Tic Tac Toe

example.ts

let game = createGame();
const moves = [0, 4, 1, 3, 2];
for (const index of moves) {
  if (game.winner || game.moves >= 9) break;
  game = makeMove(game, index);
}
console.log(game);

tictactoe.ts

type Player = "X" | "O";
type Cell = Player | null;

export type TicTacToeState = {
  board: Cell[];
  currentPlayer: Player;
  winner: Player | null;
  moves: number;
};

const initialState: TicTacToeState = {
  board: Array.from({ length: 9 }, () => null),
  currentPlayer: "X",
  winner: null,
  moves: 0,
};

// 0 | 1 | 2
// ---------
// 3 | 4 | 5
// ---------
// 6 | 7 | 8
// winningLines encodes:
// - Rows: [0,1,2], [3,4,5], [6,7,8]
// - Columns: [0,3,6], [1,4,7], [2,5,8]
// - Diagonals: [0,4,8], [2,4,6]

const winningLines: [number, number, number][] = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8],
  [0, 3, 6],
  [1, 4, 7],
  [2, 5, 8],
  [0, 4, 8],
  [2, 4, 6],
];

export const getWinner = (board: Cell[]): Player | null => {
  for (const [a, b, c] of winningLines) {
    const cell = board[a];
    if (cell && cell === board[b] && cell === board[c]) {
      return cell;
    }
  }
  return null;
};

export const createGame = (): TicTacToeState => ({
  board: initialState.board.slice(),
  currentPlayer: initialState.currentPlayer,
  winner: initialState.winner,
  moves: initialState.moves,
});

const isDraw = (state: TicTacToeState): boolean =>
  state.moves >= 9 && getWinner(state.board) === null;

const togglePlayer = (player: Player): Player => (player === "X" ? "O" : "X");

const assertValidMove = (state: TicTacToeState, index: number): void => {
  if (state.winner) {
    throw new Error(`Game is over: ${state.winner} won.`);
  }
  if (isDraw(state)) {
    throw new Error("Game is over: the board is full and it's a draw.");
  }
  if (index < 0 || index >= state.board.length) {
    throw new Error(`Invalid move: index ${index} is out of bounds.`);
  }
  if (state.board[index] !== null) {
    throw new Error(`Invalid move: cell ${index} is already occupied.`);
  }
};

export const makeMove = (
  state: TicTacToeState,
  index: number,
): TicTacToeState => {
  assertValidMove(state, index);

  const board = state.board.slice();
  board[index] = state.currentPlayer;

  const nextWinner = getWinner(board);
  const moves = state.moves + 1;

  return {
    board,
    currentPlayer: nextWinner || moves >= 9
      ? state.currentPlayer
      : togglePlayer(state.currentPlayer),
    winner: nextWinner,
    moves,
  };
};