Masyu

Masyu (ましゅ, Mashu, IPA [maɕu͍]; translates as "evil influence") is a type of path-drawing logic puzzle designed and published by Nikoli. It is played on a rectangular grid of squares, some of which contain circles; each circle is either "white" (empty) or "black" (filled). The goal is to draw a single continuous non-intersecting loop that properly passes through all circled cells. The loop must "enter" each cell it passes through from the center of one of its four sides and "exit" from a different side; all turns are therefore 90 degrees. The two varieties of circle have differing requirements for how the loop must pass through them:


 * White circles must be traveled straight through, but the loop must turn in the previous and/or next cell in its path.
 * Black circles must be turned upon, but the loop must travel straight through the next and previous cells in its path.

Variations include those that involve grey circles, which either are ambiguously white or black, or signify a turn that is not 90 degrees, and puzzles in which certain spaces are required to be visited by the path.