Constraint Propagation on a 27-node graph coloring problem

Animation by Andrew Moore

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The CONSTRAINT PROPAGATION algorithm on a 3-color 
graph-coloring problem with 27 nodes.

Tries BLUE then RED then BLACK.

Little dots denote the availability lists
for the nodes.

Notice that unlike forward checking search, Constraint
Propagation realizes very early on (on its third step)
that (row=bottom+1,col=rightmost-1) must not be black
and so (row=bottom,col=4) must not be red. It does
better than forward checking and MUCH better than

See Constraint Satisfaction Lecture notes at

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