This text begins a series of blog posts with a common theme: that of explaining what the proverb “sente gains nothing” means. My design is to start the explanation from easier, endgame-technical examples and little by little work towards more difficult concepts. If everything goes according to plan, at the end of the series I should have conveyed the idea of the proverb without (hopefully) having to explain it in abstract terms at all! (Having read essays on the art of human rationality on LessWrong, I wanted to try a similar concept on Go.)
At the same time, through the series I hope to improve on my writing skills. If the blog posts turn out well, I will probably refine them further into a part of an endgame theory book I have been planning for some time.
For readers unfamiliar with the terminology: please bear with me! I will also explain the technical meanings of sente and its opposite, gote in following posts.
Part 1: Expected territory
Dia 1 shows a shape that could often come up in the late parts of a Go game. At some point late in the game, either White or Black plays at A, and then play switches elsewhere and soon the game is over.
For the purposes of this blog post, we will assume all black and white stones are alive at all times.
When asked, “How big is a move at A?” the most common answer seems to be: “One point! If Black plays A, Black has one point of territory, and if White plays A, Black has zero. The difference between these scenarios (Dia 2 and Dia 3) is one point, and thus A is a one-point move!”
If the same person is pressed for how much territory Black should expect back in Dia 1, the answer we are likely to get is, “Either Black has 1 or 0 points, but we cannot know which.”
Neither of these answers is wrong, but they are also not exact, and I think the way of thinking they represent is prone to confusion.
If Dia 1 was the last endgame point available on the board, then indeed Black would either get 1 or 0 point, depending on whose turn it is, and the game would end there. Throughout most of the game, however, there are a big number of valuable moves available, many of which are of equal sizes.
Enter my favourite trick for endgame analysis: shape duplication!
Consider Dia 4: A and B are clearly moves of equal size, both surrounding “either 1 or 0 points of black territory”. Inside the local situation of Dia 4, neither player can expect to get both A and B for themselves. Remember, a player gets only one move at a time! Thus, either Black gets A and White gets B, or Black gets B and White gets A. This does not change whether Black or White has the turn to move first. In Dia 4, then, Black always has exactly one point of territory.
Going back to Dia 1, which holds half the territory of the black shape in Dia 4, can you guess how much territory Black has?
That’s right! Half a point!
But wait, the counting gets trickier from here! If Black already has half a point in Dia 1, what does that make the value of a move at A?
Comparing the final points difference between White or Black having gotten A indeed gives an idea of the size of an endgame move, but that number does not tell us how the territory balance of the game changes. We should originally value the black territory in Dia 1 at 0.5 points. If White plays A, the black territory shrinks to zero, meaning a change of 0.5 points in the game score. Likewise, if Black plays A, the black territory increases to one point, similarly meaning a change of 0.5 points. In other words, the value of a single stone at A in Dia 1 is half a point!
This concludes Part 1 of the series, but I have homework for you!
How many points does Black expect in Dia 5? You will not need any tips other than the techniques above!