# Type B Strategy

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The Type B Strategy, proposed in 1949 by Claude Shannon in his groundbreaking publication Programming a Computer for Playing Chess [1], is a selective approach to search minimax trees considering only a subset of plausible moves in contrast to the brute-force Type A strategy.

# Quotes

from Shannon's Programming a Computer for Playing Chess:

```From these remarks it appears that to improve the speed and strength of play the machine must:
```
1. `Examine forceful variations out as far as possible and evaluate only at reasonable positions, where some quasi-stability has been established.`
2. `Select the variations to be explored by some process so that the machine does not waste its time in totally pointless variations.`
```A strategy with these two improvements will be called a type B strategy. It is not difficult to construct programs incorporating these features. For the first we define a function g(P) of a position which determines whether approximate stability exists (no pieces en prise, etc.). A crude definition might be:
```
```                | 1 if any piece is attacked by a piece of lower value,
g(P) =    /    or by more pieces then defences of if any check exists
\    on a square controlled by opponent.
| 0 otherwise.
```
```Using this function, variations could be explored until g(P)=0, always, however, going at least two moves and never more say, 10.
```

# Type B programs

Most early chess programs were Type B and used a selective width for a maximum amount of plausible moves to be tried. Bernstein used {7, 7, 7, 7}, later programs chose width dependent from depth, Kotok-McCarthy had {4, 3, 2, 2, 1, 1, 1, 1, 0, 0}, while Greenblatt's Mac Hack used {15, 15, 9, 9, 7, ...}, and CHAOS carried out a selective search with iterative widening. With the advent of brute-force alpha-beta, and programs like Tech, Kaissa and Chess 4.5 in the early 70s, the era of the former dominating Type B programs drew to a close. Today most programs are closer to Type A, but have some characteristics of a Type B due to selectivity.

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