Author: David Blackman
Date: 03:29:22 08/11/99
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Start with an ACTUAL branching factor B (ie number of legal moves in each position). For depth N, minimax needs B ** N, apparent branching factor B. Plain alpha-beta with perfect move ordering needs k * B ** (N/2) where k is some small constant. So apparent branching factor is SQRT(B). With alpha-beta, nullmove pruning factor R, assume perfect move ordering, and every move that fails low gets cut by null move pruning, k * B ** (N/(R+2)) so apparent branching factor is B ** ( 1/(R+2) ) . For R=3 that is fifth root of B. With B=32 (probably a bit low for a midgame) that would be branching factor 2. I don't think you're going to get quite that good for most positions. First it assumes perfect move ordering. There might be tricks to get close to that. But also it assumes null move pruning works for almost all failing low moves, and i don't think that is true. A tempo is often worth a lot. Working out a model for effect of transposition tables on branching factor makes my head hurt. For now i'll just note that in practice it doesn't seem to help much in the middlegame. So i think apparent branching factors in the range 2 to 4 will be the rule for the moment, for normal mid-game positions. If you could get down close to 2 that would make depths of 15 to 20 ply feasible at normal tournament controls. I guess most of us aren't there yet.
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