Mobility Based Evaluation

So far we have introduced the principles of board location evaluation and, along the way, we touched on some aspects of pattern evaluation. Mobility based evaluation considers the number of potential moves as a means for determining move strength. Consider this example:

board8

As you can see, black has 2 possible moves [A,B]. The most important issue in choosing between these two moves turns out to be a mobility issue rather than board location values. Recalling from the previous section on board value locations, owning board position A has a value of 50 while board position B has a value of -1. So it would seem that a move to position A would clearly be the best. However, it can be shown that a move to location A is not the best option.

A:   board9 arrow board11

White now has two possible moves available to him which are denoted as the red X's. In fact, one of the moves allows white to gain a corner along with 5 irrecoverable pieces along the top side, making white the clear winner of this exchange. Now, lets consider the alternate move to location B :
B: board10

Notice that this move leaves white absolutely no counter moves. In this example, making the move decision based on minimizing how many moves the opponent is left with is clearly the way to go. This example translates into an important rule: Force your opponent to forfeit their next turn whenever possible. This idea can also be generalized into a set of central mobility goals:
  1. Minimize your opponents move options.
  2. Less importantly, maximize your own move options.
Evaluating the mobility issues of each move option will not only increase the general strength of an algorithm but it could also be used to help parse decision trees as well.

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Copyright© Marc Mandt, 2001
mmandt@mindspring.com