White-Black Pearls 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.