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GNU Go Documentation

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5.3 Checking the reading code

The `--decide-string' option is used to check the tactical reading code (see section 14. 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.

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