Factors that make difficult for Go progams to be strong include:
- Branching factor - simply put, there are 20 choices for the first half-move of a chess game, followed by 20 choices for the second half-move. For Go, these numbers are 361 and 360 respectively.
- Length of game - a typical chess game lasts 60 to 100 half moves; a typical go game can last for over 300.
This makes devising effective pruning strategies for Go very challenging.
Certain simpler games, like Othello/Reversi, with more limited move trees have programs that are almost impossible for a human to beat, picking the optimum play for a look-ahead of 24 half-moves, and perfect play for the last 40 half-moves.
