What is Principal variation search?

Principal variation search is a negamax algorithm that can be faster than alpha-beta pruning. Like alpha-beta pruning, NegaScout is a directional search algorithm for computing the minimax value of a node in a tree. It dominates alpha-beta pruning in the sense that it will never examine a node that can be pruned by alpha-beta; however, it relies on accurate node ordering to capitalize on this advantage.

NegaScout works best when there is a good move ordering. In practice, the move ordering is often determined by previous shallower searches. It produces more cutoffs than alpha-beta by assuming that the first explored node is the best. In other words, it supposes the first node is in the principal variation. Then, it can check whether that is true by searching the remaining nodes with a null window , which is faster than searching with the regular alpha-beta window. If the proof fails, then the first node was not in the principal variation, and the search continues as normal alpha-beta. Hence, NegaScout works best when the move ordering is good. With a random move ordering, NegaScout will take more time than regular alpha-beta; although it will not explore any nodes alpha-beta did not, it will have to re-search many nodes.

Alexander Reinefeld invented NegaScout several decades after the invention of alpha-beta pruning. He gives a proof of correctness of NegaScout in his book.

Another search algorithm called SSS* can theoretically result in fewer nodes searched. However, its original formulation has practical issues and nowadays most chess engines still use a form of NegaScout in their search. Most chess engines use a transposition table in which the relevant part of the search tree is stored. This part of the tree has the same size as SSS*'s OPEN list would have. A reformulation called MT-SSS* allowed it to be implemented as a series of null window calls to Alpha-Beta that use a transposition table, and direct comparisons using game playing programs could be made. It did not outperform NegaScout in practice. Yet another search algorithm, which does tend to do better than NegaScout in practice, is the best-first algorithm called MTD, although neither algorithm dominates the other. There are trees in which NegaScout searches fewer nodes than SSS* or MTD and vice versa.