Blockage Detection in King and Pawn Endings Omid David Tabibi 1 , Ariel Felner 1 , and Nathan S. Netanyahu 1,2 1 Department of Computer Science, Bar-Ilan University, Ramat-Gan 52900, Israel {davoudo,felner,nathan}@cs.biu.ac.il http://www.cs.biu.ac.il/∼{davoudo,felner} 2 Center for Automation Research, University of Maryland, College Park, MD 20742, USA nathan@cfar.umd.edu Abstract. The conventional search methods using static evaluation at leaf nodes are liable to missing the fact that no win goal exists in certain positions. Blockage positions, in which neither side can penetrate into the opponent’s camp, are a prominent example of such positions. Deep search cannot detect the existence of a blockage, since its judgment is based solely on static evaluation, without taking the goals into consideration. In this paper we introduce a blockage detection method, which man- ages to detect a large set of blockage positions in pawn endgames, with practically no additional overhead. By examining different aspects of the pawn structure, it checks whether the pawns form a blockage which pre- vents the king from penetrating into the opponent’s camp. It then checks several criteria to find out whether the blockage is permanent. 1 Introduction In the early days of computer-chess, many were pessimistic about the prospects of creating grandmaster level chess playing programs through the conventional search methods which incorporate chess knowledge only in the evaluation func- tion applied to the leaf nodes. As de Groot [3] mentioned, there are positions which computers have trouble evaluating correctly through exhaustive search, yet humans recognize at a glance. For many years the pawn endgames remained the toughest stage of the game for computers, as many combinations require very deep searches, infea- sible for the computers to conduct. There have been several efforts to improve the pawn endgame play of computers in these positions, notably PEASANT [8], co-ordinate squares [2], and chunking [1]. But since then, most such positions have succumbed to the sheer processing power of computers. Nowadays, tour- nament playing programs search to depths of tens of plies in pawn endgames, and thus, manage to solve many positions once thought as never solvable by computers. For example, Newborn, the author of PEASANT, estimated that the position shown in Figure 1 (Fine #70 [4]) required 25,000 hours for the com- puters to solve [8]. Yet today it takes less than a second for most programs to search deep enough to find the correct 1.Kb1 move.