Gomoku

The board, and how you win

The board is a 15×15 grid of lines, which makes 225 intersections. Stones sit on the crossings rather than inside the squares, and the wood carries nothing but the grid lines and 5 star points.

You win the moment 5 or more of your stones stand in an unbroken line — across, down, or on either diagonal. More than 5 still wins. A run of 6 is a victory here, not a foul. If all 225 intersections fill and neither side has a line, the match is a draw.

• Stones carry no letters and no numbers. They are black and white, so the position reads identically in every language.
• The winning line is ringed and every other stone is dimmed, so the result never rests on colour alone.
• The last stone played carries a small mark. It is how you find the CPU’s move without having to watch for it.
• There is no score in this game and no clock on the screen. A match is a win, a loss or a draw, and your time is recorded quietly for the statistics only.

These are freestyle rules. The renju restrictions on the opening player — three-three, four-four and the overline — are not implemented. They exist to trim the first player’s advantage in a game between two people; against a CPU that job belongs to the difficulty, so there is nothing forbidden to learn before you can play.

Tap once to aim, again to place

The first tap marks the intersection with a ghost of the stone you are about to play. A second tap on the same point commits it. Tapping a different point simply moves the mark there.

The reason is physical. On the narrowest screen this series supports — 320px wide — the intersections sit about 19px apart, and a single tap that committed immediately would be a machine for producing stones you never meant to play. It is the same two-step language as choosing a cell and then typing a number in Sudoku, and it is not a setting: if only one of the two is right, offering the choice would just be handing you the problem.

• Aiming makes no sound. Nothing has happened yet; the sound belongs to the stone landing.
• An intersection that already holds a stone cannot be tapped. It is disabled rather than quietly ignored, and nothing scolds you for trying.
• A stone pops into place over 120ms. Reduced Motion — from the app’s settings or from the system — turns that off, and the stone is simply drawn where it was played.
• Every intersection is a button, and a screen reader reads its row and column along with what is there: empty, your stone, the CPU’s stone, or waiting for the second tap.
• The line that says whose turn it is keeps a fixed height, so the board never shifts as it changes between your turn, the prompt for your second tap, and the CPU’s.

Black opens, so your colour decides who starts

Black always moves first. That is a rule of the game rather than a setting, and nobody can pass: every turn adds exactly one stone, so whose turn it is follows from the stones already on the board, and the game never has to remember it.

So choosing a colour is choosing whether you open, and the home screen says so in those words — Black · first and White · second. Black is the default. Take White and the CPU opens; on an empty board its first stone goes to the centre.

• The colour applies to your next match. Changing it while a match is in progress asks nothing and breaks nothing — a match keeps the colour it started on until it ends.
• Your opponent is always the CPU on your own device. There is no online play, no two-player mode, and no rating or ranking anywhere.
• Your record is kept separately for each difficulty: matches played, wins, losses and draws, with your total play time across all of them. No streak is counted and none is shown.
• A match you abandon in order to start another is not recorded as a loss. It counted as played the moment it began, and only a match that actually reached an end changes the record.

The help you get, and why there is no Hint

Undo is unlimited and free, and it works from the first launch. No game feature here is ever behind an ad or a purchase, and strength is not something you unlock — all 3 difficulties are selectable from the start.

One press takes the board back to your previous decision point, which means your stone and the CPU’s answer to it come off together. It never leaves you standing in the middle of the CPU’s turn, because that is not a position you ever decided anything from.

• It is a take-back, not a reroll. The CPU’s reply is fixed by the match’s seed and by how many stones are on the board, so undoing and playing the same point brings back exactly the same move. A golden test holds that steady across updates.
• There is no Hint. Nothing on this board is hidden, so a hint would have nothing to reveal — it could only name the point it would play itself, which is the CPU playing your side as well as its own. Reversi and Connect Four are built on the same judgement.
• There is no display of legal moves either. Every empty intersection is legal, so there would be no information in showing them; the help against a misplaced stone is the second tap.
• The board is saved as you play, so leaving the screen or the app is safe. There is one match slot: on the home screen the difficulty you are playing says Resume, and picking a different one asks before it replaces the match.
• The undo history is not saved. A match you come back to starts from the position on the board rather than from the moves that built it.
• New Game sits on the board screen and on the result screen. Starting one in the middle of a match asks first, because the position would be gone.

What Easy, Normal and Hard mean

The home screen calls this choosing your opponent. All 3 are there from the first launch and each keeps its own record, and the board, the rules and the help are identical at every setting. The only thing that changes is how the CPU chooses its stone.

• Easy looks one ring out and no further. It plays a random empty point beside a stone already down, and it does not check for a five — yours or its own. Seeing only what is next to it is the whole of its weakness.
• Normal and Hard settle a five before they weigh anything. If a stone completes 5 for them they play it; if your next stone would complete 5 they block it. Both behaviours are pinned by tests.
• That rule has to sit outside the scoring, because the score cannot express it. Blocking your four barely improves the CPU’s own shape, while building an open four of its own scores roughly ten times as much — so an opponent that only compared numbers would build its four and lose to the five that landed first.
• Hard stops at a budget of 8,000 positions and plays the best move of the last depth it finished. The first depth always finishes, so Hard is never weaker for having run out of room.
• It weighs a threat against it slightly heavier than one of its own — 1.15 times. Valuing both the same produces a CPU that would rather build than defend, and that CPU loses on the following move.
• The CPU thinks only on its own turn. Nothing runs in the background while you sit and look at the board, and its stone lands roughly 450ms after its turn begins, plus whatever the search cost.

Hard is deliberately not the strongest opponent a computer can be. Freestyle Gomoku is known to be a theoretical win for the side that opens, so a CPU that read to the end would not be an opponent — it would be a wall. That result assumes perfect play, and a search stopped at 8,000 positions is nowhere near perfect: taking White is a handicap here, not a lost cause. The budget was chosen so a reply costs about what the CPU opponents already shipped in Simple Games cost, and there is no opening database behind it — the first stones are played by the same search as the last ones.

Tips

• Play near the stones already down. A stone dropped into the empty half of the board builds nothing this turn and blocks nothing next turn. The CPU agrees with you about this — it will not even consider a point more than 2 rings from a stone.
• Count the ends, not the stones. Whether both ends of a run are open is nearly the whole game: four in a row with both ends free cannot be stopped, while four with one end blocked has a single completing point and is merely a move you have to answer.
• Block the open three. Three of your opponent’s stones with both ends free become that unstoppable four on the next stone, so the moment to act is one move earlier than it feels. It is the one piece of advice the in-app tutorial spends a whole step on.
• Make two threats at once. Nobody can pass and nobody plays two stones in a turn, so if one stone leaves you with two open threes — one along a row, one on a diagonal — only one of them can be answered. Look for the intersection where two of your own lines cross; that is where a single stone does two jobs.
• Because 6 in a row also wins, you never have to hold a line back to keep it at exactly 5. Extend freely — there is no way to overshoot and no shape here that can be too long.
• Against Hard, use Undo to read rather than to escape. Within a match the CPU’s answer is fixed, so a point you take back and play again draws the same reply. You can go back, try the other point instead, and compare the two lines rather than guess between them.

Questions people ask