5. Analyzing GNU Go’s moves

In this chapter we will discuss methods of finding out how GNU Go understands a given position. These methods will be of interest to anyone working on the program, or simply curious about its workings.

In practice, most tuning of GNU Go is done in conjunction with maintaining the ‘regression/’ directory (see section Regression testing).

We assume that you have a game GNU Go played saved as an sgf file, and you want to know why it made a certain move.

5.1 Interpreting Traces

A quick way to find out roughly the reason for a move is to run

gnugo -l filename -t -L move number

(You may also want to add ‘--quiet’ to suppress the copyright message.) In GNU Go 3.6, the moves together with their reasons are listed, followed by a numerical analysis of the values given to each move.

If you are tuning (see section Tuning the Pattern databases) you may want to add the ‘-a’ option. This causes GNU Go to report all patterns matched, even ones that cannot affect the outcome of the move. The reasons for doing this is that you may want to modify a pattern already matched instead of introducing a new one.

If you use the ‘-w’ option, GNU Go will report the statuses of worms and dragons around the board. This type of information is available by different methods, however (see section Debugging on a Graphical Board, see section Colored Display).

5.2 The Output File

If GNU Go is invoked with the option ‘-o filename’ it will produce an output file. This option can be added at the command line in the Go Modem Protocol Setup Window of CGoban. The output file will show the locations of the moves considered and their weights. It is worth noting that by enlarging the CGoban window to its fullest size it can display 3 digit numbers. Dragons with status DEAD are labelled with an ‘X’, and dragons with status CRITICAL are labelled with a ‘!’.

If you have a game file which is not commented this way, or which was produced by a non-current version of GNU Go you may ask GNU Go to produce a commented version by running:

gnugo --quiet -l <old file> --replay <color> -o <new file>

Here <color> can be ’black,’ ’white’ or ’both’. The replay option will also help you to find out if your current version of GNU Go would play differently than the program that created the file.

5.3 Checking the reading code

The ‘--decide-string’ option is used to check the tactical reading code (see section Tactical reading). This option takes an argument, which is a location on the board in the usual algebraic notation (e.g. ‘--decide-string C17’). This will tell you whether the reading code (in ‘engine/reading.c’) believes the string can be captured, and if so, whether it believes it can be defended, which moves it finds to attack or defend the move, how many nodes it searched in coming to these conclusions. Note that when GNU Go runs normally (not with ‘--decide-string’) the points of attack and defense are computed when make_worms() runs and cached in worm.attack and worm.defend.

If used with an output file (‘-o filename’) ‘--decide-string’ will produce a variation tree showing all the variations which are considered. This is a useful way of debugging the reading code, and also of educating yourself with the way it works. The variation tree can be displayed graphically using CGoban.

At each node, the comment contains some information. For example you may find a comment:

attack4-B at D12 (variation 6, hash 51180fdf)
break_chain D12: 0
defend3 D12: 1 G12 (trivial extension)

This is to be interpreted as follows. The node in question was generated by the function attack3() in ‘engine/reading.c’, which was called on the string at D12. The data in parentheses tell you the values of count_variations and hashdata.hashval.

The second value (“hash”) you probably will not need to know unless you are debugging the hash code, and we will not discuss it. But the first value (“variation”) is useful when using the debugger gdb. You can first make an output file using the ‘-o’ option, then walk through the reading with gdb, and to coordinate the SGF file with the debugger, display the value of count_variations. Specifically, from the debugger you can find out where you are as follows:

(gdb) set dump_stack()
B:D13 W:E12 B:E13 W:F12 B:F11  (variation 6)

If you place yourself right after the call to trymove() which generated the move in question, then the variation number in the SGF file should match the variation number displayed by dump_stack(), and the move in question will be the last move played (F11 in this example).

This displays the sequence of moves leading up to the variation in question, and it also prints count_variations-1.

The second two lines tell you that from this node, the function break_chain() was called at D12 and returned 0 meaning that no way was found of rescuing the string by attacking an element of the surrounding chain, and the function defend3() was called also at D12 and returned 1, meaning that the string can be defended, and that G12 is the move that defends it. If you have trouble finding the function calls which generate these comments, try setting sgf_dumptree=1 and setting a breakpoint in sgf_trace.

5.4 Checking the Owl Code

You can similarly debug the Owl code using the option ‘--decide-dragon’. Usage is entirely similar to ‘--decide-string’, and it can be used similarly to produce variation trees. These should be typically much smaller than the variation trees produced by ‘--decide-string’.

5.5 GTP and GDB techniques

You can use the Go Text Protocol (see section The Go Text Protocol) to determine the statuses of dragons and other information needed for debugging. The GTP command dragon_data P12 will list the dragon data of the dragon at P12 and worm_data will list the worm data; other GTP commands may be useful as well.

You can also conveniently get such information from GDB. A suggested ‘.gdbinit’ file may be found in See section Debugging the reading code. Assuming this file is loaded, you can list the dragon data with the command:

(gdb) dragon P12

Similarly you can get the worm data with worm P12.

5.6 Debugging on a Graphical Board

The quickest way to analyze most positions is to use the tool ‘view.pike’ in the ‘regression’ directory. It can be started with a testcase specified, e.g. pike view.pike strategy:40 or at a move in an sgf file, e.g. pike view.pike mistake.sgf:125. When started it shows the position on a grapical board on which it also marks information like move values, dragon status, and so on. By clicking on the board further information about the valuation of moves, contents of various data structures, and other data can be made available.

Specific information on how to use ‘view.pike’ for influence tuning can be found in See section Influence Tuning with view.pike.

5.7 Scoring the game

GNU Go can score the game. Normally GNU Go will report its opinion about the score at the end of the game, but if you want this information about a game stored in a file, use the ‘--score’ option (see section Invoking GNU Go: Command line options).

5.8 Colored Display

Various colored displays of the board may be obtained in a color xterm or rxvt window. Xterm will only work if xterm is compiled with color support. If the colors are not displayed on your xterm, try rxvt. You may also use the Linux console. The colored display will work best if the background color is black; if this is not the case you may want to edit your ‘.Xdefaults’ file or add the options ‘-bg black -fg white’ to xterm or rxvt. On Mac OS X put setenv TERM xterm-color in your ‘.tcshrc’ file to enable color in the terminal.

5.8.1 Dragon Display

You can get a colored ASCII display of the board in which each dragon is assigned a different letter; and the different matcher_status values (ALIVE, DEAD, UNKNOWN, CRITICAL) have different colors. This is very handy for debugging. Actually two diagrams are generated. The reason for this is concerns the way the matcher status is computed. The dragon_status (see section Dragons) is computed first, then for some, but not all dragons, a more accurate owl status is computed. The matcher status is the owl status if available; otherwise it is the dragon_status. Both the dragon_status and the owl_status are displayed. The color scheme is as follows:

green = alive
cyan = dead
red = critical
yellow = unknown
magenta = unchecked

To get the colored display, save a game in sgf format using CGoban, or using the ‘-o’ option with GNU Go itself.

Open an xterm or rxvt window.

Execute gnugo -l [filename] -L [movenum] -T to get the colored display.

Other useful colored displays may be obtained by using instead:

5.8.2 Eye Space Display

Instead of ‘-T’, try this with ‘-E’. This gives a colored display of the eyespaces, with marginal eye spaces marked ‘!’ (see section Eyes and Half Eyes).

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