using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;

namespace DDD
{
    public class AutoScrollToSelected : MonoBehaviour
    {
        private ScrollRect _scrollRect;
        private RectTransform _viewport;
        private RectTransform _content;

        private GameObject _lastSelected;
        private bool _wasScrolling;

        private void Awake()
        {
            _scrollRect = GetComponent<ScrollRect>();
            _viewport = _scrollRect.viewport;
            _content = _scrollRect.content;
        }

        private void Update()
        {
            if (_scrollRect.velocity.sqrMagnitude > 0.01f)
            {
                _wasScrolling = true;
                return;
            }
            
            if (_wasScrolling)
            {
                _wasScrolling = false;
                return;
            }
            
            var selected = EventSystem.current.currentSelectedGameObject;
            if (selected == null || selected == _lastSelected) return;
            _lastSelected = selected;

            if (!selected.TryGetComponent<IAutoScrollItem>(out var autoScrollItem)) return;

            var target = autoScrollItem.RectTransform;
            if (!target.IsChildOf(_content)) return;

            if (!IsFullyVisible(target))
            {
                ScrollToMakeFullyVisible(target);
            }
        }

        private bool IsFullyVisible(RectTransform target)
        {
            var worldCorners = new Vector3[4];
            target.GetWorldCorners(worldCorners);

            for (int i = 0; i < 4; i++)
            {
                Vector3 local = _viewport.InverseTransformPoint(worldCorners[i]);
                if (!_viewport.rect.Contains(local)) return false;
            }

            return true;
        }

        private void ScrollToMakeFullyVisible(RectTransform target)
        {
            Canvas.ForceUpdateCanvases();

            float scrollRange = _content.rect.height - _viewport.rect.height;
            if (scrollRange <= 0f) return;

            Vector3[] itemCorners = new Vector3[4];
            target.GetWorldCorners(itemCorners);
            Vector3[] viewportCorners = new Vector3[4];
            _viewport.GetWorldCorners(viewportCorners);

            float itemTop = itemCorners[1].y;
            float itemBottom = itemCorners[0].y;
            float viewTop = viewportCorners[1].y;
            float viewBottom = viewportCorners[0].y;

            float offset = 0f;
            float normalizedDelta = 0f;

            if (itemTop > viewTop)
            {
                offset = itemTop - viewTop;
                normalizedDelta = offset / scrollRange;
                _scrollRect.verticalNormalizedPosition += normalizedDelta;
            }
            else if (itemBottom < viewBottom)
            {
                offset = viewBottom - itemBottom;
                normalizedDelta = offset / scrollRange;
                _scrollRect.verticalNormalizedPosition -= normalizedDelta;
            }
            else
            {
                return;
            }

            _scrollRect.verticalNormalizedPosition = Mathf.Clamp01(_scrollRect.verticalNormalizedPosition);
        }
    }
}