#nullable disable
using System;
using System.Linq;
using CoreGraphics;
using Foundation;
using ObjCRuntime;
using UIKit;
 
namespace Microsoft.Maui.Controls.Handlers.Items
{
	public class ItemsViewDelegator<TItemsView, TViewController> : UICollectionViewDelegateFlowLayout
		where TItemsView : ItemsView
		where TViewController : ItemsViewController<TItemsView>
	{
		readonly WeakReference<TViewController> _viewController;
 
		public ItemsViewLayout ItemsViewLayout { get; }
		public TViewController ViewController => _viewController.TryGetTarget(out var vc) ? vc : null;
 
		protected float PreviousHorizontalOffset, PreviousVerticalOffset;
 
		public ItemsViewDelegator(ItemsViewLayout itemsViewLayout, TViewController itemsViewController)
		{
			ItemsViewLayout = itemsViewLayout;
			_viewController = new(itemsViewController);
		}
 
		public override void Scrolled(UIScrollView scrollView)
		{
			var (visibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex) = GetVisibleItemsIndex();
 
			if (!visibleItems)
				return;
 
			var contentInset = scrollView.ContentInset;
			var contentOffsetX = scrollView.ContentOffset.X + contentInset.Left;
			var contentOffsetY = scrollView.ContentOffset.Y + contentInset.Top;
 
			var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs
			{
				HorizontalDelta = contentOffsetX - PreviousHorizontalOffset,
				VerticalDelta = contentOffsetY - PreviousVerticalOffset,
				HorizontalOffset = contentOffsetX,
				VerticalOffset = contentOffsetY,
				FirstVisibleItemIndex = firstVisibleItemIndex,
				CenterItemIndex = centerItemIndex,
				LastVisibleItemIndex = lastVisibleItemIndex
			};
 
			var viewController = ViewController;
			if (viewController is null)
				return;
 
			var itemsView = viewController.ItemsView;
			var source = viewController.ItemsSource;
			itemsView.SendScrolled(itemsViewScrolledEventArgs);
 
			PreviousHorizontalOffset = (float)contentOffsetX;
			PreviousVerticalOffset = (float)contentOffsetY;
 
			switch (itemsView.RemainingItemsThreshold)
			{
				case -1:
					return;
				case 0:
					if (lastVisibleItemIndex == source.ItemCount - 1)
						itemsView.SendRemainingItemsThresholdReached();
					break;
				default:
					if (source.ItemCount - 1 - lastVisibleItemIndex <= itemsView.RemainingItemsThreshold)
						itemsView.SendRemainingItemsThresholdReached();
					break;
			}
		}
 
		public override UIEdgeInsets GetInsetForSection(UICollectionView collectionView, UICollectionViewLayout layout,
			nint section)
		{
			if (ItemsViewLayout == null)
			{
				return default;
			}
 
			return ItemsViewLayout.GetInsetForSection(collectionView, layout, section);
		}
 
		public override nfloat GetMinimumInteritemSpacingForSection(UICollectionView collectionView,
			UICollectionViewLayout layout, nint section)
		{
			if (ItemsViewLayout == null)
			{
				return default;
			}
 
			return ItemsViewLayout.GetMinimumInteritemSpacingForSection(collectionView, layout, section);
		}
 
		public override nfloat GetMinimumLineSpacingForSection(UICollectionView collectionView,
			UICollectionViewLayout layout, nint section)
		{
			if (ItemsViewLayout == null)
			{
				return default;
			}
 
			return ItemsViewLayout.GetMinimumLineSpacingForSection(collectionView, layout, section);
		}
 
		public override void CellDisplayingEnded(UICollectionView collectionView, UICollectionViewCell cell, NSIndexPath indexPath)
		{
			ViewController?.CellDisplayingEndedFromDelegate(cell, indexPath);
		}
 
		protected virtual (bool VisibleItems, NSIndexPath First, NSIndexPath Center, NSIndexPath Last) GetVisibleItemsIndexPath()
		{
			var collectionView = ViewController?.CollectionView;
			if (collectionView is null)
				return default;
 
			var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Row).ToList();
 
			var visibleItems = indexPathsForVisibleItems.Count > 0;
			NSIndexPath firstVisibleItemIndex = null, centerItemIndex = null, lastVisibleItemIndex = null;
 
			if (visibleItems)
			{
				firstVisibleItemIndex = indexPathsForVisibleItems.First();
				centerItemIndex = GetCenteredIndexPath(collectionView);
				lastVisibleItemIndex = indexPathsForVisibleItems.Last();
			}
 
			return (visibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex);
		}
 
		protected virtual (bool VisibleItems, int First, int Center, int Last) GetVisibleItemsIndex()
		{
			var (VisibleItems, First, Center, Last) = GetVisibleItemsIndexPath();
			int firstVisibleItemIndex = -1, centerItemIndex = -1, lastVisibleItemIndex = -1;
			if (VisibleItems)
			{
				firstVisibleItemIndex = (int)First.Item;
				centerItemIndex = (int)Center.Item;
				lastVisibleItemIndex = (int)Last.Item;
			}
			return (VisibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex);
		}
 
		static NSIndexPath GetCenteredIndexPath(UICollectionView collectionView)
		{
			NSIndexPath centerItemIndex = null;
 
			var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Row).ToList();
 
			if (indexPathsForVisibleItems.Count == 0)
				return centerItemIndex;
 
			var firstVisibleItemIndex = indexPathsForVisibleItems.First();
 
			var centerPoint = new CGPoint(collectionView.Center.X + collectionView.ContentOffset.X, collectionView.Center.Y + collectionView.ContentOffset.Y);
			var centerIndexPath = collectionView.IndexPathForItemAtPoint(centerPoint);
			centerItemIndex = centerIndexPath ?? firstVisibleItemIndex;
			return centerItemIndex;
		}
 
		public override CGSize GetSizeForItem(UICollectionView collectionView, UICollectionViewLayout layout, NSIndexPath indexPath)
		{
			return ViewController?.GetSizeForItem(indexPath) ?? CGSize.Empty;
		}
	}
}