Neon Tac: 6×6 tic-tac-toe, 4 in a row

Neon Tac is tic-tac-toe played on a six-by-six grid. You play as X (glowing cyan) and the computer plays as O (glowing magenta). The first player to place four marks in a row (across, up-and-down or diagonally) wins.

Click or tap any empty cell to place your mark, or use the arrow keys to move the highlighted cell and press Enter. The computer responds immediately. The winning line lights up when the game is over, and you hear a win or lose sound. Press Restart (or R) to play again.

Unlike classic three-by-three tic-tac-toe, a six-by-six board rarely ends in a draw. There are many more lines to threaten, so you need to attack and defend at the same time.

• Control the centre columns and rows. Lines through the middle of a 6×6 board cover more potential winning runs than edge cells.
• Build two threats at once. If you can create two open-ended three-in-a-row lines simultaneously, the computer can only block one.
• Watch diagonals. Diagonal wins are easy to miss when you are focused on rows and columns. Check all four diagonal directions before each move.
• Block early. The computer plans ahead. If you see it build two in a row, stop it before it reaches three: a three-in-a-row with open ends is almost impossible to stop.

The computer always takes a winning move when it has one and always blocks a line where you are one mark away from four. Otherwise it searches a few moves ahead (a minimax search with alpha-beta pruning) over the empty cells next to existing marks, and scores each position by how many open lines of four each side still has.

That makes it good at stopping single threats but beatable: it does not look deep enough to see every double threat coming. Set up two lines that each need one more mark, and it can only block one of them. Each move takes a fraction of a second, even on an older phone.