Overlapping Alldifferent Constraints and the Sudoku puzzle

Combinatorial problems can be modeled as constraint satisfaction problems (CSP). Modeling and resolution of CSP is often strengthened by global constraints (e.g., Alldiff constraint). We propose a formal system and some propagation rules for reducing domains of variables appearing in several Alldiff global constraints. We illustrate our approach on the well-known Sudoku puzzle which presents 27 overlapping Alldiff constraints in its 9× 9 standard size. We also present some preliminary results we obtained in CHR and GeCode.