Tic Tac Toe - A Two-Player Game Class in Scala

Build a TicTacToe class for two players, X and O, on a 3 by 3 board. One object is one game: it remembers the board and whose turn it is, lets the players take turns, and tells who has won.

The board: rows and columns are numbered 0, 1 and 2. Cell (0, 0) is the top-left corner, (0, 2) the top-right, (2, 0) the bottom-left and (2, 2) the bottom-right. A cell holds 'X', 'O' or '.' when it is still empty.

Build:

• A new game. The class has two private fields, set up when the object is created: board, a 3 by 3 array of Char filled with '.' (read as board(r)(c)), and var player = 'X'. Every row must be its own array. All nine cells are empty and X moves first.
• def move(r: Int, c: Int): Unit plays one turn at the given row and column (always between 0 and 2, so you do not need to check the range).
  • If the cell is not empty, ignore the call: nothing changes and it is still the same player's turn.
  • Otherwise put the current player's mark in the cell, then give the turn to the other player (X becomes O, O becomes X).
• def winner(): Char returns the winner as a Char: 'X', 'O' or '.'.
  • A player wins by having the same mark in all three cells of a row, a column, or one of the two diagonals ((0,0), (1,1), (2,2) and (0,2), (1,1), (2,0)). Three empty cells in a line are not a win.
  • If nobody has three in a row, return '.'. That covers a game still in progress, an empty board and a draw.

You start with this code, which plays six games and prints the result of each one:

class Solution {
    def mainFunction(): Unit = {
        var game = TicTacToe()
        game.move(0, 0)
        game.move(1, 0)
        game.move(0, 1)
        game.move(1, 1)
        game.move(0, 2)
        println(s"${game.winner()} wins!")
        game = TicTacToe()
        game.move(0, 0)
        game.move(0, 1)
        game.move(0, 0)
        game.move(1, 0)
        game.move(1, 1)
        game.move(2, 2)
        game.move(2, 1)
        println(s"${game.winner()} wins!")
        game = TicTacToe()
        game.move(0, 0)
        game.move(0, 1)
        game.move(1, 1)
        game.move(0, 2)
        game.move(2, 2)
        println(s"${game.winner()} wins!")
        game = TicTacToe()
        game.move(0, 2)
        game.move(0, 0)
        game.move(1, 1)
        game.move(0, 1)
        game.move(2, 0)
        println(s"${game.winner()} wins!")
        game = TicTacToe()
        game.move(0, 0)
        game.move(0, 1)
        game.move(0, 2)
        game.move(1, 1)
        game.move(1, 0)
        game.move(1, 2)
        game.move(2, 1)
        game.move(2, 0)
        game.move(2, 2)
        println(s"No winner: ${game.winner()}")
        game = TicTacToe()
        println(s"No winner: ${game.winner()}")
    }
}

Games played: X fills the top row; then a game where X tries to play on a cell that is already taken (ignored, and still X's turn) and O wins the middle column; X on the main diagonal; X on the other diagonal; a full board with no three in a row; and a brand-new game.

Expected output:

X wins!
O wins!
X wins!
X wins!
No winner: .
No winner: .