Author: Steven Edwards
Date: 07:31:03 02/16/04
Symbolic: The TNS (Thousand Node Search) The idea of limiting the cognitive search in Symbolic to under a thousand nodes is based upon psychological studies that suggest top level human chessplayers usually visualize between 100 and 1,000 positions per move in complex middlegames. My personal time control upper limit preference for non-blitz chess is a minute per move, and so the resulting target figure for node frequency is about 20 Hz. One idea here is that the target frequency remain somewhat invariant of the host hardware. On faster machines, the effort expended on non-search chess knowledge can be increased, while on slower hardware, it can be lessened. Similar throttling can be applied for different time controls on the same hardware. A distinction here between the large pool of iterative A/B searchers (and their hardware brethren) vs programs like Paradise and Symbolic is the purpose of the search itself. For the descendants of Slate and Atkin program Chess 4.x, the main purpose is discovery. For Paradise, and for Symbolic to a slightly lesser extent, the main purpose of the search is plan verification. It is important to note that Symbolic is not a "selective search" program is the commonly used sense of the phrase. A selective search program is one that employs the Shannon type B strategy of reducing the full width search at each node by applying a plausibility filter or by having a plausible move generator. Shannon type B is the same as type A in that the purpose of the search is discovery; the topography of the resulting depth first search trees may differ in mean height and width, but the reasons for searching any particular node are the same. Symbolic, like Paradise, should only expand a search node (i. e, move generation plus selection) if it has a good reason to do so. The phrase "good reason" is somewhat vague (at this point in development), but the one thing it does not mean is "there's still time on the clock, so let's try another move/another iteration". Instead, Symbolic will always have an active plan that will determine which nodes to expand. In some cases, multiple moves at a given node will be conforming to the current plan. In others, perhaps no move will suffice, so the plan must be modified or abandoned. As Symbolic keeps the entire search tree available at all times, a node may be revisited with different plans. And although the exact implementation details of the above phrase "good reason" are not well defined at this point, it is guaranteed that each "good reason" will have a natural language representation. Thus, another reason for the TNS node count limitation: Symbolic will produce an explanation audit trail with all of the good reasons, in English, for each decision made during the search and this document has to be easily readable (by me) for the purposes of tutoring the program. A multi-megabyte dump will not be useful, but a five or six page synopsis should work well.
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