TTT.cs

#define TTT_USELINQ //If you feel that Linq is cheating, you can remove this line.

using System;
#if TTT_USELINQ
using System.Linq;
#endif

namespace UglyRestrictions
{
	/// <summary>
	/// Provides a minimalistic Tic Tac Toe implementation.
	/// Requirements:
	/// - Do not use arrays
	/// - Do not declare and use user defined functions
	/// Features:
	/// - No lists either. Doesn't uses Indexer syntax (square brackets) at all
	/// - (Optional) No Linq, just System namespace
	/// - Fully commented
	/// - Using iterations for win condition check
	/// - Only tests win against the player that made the last move
	/// - No redundant code
	/// - Shows current player
	/// - Detect draw situation
	/// </summary>
	/// <seealso cref="https://old.reddit.com/r/programminghorror/comments/q8hptf/_/hgr2mo9/" />
	public static class TTT
	{
#if TTT_USELINQ
		public static void Run()
		{
			//Using the characters '1' and '2' doubles as "Player X" output
			char Player = '1';
			//char* is just memory
			string Field = "000000000";
			//char* is also just indexes
			string Label = "ABCDEFGHI";

			Console.Clear();
			while (Player != '0')
			{
				//Show field
				Console.SetCursorPosition(0, 0);
				for (int i = 0; i < 9; i += 3)
				{
					Console.WriteLine("{0} {1} {2} | {3} {4} {5}",
						Field.Substring(i + 0, 1),
						Field.Substring(i + 1, 1),
						Field.Substring(i + 2, 1),
						Label.Substring(i + 0, 1),
						Label.Substring(i + 1, 1),
						Label.Substring(i + 2, 1)
						);
				}
				Console.Write("Player {0}, make your move (A-I): ", Player);
				char Key = Console.ReadKey(true).KeyChar.ToString().ToUpper().First();
				int Pos = Label.IndexOf(Key);
				//Validate input
				if (Pos < 0 || Field.Substring(Pos, 1).First() != '0')
				{
					//Escape key
					if (Key == '\x1B')
					{
						Console.WriteLine("Game ended by [ESC] key");
						Player = '0';
					}
					Console.Beep();
				}
				else
				{
					Console.WriteLine();
					//make move
					Field = Field.Substring(0, Pos) + Player + Field.Substring(Pos + 1);

					for (int i = 0; i < 3; i++)
					{
						if (
							//Check horizontals
							Field.Substring(i, 3).All(m => m == Player) ||
							//Verticals
							(Field.Substring(i, 1) + Field.Substring(i + 3, 1) + Field.Substring(i + 6, 1)).All(m => m == Player))
						{
							Console.WriteLine("Player {0} wins", Player);
							Player = '0';
						}
					}
					//Check diagonals
					if (
						(Field.Substring(0, 1) + Field.Substring(4, 1) + Field.Substring(8, 1)).All(m => m == Player) ||
						(Field.Substring(2, 1) + Field.Substring(4, 1) + Field.Substring(6, 1)).All(m => m == Player))
					{
						Console.WriteLine("Player {0} wins", Player);
						Player = '0';
					}
					//It's a draw if all fields occupied and nobody won
					if (!Field.Contains("0"))
					{
						Console.WriteLine("It's a draw");
						Player = '0';
					}
					//Switch player
					If(Player != '0')
					{
						Player = Player == '1' ? '2' : '1';
					}
				}
			}
		}
#else
		public static void Run()
		{
			//Using the characters '1' and '2' doubles as "Player X" output
			char Player = '1';
			//char* is just memory
			string Field = "000000000";
			//char* is also just indexes
			string Label = "ABCDEFGHI";

			Console.Clear();
			while (Player != '0')
			{
				//Show field
				Console.SetCursorPosition(0, 0);
				for (int i = 0; i < 9; i += 3)
				{
					Console.WriteLine("{0} {1} {2} | {3} {4} {5}",
						Field.Substring(i + 0, 1),
						Field.Substring(i + 1, 1),
						Field.Substring(i + 2, 1),
						Label.Substring(i + 0, 1),
						Label.Substring(i + 1, 1),
						Label.Substring(i + 2, 1)
						);
				}
				Console.Write("Player {0}, make your move (A-I): ", Player);
				string Key = Console.ReadKey(true).KeyChar.ToString().ToUpper();
				int Pos = Label.IndexOf(Key);
				//Validate input
				if (Pos < 0 || Field.Substring(Pos, 1) != "0")
				{
					//Escape key
					if (Key == "\x1B")
					{
						Console.WriteLine("Game ended by [ESC] key");
						Player = '0';
					}
					Console.Beep();
				}
				else
				{
					//This is either "111" or "222". Used later for comparison against the field.
					string PPP = string.Format("{0}{0}{0}", Player);
					Console.WriteLine();
					//make move
					Field = Field.Substring(0, Pos) + Player + Field.Substring(Pos + 1);

					for (int i = 0; i < 3; i++)
					{
						if (
							//Check horizontals
							Field.Substring(i, 3) == PPP ||
							//Verticals
							(Field.Substring(i, 1) + Field.Substring(i + 3, 1) + Field.Substring(i + 6, 1)) == PPP)
						{
							Console.WriteLine("Player {0} wins", Player);
							Player = '0';
						}
					}
					//Check diagonals
					if (
						(Field.Substring(0, 1) + Field.Substring(4, 1) + Field.Substring(8, 1)) == PPP ||
						(Field.Substring(2, 1) + Field.Substring(4, 1) + Field.Substring(6, 1)) == PPP)
					{
						Console.WriteLine("Player {0} wins", Player);
						Player = '0';
					}
					//It's a draw if all fields occupied and nobody won
					if (!Field.Contains("0"))
					{
						Console.WriteLine("It's a draw");
						Player = '0';
					}
					//Switch player
					If(Player != '0')
					{
						Player = Player == '1' ? '2' : '1';
					}
				}
			}
		}
#endif
	}
}