Nurikabe

Overview

Nurikabe divides a grid into numbered islands and a continuous sea. Each island consists of orthogonally connected white squares and contains exactly one clue equal to the island's size. Different islands cannot share an edge. The black sea must remain connected and may not contain a solid two-by-two block. Islands may touch at a corner without merging. These rules make the shape of empty space as important as the numbered regions themselves: most expert deductions are about where the sea is forced to flow, not where the islands want to grow. The locally hosted pzpr player supplies one introductory five-by-five puzzle with a checked reference solution. Its practice selector rotates or reflects that board; these are clearly labelled transformations of one puzzle, not a collection of independent levels. Check answer provides feedback without requiring an account or a remote service.

How to Play

Click or drag through unnumbered squares to shade them as sea. Secondary input marks squares that belong to white islands; on a phone, use the Secondary toggle for that action. Leave numbered cells as part of their islands. Build each island to exactly its clue size, then ensure the surrounding sea joins into one connected region and contains no two-by-two black block. Marking squares as known-white is not decoration: on any board beyond the smallest size, distinguishing confirmed island cells from undecided ones is what keeps the sea-connectivity checks manageable. Press Check answer to inspect the result. Undo revises a mark, while Restart clears the current orientation without changing its clues.

Tips & Strategy

A clue of one is a complete island, so all its edge neighbours must be sea. Two different clues separated by a single square often force that intervening square black to prevent their islands merging. Look for a two-by-two area with three confirmed sea cells: the fourth must remain white. At the same time, protect a route connecting separate patches of sea. A tempting island extension may seal a black cell into an isolated pocket. Count an island's available room as well as its current size; a large clue trapped in a small space exposes a mistaken sea boundary. Work from certainties outward and re-examine the sea after every few placements, because the black region's needs change as islands finish.

Opening Patterns

Certain clue arrangements resolve themselves before any growth begins:

• A clue of one is finished the moment you read it. Shade every square sharing an edge with it.
• Two clues touching diagonally at a corner force the two squares that share an edge with both clues to be sea; leaving either white would merge the islands.
• Two clues with exactly one empty square between them force that square black for the same reason.
• A square too far away to be reached by any island, counting one new cell per unit of remaining size, must belong to the sea.

These four patterns often shade a third of a small board immediately and give the larger clues their first real boundaries.

Protecting the Sea

The sea has two silent requirements, and most failed boards break one of them. First, connectivity: imagine the black region as water that must flow through every shaded square. Before extending an island, check that the black cells behind it still have a corridor to the rest of the sea. Second, the two-by-two rule: whenever three corners of a two-by-two block are sea, the fourth is forced white, and that white cell must belong to some island that can still reach it.

A concrete example shows the interaction. Suppose a completed island of size three sits as an L-shape, and the sea has filled three cells of a two-by-two block beside it. The fourth cell must be white, yet the finished island cannot absorb it without exceeding its clue. Some other island must reach that square, which can force its growth direction immediately. Beginners typically err by treating white as the default filler, forgetting that diagonal contact between islands is legal, or shading a narrow passage that quietly splits the sea in two.