programming-for-dummies

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Adversarial Search
This is how many games offer beginner, intermediate, and expert modes. The
expert mode may search 24 levels in each tree, the intermediate mode may
only search 12 levels, and the beginner mode may only search 4 levels. Because
the beginner mode searches far fewer levels than the expert mode, it runs
faster and doesn’t appear as smart as the intermediate or expert modes.
When a computer doesn’t search beyond a fixed number of levels, it can
miss potential problems that it might have discovered if it had just searched
a little bit deeper. This event is dubbed the horizon effect because the computer
doesn’t see the consequences beyond a certain move, so the problem
appears to lie outside the computer’s sight or beyond the horizon.
Alpha-beta pruning
Another way to speed up searching is to use alpha-beta pruning. The idea
behind this tactic is that it’s relatively pointless to keep searching a tree if a
potential move would represent a horrible choice. For example, in a chess
game, two possible moves might be moving the king into a position where
it’ll get checkmated in two moves or moving a pawn to protect the king.
If a computer always searches every possible move down to 12 levels, it
wastes time evaluating the bad move that results in the king getting checkmated
in two moves. To save time, alpha-beta pruning immediately stops
searching a tree the moment it detects a losing move and makes the computer
focus on studying good moves instead. As a result, the computer’s
time can be spent more profitably examining good moves.
For example, consider the tree in Figure 2-10; the boxes represent possible
moves for the computer, and the circles represent possible counter-moves
by a human opponent. The higher the value in each box or circle, the better
the move. So the human opponent will most likely choose moves with high
values. In response, the computer must look at the best possible countermoves
based on what the human opponent is likely to choose.
Figure 2-10:
Assigning
values to
possible
moves helps
the
computer
evaluate the
best
possible
move.
–45 7
102 87 52
62
6
–6
68 12
8
41
39