Search index in grid under mouse

CellIndexSearcher.cs

using UnityEngine;
using UnityEngine.UI;

public class CellIndexSearcher
{
	private readonly RectTransform _cellsContainer;

	private readonly float _cellW;
	private readonly float _cellH;
	private readonly float _spaceX;
	private readonly float _spaceY;
	
	private readonly int _rowCount;
	private readonly int _columnCount;

	public CellIndexSearcher(RectTransform cellsContainer, GridLayoutGroup grid, int rowCount, int columnCount)
	{
		_cellsContainer = cellsContainer;

		_cellW = grid.cellSize.x;
		_cellH = grid.cellSize.y;
		_spaceX = grid.spacing.x;
		_spaceY = grid.spacing.y;
		_rowCount = rowCount;
		_columnCount = columnCount;
	}

	public (int row, int column) GetCellIndexUnderMouse(Vector2 mousePosition)
	{
		if (RectTransformUtility.ScreenPointToLocalPointInRectangle(
				_cellsContainer, mousePosition, null, out Vector2 localPoint))
		{
			int column = -(Mathf.FloorToInt(-localPoint.x / (_cellW + _spaceX)) - 3);
			int row = -(Mathf.FloorToInt(localPoint.y / (_cellH + _spaceY)) - 3);

			if (row >= 0 && row < _rowCount &&
				column >= 0 && column < _columnCount)
			{
				return (row, column);
			}
		}

		return (-1, -1);
	}
}