AdventOfCode/src/core/SymbolGrid.cs

using System.Diagnostics.CodeAnalysis;
using System.IO.MemoryMappedFiles;
using System.Numerics;

namespace core;

public class SymbolGrid
{
	private char[]? _map;

	[DisallowNull]
	public char? this[int x, int y]
	{
		get
		{
			EnsureMap();
			return IsValid(x, y) ? _map[y * Height + x] : null;
		}
		set
		{
			EnsureMap();
			if (IsValid(x, y))
			{
				_map[y * Height + x] = value.Value;
			}
		}
	}

	[DisallowNull]
	public char? this[Vector2I i]
	{
		get => this[i.X, i.Y];
		set => this[i.X, i.Y] = value;
	}

	public int Width { get; private set; }

	public int Height { get; private set; }

	public void Load(string file)
	{
		using var fileView = MemoryMappedFile.CreateFromFile(file, FileMode.Open);
		using var accessor = fileView.CreateViewAccessor();
		(Width, var stride, Height) = Parse(accessor);
		_map = new char[Width * Height];
		for (int x = 0; x < Width; x++)
		{
			for (int y = 0; y < Height; y++)
			{
				_map[y * Height + x] = (char)accessor.ReadByte(y * stride + x);
			}
		}

		static (int Width, int Stride, int Height) Parse(MemoryMappedViewAccessor map)
		{
			bool hasNewLine = false;
			int? width = null, stride = null;
			for (int i = 0; i < map.Capacity; i++)
			{
				if ((char)map.ReadByte(i) is '\r' or '\n')
				{
					if (!hasNewLine)
					{
						width = i;
					}

					hasNewLine = true;
				}
				else if (hasNewLine)
				{
					stride = i;
					break;
				}
			}

			return (width!.Value, stride!.Value, (int)((map.Capacity + width.Value) / stride.Value));
		}
	}

	public SymbolEnumerator GetEnumerator() => new(this);

	public bool IsValid(Vector2I p) => IsValid(p.X, p.Y);

	public bool IsValid(int x, int y)
	{
		EnsureMap();
		return x >= 0 && x < Width && y >= 0 && y < Height;
	}

	public ref struct SymbolEnumerator
	{
		private readonly SymbolGrid _grid;
		private readonly int _length;
		private int? _current;

		internal SymbolEnumerator(SymbolGrid grid)
		{
			_grid = grid;
			_length = grid.Width * grid.Height;
		}

		public (Vector2I Position, char Symbol) Current { get; private set; }

		public bool MoveNext()
		{
			_current = _current.HasValue ? _current + 1 : 0;
			if (_current == _length)
			{
				return false;
			}

			Current = (_grid.ToVector(_current.Value), _grid._map![_current.Value]);

			return true;
		}
	}

	private Vector2I ToVector(int index) => new()
	{
		Y = index / Height,
		X = index % Height
	};

	[MemberNotNull(nameof(_map))]
	private void EnsureMap() => ArgumentNullException.ThrowIfNull(_map);
}