Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;

char[,] board = new char[20, 20];
List<(int Left, int Top)> showWordSelections = [];
List<(int Left, int Top)> selections = [];

string[] wordArray = default!;
string currentWord = default!;
(int Left, int Top) cursor = (0, 0);

InitializeWords();
PlayAgain:
InitializeBoard();
Console.Clear();
while (true)
{
	RenderBoard();
	Console.Write($"""

		Highlight the word "{currentWord}" above.

		Controls:
		- arrow keys: move cursor
		- enter: highlight characters
		- backspace: clear highlighted characters
		- home: new word search
		- end: give up and show word
		- escape: close game
		""");
	Console.SetCursorPosition(2 * cursor.Left, cursor.Top);
	Console.CursorVisible = true;
	switch (Console.ReadKey(true).Key)
	{
		case ConsoleKey.LeftArrow: cursor.Left = cursor.Left <= 0 ? board.GetLength(0) - 1 : cursor.Left - 1; break;
		case ConsoleKey.RightArrow: cursor.Left = cursor.Left >= board.GetLength(0) - 1 ? 0 : cursor.Left + 1; break;
		case ConsoleKey.UpArrow: cursor.Top = cursor.Top <= 0 ? board.GetLength(1) - 1 : cursor.Top - 1; break;
		case ConsoleKey.DownArrow: cursor.Top = cursor.Top >= board.GetLength(1) - 1 ? 0 : cursor.Top + 1; break;
		case ConsoleKey.Backspace: selections.Clear(); break;
		case ConsoleKey.Home: goto PlayAgain;
		case ConsoleKey.End:
			selections.Clear();
			selections.AddRange(showWordSelections);
			Console.Clear();
			RenderBoard();
			Console.Write($"""

				Here is where "{currentWord}" was hiding.

				Controls:
				- enter/home: play again
				- escape: close game
				""");
			while (true)
			{
				switch (Console.ReadKey(true).Key)
				{
					case ConsoleKey.Enter or ConsoleKey.Home: goto PlayAgain;
					case ConsoleKey.Escape: goto Close;
				}
			}
		case ConsoleKey.Escape: goto Close;
		case ConsoleKey.Enter:
			if (!selections.Remove(cursor))
			{
				selections.Add(cursor);
			}
			selections.Sort();
			if (UserFoundTheWord())
			{
				Console.Clear();
				RenderBoard();
				Console.Write($"""

						You found "{currentWord}"! You win!

						Controls:
						- enter/home: play again
						- escape: close game
						""");
				while (true)
				{
					switch (Console.ReadKey(true).Key)
					{
						case ConsoleKey.Enter or ConsoleKey.Home: goto PlayAgain;
						case ConsoleKey.Escape: goto Close;
					}
				}
			}
			break;
	}
}
Close:
Console.Clear();
Console.WriteLine("Word Search was closed.");

void InitializeWords()
{
	const string wordsResource = "Word_Search.Words.txt";
	Assembly assembly = Assembly.GetExecutingAssembly();
	using Stream? stream = assembly.GetManifestResourceStream(wordsResource);
	using StreamReader streamReader = new(stream!);
	List<string> words = [];
	while (!streamReader.EndOfStream)
	{
		string? word = streamReader.ReadLine();
		if (!string.IsNullOrWhiteSpace(word) &&
			word.Length < board.GetLength(0) &&
			word.Length < board.GetLength(1))
		{
			words.Add(word.ToUpper());
		}
	}
	wordArray = [.. words];
}

void InitializeBoard()
{
	selections.Clear();

	for (int i = 0; i < board.GetLength(1); i++)
	{
		for (int j = 0; j < board.GetLength(0); j++)
		{
			board[j, i] = (char)('A' + Random.Shared.Next(26));
		}
	}

	currentWord = wordArray[Random.Shared.Next(wordArray.Length)];

	// choose a random orientation for the word (down, right, left, up, down-right, down-left, up-right, or up-left)
	bool r((int Left, int Top) location) => location.Left + currentWord.Length < board.GetLength(0);
	bool d((int Left, int Top) location) => location.Top + currentWord.Length < board.GetLength(1);
	bool l((int Left, int Top) location) => location.Left - currentWord.Length >= 0;
	bool u((int Left, int Top) location) => location.Top - currentWord.Length >= 0;
	bool dr((int Left, int Top) location) => d(location) && r(location);
	bool dl((int Left, int Top) location) => d(location) && l(location);
	bool ur((int Left, int Top) location) => u(location) && r(location);
	bool ul((int Left, int Top) location) => u(location) && l(location);
	(Func<(int Left, int Top), bool> Validator, (int Left, int Top) Adjustment) orientation = Random.Shared.Next(8) switch
	{
		0 => (d,  ( 0,  1)),
		1 => (r,  ( 1,  0)),
		2 => (u,  ( 0, -1)),
		3 => (l,  (-1,  0)),
		4 => (dr, ( 1,  1)),
		5 => (dl, (-1,  1)),
		6 => (ur, ( 1, -1)),
		7 => (ul, (-1, -1)),
		_ => throw new NotImplementedException(),
	};

	// choose a random starting location that is valid for the orientation
	List<(int Left, int Top)> possibleLocations = [];
	for (int i = 0; i < board.GetLength(1); i++)
	{
		for (int j = 0; j < board.GetLength(0); j++)
		{
			if (orientation.Validator((j, i)))
			{
				possibleLocations.Add((j, i));
			}
		}
	}
	(int Left, int Top) randomLocation = possibleLocations[Random.Shared.Next(possibleLocations.Count)];

	showWordSelections.Clear();
	for (int i = 0; i < currentWord.Length; i++)
	{
		showWordSelections.Add(randomLocation);
		board[randomLocation.Left, randomLocation.Top] = currentWord[i];
		randomLocation = (randomLocation.Left + orientation.Adjustment.Left, randomLocation.Top + orientation.Adjustment.Top);
	}
}

void RenderBoard()
{
	Console.CursorVisible = false;
	Console.SetCursorPosition(0, 0);
	for (int i = 0; i < board.GetLength(1); i++)
	{
		for (int j = 0; j < board.GetLength(0); j++)
		{
			if (selections.Contains((j, i)))
			{
				(Console.ForegroundColor, Console.BackgroundColor) = (Console.BackgroundColor, Console.ForegroundColor);
			}
			Console.Write(board[j, i]);
			if (selections.Contains((j, i)))
			{
				(Console.ForegroundColor, Console.BackgroundColor) = (Console.BackgroundColor, Console.ForegroundColor);
			}
			if (j < board.GetLength(1) - 1)
			{
				Console.Write(' ');
			}
		}
		Console.WriteLine();
	}
}

bool UserFoundTheWord()
{
	// make sure all the selections are in a straight line
	if (selections.Count > 1)
	{
		(int Left, int Top) adjustment = (selections[1].Left - selections[0].Left, selections[1].Top - selections[0].Top);
		if (adjustment.Left > 1 || adjustment.Left < -1 || adjustment.Top > 1 || adjustment.Top < -1)
		{
			return false;
		}
		for (int i = 2; i < selections.Count; i++)
		{
			if ((selections[i].Left - selections[i - 1].Left, selections[i].Top - selections[i - 1].Top) != adjustment)
			{
				return false;
			}
		}
	}

	char[] chars = selections.Select(location => board[location.Left, location.Top]).ToArray();
	string charsString = new(chars);
	Array.Reverse(chars);
	string charsStringReverse = new(chars);
	return charsString == currentWord || charsStringReverse == currentWord;
}