Decomposition method (constraint satisfaction): Difference between revisions

Revision as of 14:39, 2 January 2023

Revision as of 00:23, 12 May 2024

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In a tree, every edge breaks the graph in two parts. The constraint passed along an edge tells how the part of the originating end of the edge affects the variables of the destination node. In other words, a constraint passed from node <math>i</math> to node <math>j</math> tells how the nodes "on the side of <math>i</math>" constrain the variables of node <math>j</math>.

If the variables of these two nodes are <math>X_i</math> and <math>X_j</math>, the nodes on the sizeside of <math>i</math> do not affect all variables <math>X_j</math> but only the shared variables <math>X_i \cap X_j</math>. As a result, the influence on <math>X_j</math> of the nodes on the side of <math>i</math> can be represented as a constraint on variables <math>X_i \cap X_j</math>. Such a constraint can be seen as a "summary" of how a set of nodes affects another node.

The algorithm proceeds from the leaves of the tree. In each node, the summaries of its children (if any) are collected. These summary and the constraint of the node are used to generate the summary of the node for its parent. When the root is reached, the process is inverted: the summary of each node for each child is generated and sent it. When all leaves are reached, the algorithm stops.