using System;

namespace IslaApocalypse.Core
{
	/// <summary>
	/// The world as a 2D GRID OF COLUMNS — one per horizontal cell.
	/// → `Design - Data - Column Model.md`, D-053.
	///
	/// The grid is square and its side is a GENERATION PARAMETER, never a constant. It is carried
	/// as a <see cref="GenerationScale"/> so that every derived distance in the project is forced
	/// through the one scaling rule. → `Design - Tooling - Scaling Discipline.md`.
	///
	/// ⚠ SHAPE ONLY, PHASE 0. Chunking for streaming, region files, serialization and the mesher
	/// interface are all later phases. A flat array is the honest Phase 0 representation: it says
	/// "grid of columns" and commits to nothing about how it will be paged.
	///
	/// ⚠ NOTHING ABOUT WATER LIVES HERE. Water is an overlay over the grid, not a layer in it.
	/// </summary>
	public sealed class ColumnGrid
	{
		private readonly Column[] _columns;

		/// <summary>The scale this world was generated at. Side length, scale factor, derived distances.</summary>
		public GenerationScale Scale { get; }

		/// <summary>Grid side in columns. 1 column = 1 metre (D-002, 1:1 scale).</summary>
		public int Side => Scale.MapSize;

		public ColumnGrid(GenerationScale scale)
		{
			Scale = scale;
			_columns = new Column[(long)scale.MapSize * scale.MapSize <= int.MaxValue
				? scale.MapSize * scale.MapSize
				: throw new ArgumentOutOfRangeException(nameof(scale), scale.MapSize,
					"MapSize squared exceeds a single flat array. Chunked storage is a later phase — see the class comment.")];
		}

		/// <summary>The column at (x, y). Null until one is placed — Phase 0 allocates no columns.</summary>
		public Column this[int x, int y]
		{
			get => _columns[Index(x, y)];
			set => _columns[Index(x, y)] = value;
		}

		private int Index(int x, int y)
		{
			if ((uint)x >= (uint)Side) throw new ArgumentOutOfRangeException(nameof(x), x, $"x outside [0,{Side}).");
			if ((uint)y >= (uint)Side) throw new ArgumentOutOfRangeException(nameof(y), y, $"y outside [0,{Side}).");
			return y * Side + x;
		}

		public override string ToString() => $"ColumnGrid({Side}x{Side})";
	}
}