Gomoku (Five in a Row, vs Computer)

The rules this board follows

The game is played on a 15 x 15 grid. You play black and move first; the computer plays white. As soon as one side has five or more stones of its own color connected in a straight line (horizontal, vertical or either diagonal), that side wins immediately. If all 225 squares fill up with no winner, the game is a draw. Click an empty intersection to place a stone; the last stone played is marked so you can follow the sequence. Reading the code, the win test is simply "the connected line is at least 5 long", which means an overline of six or more also wins, and there are no forbidden moves such as the 3-3 or 4-4 bans that some rule sets impose on Black. This is usually called free-style Gomoku. The table compares it with Renju as it is generally described; Renju and tournament rules differ in details from one organization to another, so check the official rules of any event you plan to enter.

It has long been argued that the player who moves first has an edge, and forbidden moves and opening rules are commonly explained as ways to reduce that edge. No single percentage can honestly be attached to it, though. Here you always start as black and face no restrictions, so the setup is as friendly to you as the game allows.

The board in numbers

On a 15 x 15 grid each row has (15 - 4) = 11 windows of five squares, which is where the line counts below come from. The table is computed when the page is built.

Every stone you place touches several of those potential fives at once. Squares near the center belong to many more windows than squares at the edge, so they carry more value for both attack and defense. That is why the first move is normally played near the middle.

Terms: threes, fours and what "open" means

• Open three: three connected stones with both ends empty. One more stone makes an open four, so the opponent must respond immediately.
• Closed three: a three blocked at one end by an enemy stone or the edge of the board. It can still become a four, but only a closed one, which is a much weaker threat.
• Open four: four stones with both ends empty. The opponent can block only one end, so the game is effectively won.
• Closed four: four stones with just one empty end. A single block removes the threat.
• Double threat: one move that creates two threats at once (4-4, 4-3 or 3-3). No single reply can stop both.

How the computer chooses a move

The computer only considers empty squares within two squares of an existing stone. For each candidate it computes how good the square is for white (offense) and how dangerous it would be if black played there (defense). The final score is offense + defense x 1.1, so when two squares look equally valuable the computer slightly prefers the one that blocks you. For every direction the value depends on how many stones are connected and how many ends are open.

A shape blocked at both ends scores zero. Difficulty only changes how faithfully the computer follows this ranking: Easy takes the top-scored square 30% of the time and otherwise picks any candidate at random; Medium picks randomly among the five best squares; Hard always plays a highest-scoring square (random among ties). You can change the difficulty at any time and it applies from the computer's next move. The important limit is that the evaluation looks one move deep. It does not calculate several moves ahead to see a double threat coming, so even Hard can be beaten by a prepared double attack.

Three worked examples

Example 1: the square next to an open three gets blocked first. Say black has three stones in a row with both ends empty and white's stones are far away. The computer's score for the square right next to the three is 11064; for the square one step further out it is 84. Black playing next to the three would form an open four (worth 10000), so the defensive part dominates, and on Hard the computer blocks directly beside the three (either end, if tied).

Example 2: two open fours with one stone. If black has a horizontal three and a vertical three that meet at an empty square, playing there creates an open four in both directions at once. For black that square scores 10000 + 10000 + 10 + 10 = 20020 from the four directions. In free-style Gomoku this simply wins. In Renju it would be a forbidden 4-4 for Black. Here it is legal, so the computer must block that intersection beforehand or lose.

Example 3: filling the gap makes seven. If a row holds three black stones, one empty square, then three more black stones, playing in the gap produces a line of seven. The win test here is "five or more", so this wins at once. Rule sets that count only exactly five, or that forbid Black from making an overline, would not treat it as a win.

Strategy for winning

1. Start near the center. It sits on the most potential fives and lets you grow in every direction.
2. Spread your stones across directions instead of building one long line. A straight three is easy to block, but stones whose lines cross (horizontal, vertical, diagonal) set up double threats.
3. Always block a four, and block an open three as early as you can. If your own attack is only a closed four, defend first.
4. Save forcing fours for the moment they build a double threat. A four makes the opponent reply, but if you spend them randomly you lose them as weapons later.
5. A three against the edge is a closed three. The board edge counts as a blocked end, so you need not treat it as a serious threat.

Misunderstandings and limits

• Do not use results here to judge Renju skill. A move that would be illegal for Black under Renju is a normal winning move on this board.
• The computer uses a simple one-move evaluation, so beating it does not guarantee strength against a human opponent.
• Undo works only while the game is still running and the computer is not mid-move. If the computer played last, undo removes its stone and your previous stone together.
• The win/draw/loss record lives only while the page stays open. Reloading or returning later resets it to zero; nothing is stored on a server or in browser storage.
• The computer's choices use Math.random, so on Easy and Medium the same position can get different replies.

Getting more out of each game

After a loss, look back at the last few moves and find where, two or three moves before the opponent's double threat appeared, you could have blocked a key square. That is the quickest way to improve. If you win several Medium games in a row, move up to Hard, and use Undo to test a different reply from the same position.

Frequently Asked Questions

More board and logic games

For a simpler three-in-a-row game try Tic-Tac-Toe; for flipping stones to take territory there isReversi; Connect 4 in a Row uses gravity; and for a number-deduction puzzle see Number Baseball.