SSS* - Algorithm

Algorithm

There is a priority queue OPEN that stores states or the nodes, where - node identificator (Dewey's notation is used to identify nodes, is a root), - state of the node (L - the node is live, which means it's not solved yet and S - the node is solved), - value of the solved node. Items in OPEN queue are sorted descending by their value. If more than one node has the same value of, a node left-most in the tree is chosen.

OPEN := { (e,L,inf) } while (true) // repeat until stopped pop an element p=(J,s,h) from the head of the OPEN queue if J == e and s == S STOP the algorithm and return h as a result else apply Gamma operator for p

operator for is defined in the following way:

if s == L if J is a terminal node (1.) add (J,S,min(h,value(J))) to OPEN else if J is a MIN node (2.) add (J.1,L,h) to OPEN else (3.) for j=1..number_of_children(J) add (J.j,L,h) to OPEN else if J is a MIN node (4.) add (parent(J),S,h) to OPEN remove from OPEN all the states that are associated with the children of parent(J) else if is_last_child(J) // if J is the last child of parent(J) (5.) add (parent(J),S,h) to OPEN else (6.) add (parent(J).(k+1),L,h) to OPEN // add state associated with the next child of parent(J) to OPEN

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