How to play Bridges
A sea of open water with numbered islands in it. Join the islands with straight bridges, across or down, until each one has exactly as many bridges as its number and every island belongs to one connected group.
Boards come in three sizes, 7 × 7, 10 × 10 and 13 × 13, each generated in your browser with exactly one answer that you can reach without guessing. The small board is a coffee-break warm-up; the large one has about forty islands.
Rules
- A bridge runs in a straight line between two islands in the same row or column, with nothing in between.
- Two islands can share one bridge or two, never more.
- Bridges may not cross each other or pass over an island.
- Each island ends up with exactly as many bridges as its number, a double bridge counting as two.
- All islands must join into one group, so you could walk from any island to any other.
- A full island turns pale, and one with too many bridges turns red. The clock stops when the last bridge completes the board. Each size remembers its own board, and the clock pauses while the tab is hidden.
Controls
- Drag from an island
- Toward a neighbour: none → one bridge → two → none
- Click / tap two islands
- The same, between two islands that face each other
- Click / tap a bridge
- Cycle that bridge
- Arrows / WASD
- Move between islands
- Shift + arrow
- Lay a bridge from the island under the cursor
- Space / Enter
- Pick the island under the cursor; pick a neighbour next to join them
- C
- Toggle count down (each island shows what it still needs)
- Z / Ctrl+Z
- Undo
- P
- Pause
Tips
- Start with islands that leave no choice. An island whose number is twice its neighbours, such as a 4 with two neighbours or an 8 in open water, needs a double bridge to each of them.
- One less works too: a 3 with two neighbours, a 5 with three or a 7 with four has at least one bridge to every neighbour.
- Two 1s can never join each other, and two 2s can never share a double bridge: either pair would be sealed off from the rest.
- When an island has only one neighbour left that it can still reach, the bridges it is missing go there.
- Count down (C) shows what each island still needs, which makes the late game quicker to read.
Bridges strategy
Count what an island can reach
An island has at most four neighbours and takes at most two bridges from each. When its number is twice the neighbours it can still reach, every bridge is double; one less, and each of those neighbours gets at least one. A neighbour stops counting once it is full or a built bridge cuts across the way to it, so look at an island again whenever the board around it changes.
Never close a group early
A group of islands that is complete but isn’t the whole board breaks the last rule. That is why two 1s never join and two 2s never take a double bridge, and it scales up: if a bridge would use the last free bridge of a group of joined islands, it has to go somewhere else. On 10 × 10 and 13 × 13 every board needs this somewhere, since the generator only keeps boards where counting alone stalls.
The only way in
If one part of the board can reach the rest only between one pair of islands, that pair needs at least one bridge, however much choice the numbers seem to leave. Look for it along the rim and around clusters walled in by long bridges.
Let built bridges do work
A bridge that is down rules out everything it would cross. In a crowded middle, one sure bridge can take a neighbour away from two or three islands at once, and often that settles them.
One step of what if
A few 13 × 13 boards stall even then. Pick a pair, imagine the bridge, and follow only what it forces. If that leaves an island that can’t be satisfied or a group sealed off, the opposite is true. One step is always enough: the generator never deals a board that needs deeper guessing.
About Bridges
Hashiwokakero, “build bridges!”, comes from Nikoli, the Japanese puzzle publisher, whose magazine first printed it in 1990. In English it usually goes by Bridges or Hashi.
The generator here grows a network outward from one island, a bridge at a time, and reads the numbers off it. Before a board is dealt, a solver works through it the way a person would: island counts, crossings and which bridges keep everything joined. Only boards it finishes without guessing are kept, which also proves the answer is unique. The 7 × 7 falls to counting alone. The larger boards are kept only if, somewhere, counting stalls and you have to think about connections, and a 13 × 13 may ask for a short “what if” now and then.
Questions
Can I always solve it without guessing?
Yes. Every board was solved by reasoning before you saw it. The trickiest steps on the larger boards are of the “if this bridge went here, that group would be cut off” kind.
Can bridges bend or run diagonally?
No. A bridge is a straight line across or down between two islands, with nothing between them.
Why can’t I lay a bridge here?
It would cross a bridge that is already built. The bridge in the way flashes red; take it down first if you want to go through.
What does count down do?
Each island shows how many bridges it still needs instead of its number, and a ✓ once it is full. It only changes the display, and your times count either way.
Is my board saved if I leave?
Yes, in your browser, one board per size. The clock pauses while the tab is hidden.
