On: AlphaGo beats Lee Sedol again in match 2 of 5

[sciyoshi]

I wonder if AlphaGo has some specialized knowledge to handle ladders, where stones can have an effect at a distance that might only come into play after 20 moves.

[_vk_ → sciyoshi]

>I wonder if AlphaGo has some specialized knowledge to handle ladders

Yes. A representation of ladders is among the input features of its neural networks.

https://gogameguru.com/i/2016/03/deepmind-mastering-go.pdf

Stone colour 3 Player stone / opponent stone / empty

Ones 1 A constant plane filled with 1

Turns since 8 How many turns since a move was played

Liberties 8 Number of liberties (empty adjacent points)

Capture size 8 How many opponent stones would be captured

Self-atari size 8 How many of own stones would be captured

Liberties after move 8 Number of liberties after this move is played

Ladder capture 1 Whether a move at this point is a successful ladder capture

Ladder escape 1 Whether a move at this point is a successful ladder escape

Sensibleness 1 Whether a move is legal and does not fill its own eyes

Zeros 1 A constant plane filled with 0

Player color 1 Whether current player is black

(The number is how many 19x19 planes the feature consists of.)

[EGreg → sciyoshi]

20 moves may sound like a huge number of variations but when you prune things early, it can be quite manageable. The alphabeta algorithm in chess does pruning quite a lot.