.App, calendarLoadDirection: CalendarLoadDirection.Previous);
                }
            }
            catch (Exception)
            {
                Debugger.Break();
            }
            finally
            {
                _calendarLoadingSemaphore.Release();
            }
        }

        public void Receive(CalendarSettingsUpdatedMessage message)
        {
            RefreshSettings();

            // TODO: This might need throttling due to slider in the settings page for hour height.
            // or make sure the slider does not update on each tick but on focus lost.

            // Messenger.Send(new LoadCalendarMessage(DateTime.UtcNow.Date, CalendarInitInitiative.App, true));
        }

        private IEnumerable<CalendarItemViewModel> GetCalendarItems(Guid calendarItemId)
        {
            // Multi-day events are sprated in multiple days.
            // We need to find the day that the event is in, and then select the event.

            return DayRanges
                .SelectMany(a => a.CalendarDays)
                .Select(b => b.EventsCollection.GetCalendarItem(calendarItemId))
                .Where(c => c != null)
                .Cast<CalendarItemViewModel>()
                .Distinct();
        }

        private void ResetSelectedItems()
        {
            foreach (var item in AccountCalendarStateService.SelectedItems)
            {
                var items = GetCalendarItems(item.Id);

                foreach (var childItem in items)
                {
                    childItem.IsSelected = false;
                }
            }

            AccountCalendarStateService.SelectedItems.Clear();
        }

        public async void Receive(CalendarItemTappedMessage message)
        {
            if (message.CalendarItemViewModel == null) return;

            await ExecuteUIThread(() =>
            {
                var calendarItems = GetCalendarItems(message.CalendarItemViewModel.Id);

                if (!_keyPressService.IsCtrlKeyPressed())
                {
                    ResetSelectedItems();
                }

                foreach (var item in calendarItems)
                {
                    item.IsSelected = !item.IsSelected;

                    // Multi-select logic.
                    if (item.IsSelected && !AccountCalendarStateService.SelectedItems.Contains(item))
                    {
                        AccountCalendarStateService.SelectedItems.Add(message.CalendarItemViewModel);
                    }
                    else if (!item.IsSelected && AccountCalendarStateService.SelectedItems.Contains(item))
                    {
                        AccountCalendarStateService.SelectedItems.Remove(item);
                    }
                }
            });
        }

        public void Receive(CalendarItemDoubleTappedMessage message)
        {
            // TODO: Navigate to the event details page.
        }

        public void Receive(CalendarItemRightTappedMessage message)
        {
        }
    }
}