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Start writing a document with Nurikabe strategies.

These are strategies that help me as a human solver of Nurikabe, and may
help in writing solvers.

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# File docs/Nurikabe_Strategies.txt

• Ignore whitespace
`+# vim:ft=txt`
`+`
`+#. Proximity analysis - some cells can be eliminated because they are not`
`+reachable (also consider that white cells of certain islands interfere`
`+with the extent of different islands).`
`+`
`+#. A straight row or column of black cells can be tested for black cells in`
`+its middle which will place two white cells and so one can conclude that`
`+they are white.`
`+`
`+#. If you have something like:`
`+`
`+   |[B][G][B]`
`+   |[G][G][B]`
`+   -----------------`
`+`
`+Then it is evident that the upper [G] (Gray/Unknown) has to be white, or else`
`+the lower [G]s will also all be black.`
`+`
`+#. Cells between two islands (either diagonally or in a one-cell spacing) must`
`+be black, or else the two islands will be joined.`
`+`
`+#. Filling-style strategy: if past a certain geometery (that is within the`
`+limits of its number), an island can only expand into one cell, then that`
`+cell will be white.`
`+`
`+#. If an Island reaches certain points that reaching them consumes all of its`
`+number, then its white edge points can be marked with black in the opposite`
`+direction to its extent.`
`+`