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%\hypersetup{pdftitle=Meta-learning methods for analyzing Go playing trends}
%\hypersetup{pdfauthor=Josef Moudřík}

\begin{document}
%
% paper title
% can use linebreaks \\ within to get better formatting as desired
%\title{On Move Pattern Trends\\in  Large Go Games Corpus}
\title{Evaluating Go Game Records\\for Prediction of Player Attributes }

% use \thanks{} to gain access to the first footnote area
% a separate \thanks must be used for each paragraph as LaTeX2e's \thanks
% was not built to handle multiple paragraphs
\author{Josef~Moud\v{r}\'{i}k%
\thanks{J. Moud\v{r}\'{i}k is student at the Faculty of Math and Physics, Charles University, Prague, CZ.},~Petr~Baudi\v{s}%
\thanks{P. Baudi\v{s} is student at the Faculty of Math and Physics,
Charles University, Prague, CZ, and also does some of his Computer
Go research as an employee of SUSE Labs Prague, Novell CZ.}}
\maketitle

\begin{abstract}
We propose a~way of extracting a per-move evaluation of sets of Go game records.
The evaluations capture different aspects of the games such as patterns played
or statistics of sente/gote sequences (among others); using machine learning
algorithms, they can be used to predict arbitrary relevant target variables.
We apply this methodology to predict strength and playing style (e.g.
territoriality or aggressivity) of a player and make our predictor
available as an online tool, a part of the GoStyle project.
%% No, na tohle neni v clanku misto, pze to ma mit jen 8 stranek
% navic bych tyhle veci chtel zverejnit i samy o sobe, nejak dukladnejc,
% 
%By inspecting the dependencies between the evaluations and the target variable,
%we are able to tell which patterns are bad or good (in case of strength as the
%target variable), or which moves e.g. constitute the territorial style of play.
%%
We propose a number of possible applications including seeding real-work ranks
of internet players, aiding in Go study and tuning of Go-playing programs, or
contribution to Go-theoretical discussion on the scope of ``playing style''.
\end{abstract}


\section{Introduction}
The field of Computer Go usually focuses on the problem
of creating a~program to play the game, finding the best move from a~given
board position \cite{GellySilver2008}. We focus on analyzing existing game
records with the aim of helping humans to play and understand the game better
instead.

Go is a~two-player full-information board game played
on a~square grid (usually $19\times19$ lines) with black and white
stones; the goal of the game is to surround the most territory and
capture enemy stones. We assume basic familiarity with the game.

Since the game has a worldwide popularity, there exist large collections
of Go game records, both for amateur players and professionals
e.g. \citep{KGS,GoGoD}. 
So far, not much has been done in analysing these records using computers.
There are programs that serve as tools to study the opening phase of the game
by giving simple statistics of next move from professional
games~\citep{Kombilo,MoyoGo}.
The professional games have also been used in computer Go;
patterns from the professional games
are used as a heuristic to improve the tree
searching, e.g.~\citep{PatElo}. Apart from these, we are not aware of
any other uses.

Following up our initial research \citep{GoStyleArxiv},
we present a deeper approach. We extract different
kinds of information from the records to create a complex
evaluation of the game sample. The \emph{evaluation} is a vector
composed of independent features -- each of the features
captures different aspect of the sample. For example,
we use statistics of most frequent
local patterns played, statistics of high and low plays
in different game stages, etc.

Using machine learning, the evaluation of the sample
can be used to predict relevant variables. In this work
for instance,
the sample consists of games of a player
and we predict his strength or playing style.

This paper is organized as follows. Section~\ref{sec:feat}
presents the features comprising the evaluation.
Section~\ref{sec:mach} gives details about the machine
learning method we have used.
In Section~\ref{sec:expe} we give details about our
datasets -- for prediction of strength and style -- and
show how precisely can the prediction be conducted.
Section~\ref{sec:disc} discusses applications and future work.

\section{Feature Extraction}
\label{sec:feat}
This section presents the methods for extracting the evaluation 
vector (call it $ev$) from a set of games. Because we should
distinguish between both players in any particular game,
each game in the
set is accompanied by the color which specifies our player of
interest. The sample is therefore is regarded as a \emph{set
of colored games}, $GC = \{ (game_1, color_1), ...\}$. For example,
the $color_1$ specifies the player of interest in $game_1$.

The evaluation vector $ev$ is composed by concatenating several
sub-vectors we call \emph{features} -- examples include the
aforementioned local patterns or statistics of sente and gote
sequences. These will be detailed in the rest of this section.
Some of the explanations are simplified to fit the size of
the paper, please see \citep{Moudrik13} for precise details and algorithms.

\subsection{Raw Game Processing}
Firstly, we need to specify how do we process the games.
\footnote{
We use the standard \emph{.sgf} file format as input, \cite{SGF}.
}
We have used the Pachi Go
Engine~\citep{Pachi} which -- apart
from being quite a good performing Go Bot -- allows to extract
raw information from each game on a per-move basis.
For each move,
Pachi outputs a list of key-value pairs regarding the current move:

\begin{itemize}
    \item \textbf{atari flag} --- whether the move put enemy stones in atari,
    \item \textbf{atari escape flag} --- whether the move saved own stones from atari,
    \item \textbf{capture} --- number of enemy stones the move captured,
    \item \textbf{contiguity to last move} --- the gridcular
        distance \footnotemark{\ref{grid}} from the last move,
    \item \textbf{board edge distance} --- the distance from
        the nearest edge of the board,
    \item \textbf{spatial pattern} --- configuration of stones around the played move.
\end{itemize}

We use this information to compute the higher level features given below.
The spatial pattern pictures positions of stones around the current move up to
a certain distance.\footnote{
    \label{grid}
    The distance is given by the {\em gridcular} metric
$d(x,y) = |\delta x| + |\delta y| + \max(|\delta x|, |\delta y|)$, which produces
a circle-like structure on the Go board square grid \cite{SpatPat}.
Spatial patterns of sizes 2 to 6 are regarded.
}

\subsection{Patterns}
The first feature collects a statistics of $N = 400$ most frequently ocurring
spatial patterns (together with both atari flags). The list of the $N$ most frequently
played patterns is computed beforehand from the whole database of games.

Given a set of of colored games $GC$ we then count how many times was each of the $N$
patterns played -- thus obtaining a vector $c$ of counts ($|c| = 400$).
With simple occurences count however, particular counts $c_i$ increase proportionally to 
number of games in $GC$. To maintain invariancy under the number of games in the sample,
a normalization is needed. We do this by dividing the $c$ by $|GC|$, though other schemes
are possible, see \citep{Moudrik13}.

\subsection{$\omega$-local Sente and Gote Sequences}
Because the concept of sente and gote is very important in real games, we devised
a statistics which tries to capture distribution of sente and gote plays in the games
from the sample. Because deciding what moves are sente or gote can be hard even
for human players, we restricted ourselves to what we call $\omega$-local (sente
and gote) sequences. The simplification has a clear assumption -- the responses to
a sente move are always local. We say, that a move is $\omega$-local (with respect
to the previous move) if its gridcular distance from previous move
is smaller than a fixed number $\omega$ (in this work, we used $\omega=10$\footnote{
    Of course, different values are possible, see \citep{Moudrik13} for details.
}).
Of course, this assumption might not always hold, but
the feature proves to be useful nonetheless.

We than partition each game into $\omega$-local sequences (that is, each move in the
sequence is $\omega$-local with respect to its directly previous move) and observe
whether the player who started the sequence is different from the player who ended it.
If it is so, the $\omega$-local sequence is said to be sente for player who started it
because he gets to play somewhere else first (tenuki). Similarly if the player who 
started the sequence had to respond at last we say that the sequence is gote for him.
Based on this partitioning, we can count the average number of sente and gote
sequences per game from the sample $GC$. These two numbers, along with their difference,
form the second feature.

\subsection{Border Distance}
The third feature is a two dimensional histogram, counting the average number of moves
in the sample played low or high in different game stages. The original idea was to help
to distinguish between territorial and influence based moves in the opening.

The first dimension is specified by
the move's border distance, the second one by the number of the current move. The size of each
dimension is given by intervals dividing the domains.
We use
$$ByMoves = \{ \langle1, 10\rangle, \langle 11, 64\rangle, \langle 65,200\rangle, \langle 201, \infty)\}$$ 
for the move coordinate -- the motivation is to (very roughly) distinguish
between opening (say first ten moves), early middle game (moves 11-64), middle game
and endgame.
The border distance dimension is given by
$$ByDist = \{ \langle1, 2\rangle, \langle 3 \rangle, \langle4\rangle, \langle 5, \infty)\}$$
(distinguishing between first 2 lines, 3rd line of territory, 4th line of influence and
higher plays for the rest).

If we use the $ByMoves$ and $ByDist$ intervals to divide the domains, we obtain a histogram
of total $|ByMoves| * |ByDist| = 16$ field. For each move, we increase the count in the
appropriate histogram fields.  In the end, the whole histogram is normalized
to establish invariancy under the number of games scanned by dividing the 
histogram elements by $|GC|$. These 16 numbers form the third feature.

\subsection{Captured Stones}
Apart from the border distance feature, we realized a two-dimensional histogram
which counts numbers of captured stones in different game stages. The motivation is
simple -- especially beginners tend to capture stones because ``they could'' instead of
because it is the ''best move''. For example, in the opening such capture might
be a grave mistake.

As before, one of the dimensions is given by intervals 
$$ByMoves = \{ \langle1, 60\rangle, \langle 61, 240\rangle, \langle 241, \infty)\}$$
which try to specify the game stages (opening, middle game, endgame).
The second dimension has a fixed size of three bins. Along the number of captives
of the player of interest (the first bin), we also count the number of his
opponent's captives (the second bin) and a difference between the two numbers
(the third bin). Together, we obtain a histogram of $|ByMoves| * 3 = 9$ elements.
These 9 numbers (again normalized by dividing by $|GC|$) are the output of the fourth
feature.

\subsection{Win/Loss Statistics}
Finally, we came up with a simple feature which makes statistics of
wins and losses and whether they were by points or by resignation\footnote{
    We disregard forfeited, unfinished or jigo games in this feature
    because the frequency of these events is so small it would
    require a very large dataset to utilize them reliably.
}. 
For example, quite a lot of weak players continues playing already lost games
until the end, mainly because their counting is not very good (they do not
know there is no way to win), while professionals do not hesitate to resign 
if they think that nothing can be done.

For the colored games of $GC$ we count how many times did the player of interest:
\begin{itemize}
    \item win standardly,
    \item win by resignation,
    \item lost standardly,
    \item and lost by resignation.
\end{itemize}
Again, we divide these four numbers by $|GC|$ to maintain the invariancy under number of games
in $GC$. Furthermore, for the games won or lost standardly we count:
\begin{itemize}
    \item average number of points the player won by for won games,
    \item average number of points he lost by for lost games.
\end{itemize}
The six numbers form the last feature.

\section{Machine Learning}
\label{sec:mach}
So far, we have learned how we can turn a set of coloured games $GC$ into
an evaluation. Now, we are going to study how to utilize the evaulation.
If we are to predict various player attributes, we need some input data
to learn from. Suppose we have a dataset $D$ consisting
of pairs $D=\{ (GC_i, y_i),...\}$, where $GC_i$
corresponds to a set of colored games of $i$-th player and $y_i$ is the
target attribute. The $y_i$ might be fairly arbitrary, as long as it has
\emph{some} relation to the $GC_i$. For example, $y_i$ might be $i$'s strength.

Now, lets denote our evaluation process we presented before as $eval$ and
let $ev_i$ be evaluation of $i$-th player, $ev_i = eval(GC_i)$. Then,
we can transform $D$ into $D_{ev} = \{(ev_i, y_i),. \}$, which forms
our training data.
The task of our machine learning algorithm is to generalize the knowledge
from the dataset $D_{ev}$ to predict correct $y_X$ even to previously unseen $GC_X$.
In the case of strength, we might therefore be able to predict strength $y_X$
of an unknown player $X$ given a set of his games $GC_X$ (from which we can
compute the evaluation $ev_X$).

In this work, we have used a bagged artificial neural network
to learn the dependency.
Neural networks are a standard technique in machine learning. The network is
composed of simple computational units which are organized in a layered topology.
Please see the monograph by \citet{haykin_nn} to learn more.
We have used a simple feedforward neural network with 20 hidden units, trained
using the RPROP algorithm \citep{Riedmiller1993}.

The bagging \citep{breimanbag96} is a method which combines an ensemble of
$N$ models (trained on differently sampled data) to improve their
performance and robustness. In this work, we used $N=20$. Please refer to the
paper to learn more about bagging.

\subsection{Measuring the Performance}
Given a dataset $D_{ev}$, it would be nice to estimate performance of a given machine
learning algorithm (in our case the bagged neural network). A performance measure
allows to compare different algorithms and give estimates of method precision for
unseen inputs. A standard way to do this is to divide the $D_{ev}$ in to training
and testing parts and compute the error of the method on the testing part.

A commonly used measure is the mean square error ($MSE$) which estimates variance of
the error distribution. We use its square root ($RMSE$) which is an estimate of
standard deviation of the predictions.

$$ MSE = \frac{1}{|Ts|} \sum_{(ev, y) \in Ts}{ (predict(ev) - y)^2} $$

Where the machine learning model $predict$ is trained on the
training data $Tr$ and $Ts$ denotes the testing data.
Now we will describe how do we split the data into testing and training for the
error estimation to be robust.

\subsubsection*{Cross-Validation}

Cross-validation is a standard statistical technique for robust estimation of parameters.
The idea is to split the data into $k$ disjunct subsets (called \emph{folds}), and then
iteratively compose the training and testing sets and measure errors.
%In each of the $k$ iterations, $k$-th fold is chosen as the testing data, and
%all the remaining $k-1$ folds form the training data. The division into the folds is
%done randomly, and so that the folds have approximately the
%same size.
Refer to~\citep{crossval} to learn more. In this work, we have used 10-fold
cross validation.

\section{Experiments and Results}
\label{sec:expe}

\subsection{Strength}
One of two major domains we have tested our framework on is the prediction of player
strengths.
\subsubsection*{Dataset}
We have collected a large sample of games from the publicly available
archives of the Kiseido Go server~\citep{KGSArchives}.
The sample consists of over 100 000 records of games in the \emph{.sgf} format~\citep{SGF}.

For each rank $r$ in the range of 6-dan to 20-kyu, we gathered a
list of players $P_r$ of the particular rank. To avoid biases caused by
different strategies, the sample only consists of games played on $19 \times 19$ goban without
handicap stones.
The set of colored games $GC_p$ for a~player $p \in P_r$ consists of the games player $p$
played when he had the rank $r$. We only use the $GC_p$ if the number of
games is not smaller than 10 games; if the sample is larger than 50 games, we 
randomly choose a subset of the sample (the size of subset is uniformly randomly
chosen from interval $\langle 10, 50\rangle$).\footnote{
   By cutting the number of games to a fixed number (say 50) for large
   samples, we would create an artificial disproportion in sizes of $GC_p$,
   which could introduce bias into the process.
}

For each of the 26 ranks, we gathered 120 such $GC_p$.
The target variable $y$ we learn directly corresponds to the ranks:
$y=20$ for rank of 20-kyu, $y=1$ for 1-kyu, $y=0$ for 1-dan, $y=-5$
for 6-dan, other values similarly. (With increasing strength, the $y$
decreases.)

\subsubsection*{Results}

\subsection{Style}

The second domain is the prediction of different aspects of player styles.

\subsubsection*{Dataset}
The collection of games in this dataset comes from the Games of Go on Disk database by \citet{GoGoD}.
This database contains more than 70 000 games, spanning from the ancient times
to the present.

We chose a small subset of well known players (mainly from the 20th century) and
asked some experts (professional and strong amateur players)
to evaluate these players using a questionnaire. The experts (Alexander
Dinerchtein 3-pro, Motoki Noguchi 7-dan,
Vladim\'{i}r Dan\v{e}k 5-dan and V\'{i}t Brunner 4-dan)
were asked to value the players on four scales, each ranging from 1 to 10.

%\begin{table}[h!]
\begin{center}
%\caption{Styles}
\begin{tabular}{|c|c|c|}
\hline
\textbf{Style} & \textbf{1} & \textbf{10}\\ \hline
Territoriality & Moyo & Territory \\
Orthodoxity & Classic & Novel \\
Aggressivity& Calm & Fighting \\
Thickness & Safe & Shinogi \\ \hline
\end{tabular}
\end{center}
%\caption[Definition of the style scales]{
%The definition of the style scales.
%}
%\label{tab:style_def}
%\end{table}

The scales try to reflect
some of the traditionally perceived playing styles.\footnote{
    Refer to~\citet{GoGoD:styles}, or~\citet{senseis:styles} to grasp the concept deeper.
}
For example, the first scale (\emph{territoriality})
stresses whether a player prefers safe, yet inherently smaller territory (number 10 on the scale),
or roughly sketched large territory (\emph{moyo}, 1 on the scale), which is however insecure. 
For each of the selected professionals, we took 192 of his games from the GoGoD database
at random. We divided these games (at random) into 12 colored sets $GC$ of 16 games.

The target variable (for each of the four styles) $y$ is given by average of the answers of
the experts. Results of the questionnaire are published in~\citep{style_quest}.

\subsubsection*{Results}


\section{Discussion}
\label{sec:disc}

The results in both the domains showed, that our evaluations are useful in predicting
different kinds of player attributes. This might have a number of possible applications.

So far, we have utilized some of our findings in an online web
application\footnote{\url{http://gostyle.j2m.cz/webapp.html}}. Based on data submitted
by an user, it computes his evaluation and predicts his playing style
and recommends relevant professional players to review.

Other possible applications include helping the ranking algorithms to converge faster ---
usually, the ranking of a player is determined from his opponents' ranking by looking
at numbers of wins and losses (e.g. by computing an ELO rating). Our methods might improve this
by including the domain knowledge.
Similarly, a computer Go program can quickly classify the level of its
human opponent based on the evaluation from their previous games
and auto-adjust its difficulty settings accordingly
to provide more even games for beginners.

Also, it is possible to study dependencies between single elements of the evaluation vector
and the target variable $y$ directly. By pinpointing e.g. the patterns
that correlate most strongly with small strength (players who play them are weak), we can 
warn the user not to play these. We have made some initial research into this in~\citep{Moudrik13},
we do not present these results here, because of space constraints.

\section{Conclusion}
\label{sec:conc}
This article presents a method for evaluating a player based on a sample of his games.
These summary evaluations turn out to be useful in many cases --- they allow us to predict
different player attributes (such as strength, or playing style) with reasonable accuracy.
We hope, that the applications of these findings can help to improve both human and computer
understanding in the game of Go.

\section{Implementation}
\label{sec:impl}

The code used in this work
is released online as a part of GoStyle project~\citep{GoStyleWeb}.
The majority of the source code is implemented in
the Python programming language~\citep{Python27}.

The machine learnin part was realized using the
Orange Datamining suite~\citep{curk05}, with the exception of
the Fast Artificial Neural Network library FANN~\citep{Nissen2003}.
We used the Pachi Go engine~\citep{Pachi} for the raw game processing.

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\end{document}