  Su-Doku, a popular combinatorial puzzle, provides an excellent testbench for
heuristic explorations. Several interesting questions arise from its
deceptively simple set of rules. How many distinct Su-Doku grids are there? How
to find a solution to a Su-Doku puzzle? Is there a unique solution to a given
Su-Doku puzzle? What is a good estimation of a puzzle's difficulty? What is the
minimum puzzle size (the number of "givens")?
  This paper explores how these questions are related to the well-known
alldifferent constraint which emerges in a wide variety of Constraint
Satisfaction Problems (CSP) and compares various algorithmic approaches based
on different formulations of Su-Doku.
