The Move-Decision Strategy of Indigo

This paper describes the move decision strategy of Indigo. By using the example of Indigo, the paper shows that the move decision process of a Go program can be very different from the processes used in other games with lower complexity than the complexity of Go, even if the basic modules are conventional (move generator, evaluation function and tree search). Indigo uses them in a specific way, adapted to computer Go, which may be of interest for researchers on other mind games as complex as Go. The evaluation function can be “quick”, “slow” or “strategic”. It may or may not include local tree search. The move generation brings about different kinds of moves : “urgent” moves, “life and death” moves and “calm” moves. Urgent moves are statically qualified with a global urgency. A two-player quiescence search verifies that the urgent move does not decrease the position evaluation. Calm moves are used within two-player selective global search at a very low depth. Besides, Indigo also uses single-agent search to refine the strategic importance of goals. Lastly, Indigo chooses either the calm move, the life and death move or the urgent move to be the global move. keywords: computer go, quick & slow evaluation, strategic evaluation, urgent move selection, single-agent & two-agent search.