| 1 | @cindex Reading code |
| 2 | @cindex Reading process |
| 3 | @cindex Trying hypothetical moves |
| 4 | @cindex Usage of the stack in reading |
| 5 | @cindex reading DEPTH |
| 6 | @cindex Depth of reading |
| 7 | @cindex reading.c |
| 8 | @cindex reading.h |
| 9 | |
| 10 | The process of visualizing potential moves done by you and your |
| 11 | opponent to learn the result of different moves is called |
| 12 | "reading". GNU Go does three distinct types of reading: @dfn{tactical |
| 13 | reading} which typically is concerned with the life and death of |
| 14 | individual strings, @dfn{Owl reading} which is concerned |
| 15 | with the life and death of dragons, and @dfn{connection reading}. |
| 16 | In this Chapter, we document |
| 17 | the tactical reading code, which is in @file{engine/reading.c}. |
| 18 | |
| 19 | @menu |
| 20 | * Reading Basics:: Reading Basics |
| 21 | * Hashing:: Hashing of positions |
| 22 | * Persistent Cache:: Persistent Reading Cache |
| 23 | * Ko:: Ko handling |
| 24 | * A Ko Example:: A Ko Example |
| 25 | * Another Ko Example:: Another Ko Example |
| 26 | * Alternate Komaster Schemes:: Alternate Komaster Schemes |
| 27 | * Superstrings:: Superstrings |
| 28 | * Debugging:: Debugging the reading code |
| 29 | * Connection Reading:: Connection Reading |
| 30 | @end menu |
| 31 | |
| 32 | @node Reading Basics |
| 33 | @section Reading Basics |
| 34 | |
| 35 | What we call @emph{Tactical Reading} is the analysis whether there is |
| 36 | a direct capture of a single string, or whether there is a move to prevent |
| 37 | such a direct capture. |
| 38 | |
| 39 | If the reading module finds out that the string can get captured, this |
| 40 | answer should (usually) be trusted. However, if it says it can be defended, |
| 41 | this does not say as much. It is often the case that such a string has |
| 42 | no chance to make a life, but that it cannot be captured within the |
| 43 | horizon (and the cutoff heuristics) of the tactical reading. |
| 44 | |
| 45 | The tactical reading is done by the functions in @file{engine/reading.c}. |
| 46 | It is a minimax search that declares win for the attacker once he can |
| 47 | physically take the string off board, whereas the defense is considered |
| 48 | successful when the string has sufficiently many liberties. A string with |
| 49 | five liberties is always considered alive. At higher depth within the |
| 50 | search tree even fewer liberties cause GNU Go to give up the attack, |
| 51 | @xref{depthparams}. |
| 52 | |
| 53 | The reading code makes use of a stack onto which board positions can |
| 54 | be pushed. The parameter @code{stackp} is zero if GNU Go is |
| 55 | examining the true board position; if it is higher than zero, then |
| 56 | GNU Go is examining a hypothetical position obtained by playing |
| 57 | several moves. |
| 58 | |
| 59 | The most important public reading functions are @code{attack} and |
| 60 | @code{find_defense}. These are wrappers for functions @code{do_attack} and |
| 61 | @code{do_find_defense} which are declared statically in @file{reading.c}. The |
| 62 | functions @code{do_attack} and @code{do_find_defense} call each other |
| 63 | recursively. |
| 64 | |
| 65 | @subsection Organization of the reading code |
| 66 | |
| 67 | The function @code{do_attack} and @code{do_find_defense} are wrappers |
| 68 | themselves and call @code{attack1}, @code{attack2}, @code{attack3} or |
| 69 | @code{attack4} resp. @code{defend1}, @code{defend1}, @code{defend1} |
| 70 | or @code{defend1} depending on the number of liberties. |
| 71 | |
| 72 | These are fine-tuned to generate and try out the moves in an efficient |
| 73 | order. They generate a few moves themselves (mostly direct liberties |
| 74 | of the string), and then call helper functions called @code{..._moves} |
| 75 | which suggest less obvious moves. Which of these functions get called |
| 76 | depends on the number of liberties and of the current search depth. |
| 77 | |
| 78 | @subsection Return Codes |
| 79 | @anchor{Return Codes} |
| 80 | @cindex return codes |
| 81 | @cindex reading return codes |
| 82 | |
| 83 | The return codes of the reading (and owl) functions and owl can |
| 84 | be @code{0}, @code{KO_B}, @code{KO_A} or @code{WIN}. Each reading |
| 85 | function determines whether a particular player (assumed to have the |
| 86 | move) can solve a specific problem, typically attacking or defending |
| 87 | a string. |
| 88 | |
| 89 | A return code of @code{WIN} means success, 0 failure, while @code{KO_A} and |
| 90 | @code{KO_B} are success conditioned on ko. A function returns @code{KO_A} |
| 91 | if the position results in ko and that the player to move |
| 92 | will get the first ko capture (so the opponent has to make the |
| 93 | first ko threat). A return code of @code{KO_B} means that the player |
| 94 | to move will have to make the first ko threat. |
| 95 | |
| 96 | @anchor{Experimental Owl Extension} |
| 97 | If GNU Go is compiled with the configure option |
| 98 | @option{--enable-experimental-owl-ext} then the owl functions also have |
| 99 | possible return codes of @code{GAIN} and @code{LOSS}. A code of @code{GAIN} |
| 100 | means that the attack (or defense) does not succeed, but that in the process |
| 101 | of trying to attack or defend, an opponent's worm is captured. A code |
| 102 | of @code{LOSS} means that the attack or defense succeeds, but that another |
| 103 | friendly worm dies during the attack or defense. |
| 104 | |
| 105 | @subsection Reading cutoff and depth parameters |
| 106 | @anchor{depthparams} |
| 107 | |
| 108 | Depth of reading is controlled by the parameters @code{depth} |
| 109 | and @code{branch_depth}. The @code{depth} has a default value |
| 110 | @code{DEPTH} (in @file{liberty.h}), which is set to 16 in the |
| 111 | distribution, but it may also be set at the command line using |
| 112 | the @option{-D} or @option{--depth} option. If @code{depth} is |
| 113 | increased, GNU Go will be stronger and slower. GNU Go will read |
| 114 | moves past depth, but in doing so it makes simplifying |
| 115 | assumptions that can cause it to miss moves. |
| 116 | |
| 117 | Specifically, when @code{stackp > depth}, GNU Go assumes that as |
| 118 | soon as the string can get 3 liberties it is alive. This |
| 119 | assumption is sufficient for reading ladders. |
| 120 | |
| 121 | The @code{branch_depth} is typically set a little below @code{depth}. |
| 122 | Between @code{branch_depth} and @code{depth}, attacks on strings with |
| 123 | 3 liberties are considered, but branching is inhibited, so fewer |
| 124 | variations are considered. |
| 125 | |
| 126 | %@findex small_semeai |
| 127 | %Currently the reading code does not try to defend a string by |
| 128 | %attacking a boundary string with more than two liberties. Because |
| 129 | %of this restriction, it can make oversights. A symptom of this is |
| 130 | %two adjacent strings, each having three or four liberties, each |
| 131 | %classified as @code{DEAD}. To resolve such situations, a function |
| 132 | %@code{small_semeai()} (in @file{engine/semeai.c}) looks for such |
| 133 | %pairs of strings and corrects their classification. |
| 134 | |
| 135 | The @code{backfill_depth} is a similar variable with a default 12. Below |
| 136 | this depth, GNU Go will try "backfilling" to capture stones. |
| 137 | For example in this situation: |
| 138 | |
| 139 | @example |
| 140 | @group |
| 141 | |
| 142 | .OOOOOO. on the edge of the board, O can capture X but |
| 143 | OOXXXXXO in order to do so he has to first play at a in |
| 144 | .aObX.XO preparation for making the atari at b. This is |
| 145 | -------- called backfilling. |
| 146 | |
| 147 | @end group |
| 148 | @end example |
| 149 | |
| 150 | Backfilling is only tried with @code{stackp <= backfill_depth}. The |
| 151 | parameter @code{backfill_depth} may be set using the @option{-B} |
| 152 | option. |
| 153 | |
| 154 | The @code{fourlib_depth} is a parameter with a default of only 7. |
| 155 | Below this depth, GNU Go will try to attack strings with |
| 156 | four liberties. The @code{fourlib_depth} may be set using the |
| 157 | @option{-F} option. |
| 158 | |
| 159 | The parameter @code{ko_depth} is a similar cutoff. If |
| 160 | @code{stackp<ko_depth}, the reading code will make experiments |
| 161 | involving taking a ko even if it is not legal to do so (i.e., it |
| 162 | is hypothesized that a remote ko threat is made and answered |
| 163 | before continuation). This parameter may be set using the |
| 164 | @option{-K} option. |
| 165 | |
| 166 | @cindex reading.c |
| 167 | |
| 168 | @itemize @bullet |
| 169 | @item @code{int attack(int str, int *move)} |
| 170 | @findex attack |
| 171 | @quotation |
| 172 | Determines if the string at @code{str} can |
| 173 | be attacked, and if so, @code{*move} returns the attacking move, |
| 174 | unless @code{*movei} is a null pointer. (Use null pointers if |
| 175 | you are interested in the result of the attack but not the |
| 176 | attacking move itself.) Returns @code{WIN}, if the attack succeeds, |
| 177 | 0 if it fails, and @code{KO_A} or @code{KO_B} if the result depends on ko |
| 178 | @ref{Return Codes}. |
| 179 | @end quotation |
| 180 | @findex find_defense |
| 181 | @item @code{find_defense(int str, int *move)} |
| 182 | @quotation |
| 183 | Attempts to find a move that will save the string at @code{str}. It |
| 184 | returns true if such a move is found, with @code{*move} the location |
| 185 | of the saving move (unless @code{*move} is a null pointer). It is not |
| 186 | checked that tenuki defends, so this may give an erroneous answer if |
| 187 | @code{!attack(str)}. Returns @code{KO_A} or @code{KO_B} if the |
| 188 | result depends on ko @xref{Return Codes}. |
| 189 | @end quotation |
| 190 | @findex safe_move |
| 191 | @item @code{safe_move(int str, int color)} : |
| 192 | @quotation |
| 193 | The function @code{safe_move(str, color)} checks whether a move at |
| 194 | @code{str} is illegal or can immediately be captured. If @code{stackp==0} |
| 195 | the result is cached. If the move only can be captured by a ko, it's |
| 196 | considered safe. This may or may not be a good convention. |
| 197 | @end quotation |
| 198 | @end itemize |
| 199 | |
| 200 | @node Hashing |
| 201 | @section Hashing of Positions |
| 202 | |
| 203 | @cindex Hashing of positions |
| 204 | @cindex Reading optimisation |
| 205 | @cindex Speedup of reading process |
| 206 | @cindex Zobrist hashing algorithm |
| 207 | @cindex Transposition table |
| 208 | |
| 209 | To speed up the reading process, we note that a position can be |
| 210 | reached in several different ways. In fact, it is a very common |
| 211 | occurrence that a previously checked position is rechecked, often |
| 212 | within the same search but from a different branch in the recursion |
| 213 | tree. |
| 214 | |
| 215 | This wastes a lot of computing resources, so in a number of places, we |
| 216 | store away the current position, the function we are in, and which worm |
| 217 | is under attack or to be defended. When the search for this position |
| 218 | is finished, we also store away the result of the search and which |
| 219 | move made the attack or defense succeed. |
| 220 | |
| 221 | All this data is stored in a hash table, sometimes also called a |
| 222 | transposition table, where Go positions are the key and results of the |
| 223 | reading for certain functions and groups are the data. You can increase |
| 224 | the size of the Hash table using the @option{-M} or @option{--memory} |
| 225 | option @pxref{Invoking GNU Go}. |
| 226 | |
| 227 | The hash table is created once and for all at the beginning of |
| 228 | the game by the function @code{hashtable_new()}. Although hash |
| 229 | memory is thus allocated only once in the game, the table is |
| 230 | reinitialized at the beginning of each move by a call to |
| 231 | @code{hashtable_clear()} from @code{genmove()}. |
| 232 | |
| 233 | @menu |
| 234 | * Hash Calculation:: Calculation of the hash value |
| 235 | * Hash Organization:: Organization of the hash table |
| 236 | * Hash Structures:: Structures in @file{hash.h} |
| 237 | @end menu |
| 238 | |
| 239 | @node Hash Calculation |
| 240 | @subsection Calculation of the hash value |
| 241 | |
| 242 | The hash algorithm is called Zobrist hashing, and is a standard |
| 243 | technique for go and chess programming. The algorithm as used by us |
| 244 | works as follows: |
| 245 | |
| 246 | @cindex go position |
| 247 | @cindex position |
| 248 | |
| 249 | @enumerate |
| 250 | @item First we define a @dfn{go position}. This positions consists of |
| 251 | @itemize @bullet |
| 252 | @item the actual board, i.e. the locations and colors of the stones |
| 253 | @item A @dfn{ko point}, if a ko is going on. The ko point is defined as |
| 254 | the empty point where the last single stone was situated before |
| 255 | it was captured. |
| 256 | @end itemize |
| 257 | |
| 258 | It is not necessary to specify the color to move (white or black) |
| 259 | as part of the position. The reason for this is that read results |
| 260 | are stored separately for the various reading functions such as |
| 261 | @code{attack3}, and it is implicit in the calling function which |
| 262 | player is to move. |
| 263 | |
| 264 | @item For each location on the board we generate random numbers: |
| 265 | @itemize @bullet |
| 266 | @item A number which is used if there is a white stone on this location |
| 267 | @item A number which is used if there is a black stone on this location |
| 268 | @item A number which is used if there is a ko on this location |
| 269 | @end itemize |
| 270 | |
| 271 | These random numbers are generated once at initialization time and |
| 272 | then used throughout the life time of the hash table. |
| 273 | |
| 274 | @item The hash key for a position is the XOR of all the random numbers |
| 275 | which are applicable for the position (white stones, black stones, and |
| 276 | ko position). |
| 277 | @end enumerate |
| 278 | |
| 279 | @node Hash Organization |
| 280 | @subsection Organization of the hash table |
| 281 | |
| 282 | The hash table consists of 3 parts: |
| 283 | |
| 284 | @cindex Hash node |
| 285 | @cindex Read result |
| 286 | |
| 287 | @itemize @bullet |
| 288 | @item An area which contains so called @dfn{Hash Nodes}. Each hash node |
| 289 | contains: |
| 290 | @itemize @minus |
| 291 | @item A go position as defined above. |
| 292 | @item A computed hash value for the position |
| 293 | @item A pointer to Read Results (see below) |
| 294 | @item A pointer to another hash node. |
| 295 | @end itemize |
| 296 | |
| 297 | @item An area with so called Read Results. These are used to store |
| 298 | which function was called in the go position, which string was |
| 299 | under attack or to be defended, and the result of the reading. |
| 300 | |
| 301 | Each Read Result contains: |
| 302 | @itemize @minus |
| 303 | @item the function ID (an int between 0 and 255), the position of the |
| 304 | string under attack and a depth value, which is used to |
| 305 | determine how deep the search was when it was made, packed into |
| 306 | one 32 bit integer. |
| 307 | @item The result of the search (a numeric value) and a position to |
| 308 | play to get the result packed into one 32 bit integer. |
| 309 | @item A pointer to another Read Result. |
| 310 | @end itemize |
| 311 | |
| 312 | @item An array of pointers to hash nodes. This is the hash table |
| 313 | proper. |
| 314 | |
| 315 | @end itemize |
| 316 | |
| 317 | When the hash table is created, these 3 areas are allocated using |
| 318 | @code{malloc()}. When the hash table is populated, all contents are taken |
| 319 | from the Hash nodes and the Read results. No further allocation is |
| 320 | done and when all nodes or results are used, the hash table is full. |
| 321 | Nothing is deleted from the hash table except when it is totally |
| 322 | emptied, at which point it can be used again as if newly initialized. |
| 323 | |
| 324 | @findex hashtable_search |
| 325 | When a function wants to use the hash table, it looks up the current |
| 326 | position using @code{hashtable_search()}. If the position doesn't already |
| 327 | exist there, it can be entered using |
| 328 | |
| 329 | @findex hashtable_enter_position |
| 330 | @code{hashtable_enter_position()}. |
| 331 | |
| 332 | @findex hashtable_enter_position |
| 333 | Once the function has a pointer to the hash node containing a |
| 334 | function, it can search for a result of a previous search using |
| 335 | @code{hashnode_search()}. If a result is found, it can be used, and |
| 336 | if not, a new result can be entered after a search using |
| 337 | @findex hashnode_new_result |
| 338 | @code{hashnode_new_result()}. |
| 339 | |
| 340 | Hash nodes which hash to the same position in the hash table |
| 341 | (collisions) form a simple linked list. Read results for the same |
| 342 | position, created by different functions and different attacked or |
| 343 | defended strings also form a linked list. |
| 344 | |
| 345 | This is deemed sufficiently efficient for now, but the representation |
| 346 | of collisions could be changed in the future. It is also not |
| 347 | determined what the optimum sizes for the hash table, the number of |
| 348 | positions and the number of results are. |
| 349 | |
| 350 | @node Hash Structures |
| 351 | @subsection Hash Structures |
| 352 | |
| 353 | The basic hash structures are declared in @file{engine/hash.h} and |
| 354 | @file{engine/cache.c} |
| 355 | |
| 356 | @example |
| 357 | typedef struct hashposition_t @{ |
| 358 | Compacttype board[COMPACT_BOARD_SIZE]; |
| 359 | int ko_pos; |
| 360 | @} Hashposition; |
| 361 | @end example |
| 362 | |
| 363 | Represents the board and optionally the location of a ko, |
| 364 | which is an illegal move. The player whose move is next |
| 365 | is not recorded. |
| 366 | |
| 367 | @example |
| 368 | typedef struct @{ |
| 369 | Hashvalue hashval; |
| 370 | Hashposition hashpos; |
| 371 | @} Hash_data; |
| 372 | @end example |
| 373 | |
| 374 | Represents the return value of a function (@code{hashval}) and |
| 375 | the board state (@code{hashpos}). |
| 376 | |
| 377 | @example |
| 378 | typedef struct read_result_t @{ |
| 379 | unsigned int data1; |
| 380 | unsigned int data2; |
| 381 | |
| 382 | struct read_result_t *next; |
| 383 | @} Read_result; |
| 384 | @end example |
| 385 | |
| 386 | The data1 field packs into 32 bits the following fields: |
| 387 | |
| 388 | @example |
| 389 | |
| 390 | komaster: 2 bits (EMPTY, BLACK, WHITE, or GRAY) |
| 391 | kom_pos : 10 bits (allows MAX_BOARD up to 31) |
| 392 | routine : 4 bits (currently 10 different choices) |
| 393 | str1 : 10 bits |
| 394 | stackp : 5 bits |
| 395 | |
| 396 | @end example |
| 397 | |
| 398 | The data2 field packs into 32 bits the following fields: |
| 399 | |
| 400 | @example |
| 401 | |
| 402 | status : 2 bits (0 free, 1 open, 2 closed) |
| 403 | result1: 4 bits |
| 404 | result2: 4 bits |
| 405 | move : 10 bits |
| 406 | str2 : 10 bits |
| 407 | |
| 408 | @end example |
| 409 | |
| 410 | The @code{komaster} and @code{(kom_pos)} field are |
| 411 | documented in @xref{Ko}. |
| 412 | |
| 413 | When a new result node is created, 'status' is set to 1 'open'. |
| 414 | This is then set to 2 'closed' when the result is entered. The main |
| 415 | use for this is to identify open result nodes when the hashtable is |
| 416 | partially cleared. Another potential use for this field is to |
| 417 | identify repeated positions in the reading, in particular local |
| 418 | double or triple kos. |
| 419 | |
| 420 | @example |
| 421 | typedef struct hashnode_t @{ |
| 422 | Hash_data key; |
| 423 | Read_result * results; |
| 424 | struct hashnode_t * next; |
| 425 | @} Hashnode; |
| 426 | @end example |
| 427 | |
| 428 | The hash table consists of hash nodes. Each hash node consists of |
| 429 | The hash value for the position it holds, the position itself and |
| 430 | the actual information which is purpose of the table from the start. |
| 431 | |
| 432 | There is also a pointer to another hash node which is used when |
| 433 | the nodes are sorted into hash buckets (see below). |
| 434 | |
| 435 | @example |
| 436 | typedef struct hashtable @{ |
| 437 | size_t hashtablesize; /* Number of hash buckets */ |
| 438 | Hashnode ** hashtable; /* Pointer to array of hashnode lists */ |
| 439 | |
| 440 | int num_nodes; /* Total number of hash nodes */ |
| 441 | Hashnode * all_nodes; /* Pointer to all allocated hash nodes. */ |
| 442 | int free_node; /* Index to next free node. */ |
| 443 | |
| 444 | int num_results; /* Total number of results */ |
| 445 | Read_result * all_results; /* Pointer to all allocated results. */ |
| 446 | int free_result; /* Index to next free result. */ |
| 447 | @} Hashtable; |
| 448 | @end example |
| 449 | |
| 450 | The hash table consists of three parts: |
| 451 | |
| 452 | @itemize @bullet |
| 453 | @item The hash table proper: a number of hash buckets with collisions |
| 454 | being handled by a linked list. |
| 455 | @item The hash nodes. These are allocated at creation time and are |
| 456 | never removed or reallocated in the current implementation. |
| 457 | @item The results of the searches. Since many different searches can |
| 458 | be done in the same position, there should be more of these than |
| 459 | hash nodes. |
| 460 | @end itemize |
| 461 | |
| 462 | @node Persistent Cache |
| 463 | @section Persistent Reading Cache |
| 464 | |
| 465 | @cindex persistent cache |
| 466 | @findex store_persistent_reading_cache |
| 467 | @findex purge_persistent_reading_cache |
| 468 | @findex purge_persistent_connection_cache |
| 469 | @findex purge_persistent_breakin_cache |
| 470 | @findex purge_persistent_owl_cache |
| 471 | |
| 472 | @findex search_persistent_reading_cache |
| 473 | @findex store_persistent_reading_cache |
| 474 | |
| 475 | Some calculations can be safely saved from move to move. If the |
| 476 | opponent's move is not close to our worm or dragon, we do not have to |
| 477 | reconsider the life or death of that group on the next move. So |
| 478 | the result is saved in a persistent cache. Persistent caches are used for |
| 479 | are used in the engine for several types of read results. |
| 480 | |
| 481 | @itemize @bullet |
| 482 | @item Tactical reading |
| 483 | @item Owl reading |
| 484 | @item Connection reading |
| 485 | @item Breakin code |
| 486 | @end itemize |
| 487 | |
| 488 | In this section we will discuss the persistent caching of tactical |
| 489 | reading but the same principles apply to the other persistent caches. |
| 490 | |
| 491 | Persistent caching is an important performance feature. However it |
| 492 | can lead to mistakes and debugging problems---situations where GNU |
| 493 | Go generates the right move during debugging but plays a wrong move |
| 494 | during a game. If you suspect a persistent cache effect you may |
| 495 | try loading the sgf file with the @option{--replay} option and see if the |
| 496 | mistake is repeated (@pxref{Invoking GNU Go}). |
| 497 | |
| 498 | The function @code{store_persistent_cache()} is called only |
| 499 | by @code{attack} and @code{find_defense}, never from their |
| 500 | static recursive counterparts @code{do_attack} and @code{do_defend}. |
| 501 | The function @code{store_persistent_reading_cache()} attempts to |
| 502 | cache the most expensive reading results. The function |
| 503 | @code{search_persistent_reading_cache} attempts to retrieve a |
| 504 | result from the cache. |
| 505 | |
| 506 | If all cache entries are occupied, we try to replace the least useful |
| 507 | one. This is indicated by the score field, which is initially the |
| 508 | number of nodes expended by this particular reading, and later |
| 509 | multiplied by the number of times it has been retrieved from the |
| 510 | cache. |
| 511 | |
| 512 | Once a (permanent) move is made, a number of cache entries immediately become |
| 513 | invalid. These are cleaned away by the function |
| 514 | @code{purge_persistent_reading_cache().} To have a criterion |
| 515 | for when a result may be purged, the function |
| 516 | @code{store_persistent_cache()} computes the |
| 517 | @dfn{reading shadow} and @dfn{active area}. If a permanent |
| 518 | move is subsequently played in the active area, the cached |
| 519 | result is invalidated. We now explain this algorithm in detail. |
| 520 | |
| 521 | @cindex reading shadow |
| 522 | |
| 523 | The @dfn{reading shadow} is the concatenation of all moves in all |
| 524 | variations, as well as locations where an illegal move has been tried. |
| 525 | |
| 526 | Once the read is finished, the reading shadow is expanded |
| 527 | to the @dfn{active area} which may be cached. The |
| 528 | intention is that as long as no stones are played in the |
| 529 | active area, the cached value may safely be used. |
| 530 | |
| 531 | Here is the algorithm used to compute the active area. |
| 532 | This algorithm is in the function @code{store_persistent_reading_cache()}. |
| 533 | The most expensive readings so far are stored in the persistent cache. |
| 534 | |
| 535 | @itemize @bullet |
| 536 | @item |
| 537 | The reading shadow and the string under attack are marked |
| 538 | with the character @samp{1}. We also include the successful |
| 539 | move, which is most often a part of the reading shadow, but |
| 540 | sometimes not, for example with the function @code{attack1()}. |
| 541 | |
| 542 | @item |
| 543 | Next the reading shadow is expanded by marking strings and |
| 544 | empty vertices adjacent to the area marked @samp{1} with |
| 545 | the character @samp{2}. |
| 546 | |
| 547 | @item |
| 548 | Next vertices adjacent to empty vertices marked @samp{2} are |
| 549 | labelled with the character @samp{3}. |
| 550 | |
| 551 | @item |
| 552 | Next all vertices adjacent to previously marked vertices. These are |
| 553 | marked @samp{-1} instead of the more logical @samp{4} because it |
| 554 | is slightly faster to code this way. |
| 555 | |
| 556 | @item |
| 557 | If the stack pointer is >0 we add the moves already played from the |
| 558 | moves stack with mark 4. |
| 559 | @end itemize |
| 560 | |
| 561 | @node Ko |
| 562 | @section Ko Handling |
| 563 | |
| 564 | The principles of ko handling are the same for tactical reading and |
| 565 | owl reading. |
| 566 | |
| 567 | We have already mentioned (@pxref{Reading Basics}) that GNU Go |
| 568 | uses a return code of @code{KO_A} or @code{KO_B} if the result depends on |
| 569 | ko. The return code of @code{KO_B} means that the position can be won |
| 570 | provided the player whose move calls the function can come up |
| 571 | with a sufficiently large ko threat. In order to verify this, |
| 572 | the function must simulate making a ko threat and having it |
| 573 | answered by taking the ko even if it is illegal. We call such an |
| 574 | experimental taking of the ko a @dfn{conditional} ko capture. |
| 575 | |
| 576 | Conditional ko captures are accomplished by the function @code{tryko()}. |
| 577 | This function is like @code{trymove()} except that |
| 578 | it does not require legality of the move in question. |
| 579 | |
| 580 | The static reading functions, and the global functions @code{do_attack} |
| 581 | and @code{do_find_defense} consult parameters @code{komaster}, |
| 582 | @code{kom_pos}, which are declared static in @file{board.c}. These mediate ko |
| 583 | captures to prevent the occurrence of infinite loops. During |
| 584 | reading, the komaster values are pushed and popped from a stack. |
| 585 | |
| 586 | Normally @code{komaster} is @code{EMPTY} but it can also be |
| 587 | @samp{BLACK}, @samp{WHITE}, @code{GRAY_BLACK}, @code{GRAY_WHITE} or |
| 588 | @code{WEAK_KO}. The komaster is set to @code{color} when @code{color} makes a |
| 589 | conditional ko capture. In this case @code{kom_pos} is set to the location of |
| 590 | the captured ko stone. |
| 591 | |
| 592 | If the opponent is komaster, the reading functions will not try to |
| 593 | take the ko at @code{kom_pos}. Also, the komaster is normally not |
| 594 | allowed to take another ko. The exception is a nested ko, characterized |
| 595 | by the condition that the captured ko stone is at distance 1 both |
| 596 | vertically and horizontally from @code{kom_pos}, which is the location |
| 597 | of the last stone taken by the komaster. Thus in this situation: |
| 598 | |
| 599 | @example |
| 600 | |
| 601 | .OX |
| 602 | OX*X |
| 603 | OmOX |
| 604 | OO |
| 605 | |
| 606 | @end example |
| 607 | |
| 608 | Here if @samp{m} is the location of @code{kom_pos}, then the move at |
| 609 | @samp{*} is allowed. |
| 610 | |
| 611 | The rationale behind this rule is that in the case where there are |
| 612 | two kos on the board, the komaster cannot win both, and by becoming |
| 613 | komaster he has already chosen which ko he wants to win. But in the |
| 614 | case of a nested ko, taking one ko is a precondition to taking the |
| 615 | other one, so we allow this. |
| 616 | |
| 617 | If the komaster's opponent takes a ko, then both players have taken one ko. In |
| 618 | this case @code{komaster} is set to @code{GRAY_BLACK} or @code{GRAY_WHITE} and |
| 619 | after this further ko captures are even further restricted. |
| 620 | |
| 621 | If the ko at @code{kom_pos} is filled, then the komaster reverts to |
| 622 | @code{EMPTY}. |
| 623 | |
| 624 | In detail, the komaster scheme is as follows. Color @samp{O} is to move. |
| 625 | This scheme is known as scheme 5 since in versions of GNU Go through |
| 626 | 3.4, several different schemes were included. |
| 627 | |
| 628 | @itemize @bullet |
| 629 | @item 1. Komaster is EMPTY. |
| 630 | @itemize @minus |
| 631 | @item 1a. Unconditional ko capture is allowed. |
| 632 | @quotation |
| 633 | Komaster remains EMPTY if previous move was not a ko capture. |
| 634 | Komaster is set to WEAK_KO if previous move was a ko capture |
| 635 | and kom_pos is set to the old value of board_ko_pos. |
| 636 | @end quotation |
| 637 | @item 1b) Conditional ko capture is allowed. |
| 638 | @quotation |
| 639 | Komaster is set to O and kom_pos to the location of the ko, where a stone was |
| 640 | just removed. |
| 641 | @end quotation |
| 642 | @end itemize |
| 643 | @item 2. Komaster is O: |
| 644 | @itemize @minus |
| 645 | @item 2a) Only nested ko captures are allowed. Kom_pos is moved to the |
| 646 | new removed stone. |
| 647 | @item 2b) If komaster fills the ko at kom_pos then komaster reverts to |
| 648 | EMPTY. |
| 649 | @end itemize |
| 650 | @item 3. Komaster is X: |
| 651 | @quotation |
| 652 | Play at kom_pos is not allowed. Any other ko capture |
| 653 | is allowed. If O takes another ko, komaster becomes GRAY_X. |
| 654 | @end quotation |
| 655 | @item 4. Komaster is GRAY_O or GRAY_X: |
| 656 | @quotation |
| 657 | Ko captures are not allowed. If the ko at kom_pos is |
| 658 | filled then the komaster reverts to EMPTY. |
| 659 | @end quotation |
| 660 | @item 5. Komaster is WEAK_KO: |
| 661 | @itemize @minus |
| 662 | @item 5a) After a non-ko move komaster reverts to EMPTY. |
| 663 | @item 5b) Unconditional ko capture is only allowed if it is nested ko capture. |
| 664 | @quotation |
| 665 | Komaster is changed to WEAK_X and kom_pos to the old value of |
| 666 | board_ko_pos. |
| 667 | @end quotation |
| 668 | @item 5c) Conditional ko capture is allowed according to the rules of 1b. |
| 669 | @end itemize |
| 670 | @end itemize |
| 671 | |
| 672 | @node A Ko Example |
| 673 | @section A Ko Example |
| 674 | |
| 675 | To see the komaster scheme in action, consider this position |
| 676 | from the file @file{regressions/games/life_and_death/tripod9.sgf}. |
| 677 | We recommend studying this example by examining the variation file |
| 678 | produced by the command: |
| 679 | |
| 680 | @example |
| 681 | gnugo -l tripod9.sgf --decide-dragon C3 -o vars.sgf |
| 682 | @end example |
| 683 | |
| 684 | In the lower left hand corner, there are kos at A2 and B4. |
| 685 | Black is unconditionally dead because if W wins either ko |
| 686 | there is nothing B can do. |
| 687 | |
| 688 | @example |
| 689 | @group |
| 690 | |
| 691 | 8 . . . . . . . . |
| 692 | 7 . . O . . . . . |
| 693 | 6 . . O . . . . . |
| 694 | 5 O O O . . . . . |
| 695 | 4 O . O O . . . . |
| 696 | 3 X O X O O O O . |
| 697 | 2 . X X X O . . . |
| 698 | 1 X O . . . . . . |
| 699 | A B C D E F G H |
| 700 | |
| 701 | @end group |
| 702 | @end example |
| 703 | |
| 704 | This is how the komaster scheme sees this. B (i.e. X) starts by |
| 705 | taking the ko at B4. W replies by taking the ko at A1. The board |
| 706 | looks like this: |
| 707 | |
| 708 | @example |
| 709 | @group |
| 710 | |
| 711 | 8 . . . . . . . . |
| 712 | 7 . . O . . . . . |
| 713 | 6 . . O . . . . . |
| 714 | 5 O O O . . . . . |
| 715 | 4 O X O O . . . . |
| 716 | 3 X . X O O O O . |
| 717 | 2 O X X X O . . . |
| 718 | 1 . O . . . . . . |
| 719 | A B C D E F G H |
| 720 | |
| 721 | @end group |
| 722 | @end example |
| 723 | |
| 724 | Now any move except the ko recapture (currently illegal) |
| 725 | at A1 loses for B, so B retakes the ko and becomes komaster. |
| 726 | The board looks like this: |
| 727 | |
| 728 | @example |
| 729 | @group |
| 730 | |
| 731 | 8 . . . . . . . . komaster: BLACK |
| 732 | 7 . . O . . . . . kom_pos: A2 |
| 733 | 6 . . O . . . . . |
| 734 | 5 O O O . . . . . |
| 735 | 4 O X O O . . . . |
| 736 | 3 X . X O O O O . |
| 737 | 2 . X X X O . . . |
| 738 | 1 X O . . . . . . |
| 739 | A B C D E F G H |
| 740 | |
| 741 | @end group |
| 742 | @end example |
| 743 | |
| 744 | W takes the ko at B3 after which the komaster is @code{GRAY} and |
| 745 | ko recaptures are not allowed. |
| 746 | |
| 747 | @example |
| 748 | @group |
| 749 | |
| 750 | 8 . . . . . . . . komaster: GRAY |
| 751 | 7 . . O . . . . . kom_pos: B4 |
| 752 | 6 . . O . . . . . |
| 753 | 5 O O O . . . . . |
| 754 | 4 O . O O . . . . |
| 755 | 3 X O X O O O O . |
| 756 | 2 . X X X O . . . |
| 757 | 1 X O . . . . . . |
| 758 | A B C D E F G H |
| 759 | |
| 760 | @end group |
| 761 | @end example |
| 762 | |
| 763 | Since B is not allowed any ko recaptures, there is nothing |
| 764 | he can do and he is found dead. Thus the komaster scheme |
| 765 | produces the correct result. |
| 766 | |
| 767 | |
| 768 | @node Another Ko Example |
| 769 | @section Another Ko Example |
| 770 | |
| 771 | We now consider an example to show why the komaster is reset |
| 772 | to @code{EMPTY} if the ko is resolved in the komaster's favor. This |
| 773 | means that the ko is filled, or else that is becomes no longer |
| 774 | a ko and it is illegal for the komaster's opponent to play |
| 775 | there. |
| 776 | |
| 777 | The position resulting under consideration is in the file |
| 778 | @file{regressions/games/ko5.sgf}. This is the position: |
| 779 | |
| 780 | @example |
| 781 | @group |
| 782 | . . . . . . O O 8 |
| 783 | X X X . . . O . 7 |
| 784 | X . X X . . O . 6 |
| 785 | . X . X X X O O 5 |
| 786 | X X . X . X O X 4 |
| 787 | . O X O O O X . 3 |
| 788 | O O X O . O X X 2 |
| 789 | . O . X O X X . 1 |
| 790 | F G H J K L M N |
| 791 | @end group |
| 792 | @end example |
| 793 | |
| 794 | We recommend studying this example by |
| 795 | examining the variation file produced by the command: |
| 796 | |
| 797 | @example |
| 798 | gnugo -l ko5.sgf --quiet --decide-string L1 -o vars.sgf |
| 799 | @end example |
| 800 | |
| 801 | The correct resolution is that H1 attacks L1 unconditionally while K2 |
| 802 | defends it with ko (code @code{KO_A}). |
| 803 | |
| 804 | After Black (X) takes the ko at K3, white can do nothing |
| 805 | but retake the ko conditionally, becoming komaster. B cannot |
| 806 | do much, but in one variation he plays at K4 and W takes |
| 807 | at H1. The following position results: |
| 808 | |
| 809 | @example |
| 810 | @group |
| 811 | . . . . . . O O 8 |
| 812 | X X X . . . O . 7 |
| 813 | X . X X . . O . 6 |
| 814 | . X . X X X O O 5 |
| 815 | X X . X X X O X 4 |
| 816 | . O X O O O X . 3 |
| 817 | O O X O . O X X 2 |
| 818 | . O O . O X X . 1 |
| 819 | F G H J K L M N |
| 820 | @end group |
| 821 | @end example |
| 822 | |
| 823 | Now it is important the @samp{O} is no longer komaster. Were @samp{O} |
| 824 | still komaster, he could capture the ko at N3 and there would be |
| 825 | no way to finish off B. |
| 826 | |
| 827 | |
| 828 | @node Alternate Komaster Schemes |
| 829 | @section Alternate Komaster Schemes |
| 830 | |
| 831 | The following alternate schemes have been proposed. It is assumed |
| 832 | that @samp{O} is the player about to move. |
| 833 | |
| 834 | @subsection Essentially the 2.7.232 scheme. |
| 835 | |
| 836 | @itemize @bullet |
| 837 | @item Komaster is EMPTY. |
| 838 | @itemize @minus |
| 839 | @item Unconditional ko capture is allowed. Komaster remains EMPTY. |
| 840 | @item Conditional ko capture is allowed. Komaster is set to O and |
| 841 | @code{kom_pos} to the location of the ko, where a stone was |
| 842 | just removed. |
| 843 | @end itemize |
| 844 | @item Komaster is O: |
| 845 | @itemize @minus |
| 846 | @item Conditional ko capture is not allowed. |
| 847 | @item Unconditional ko capture is allowed. Komaster parameters unchanged. |
| 848 | @end itemize |
| 849 | @item Komaster is X: |
| 850 | @itemize @minus |
| 851 | @item Conditional ko capture is not allowed. |
| 852 | @item Unconditional ko capture is allowed except for a move at |
| 853 | @code{kom_pos}. Komaster parameters unchanged. |
| 854 | @end itemize |
| 855 | @end itemize |
| 856 | |
| 857 | @subsection Revised 2.7.232 version |
| 858 | |
| 859 | @itemize @bullet |
| 860 | @item Komaster is EMPTY. |
| 861 | @itemize @minus |
| 862 | @item Unconditional ko capture is allowed. Komaster remains EMPTY. |
| 863 | @item Conditional ko capture is allowed. Komaster is set to @samp{O} and |
| 864 | @code{kom_pos} to the location of the ko, where a stone was |
| 865 | just removed. |
| 866 | @end itemize |
| 867 | @item Komaster is @samp{O}: |
| 868 | @itemize @minus |
| 869 | @item Ko capture (both kinds) is allowed only if after playing the move, |
| 870 | @code{is_ko(kom_pos, X)} returns false. In that case, |
| 871 | @code{kom_pos} is updated to the new ko position, i.e. the stone |
| 872 | captured by this move. |
| 873 | @end itemize |
| 874 | @item Komaster is @samp{X}: |
| 875 | @itemize @minus |
| 876 | @item Conditional ko capture is not allowed. |
| 877 | @item Unconditional ko capture is allowed except for a move at |
| 878 | @code{kom_pos}. Komaster parameters unchanged. |
| 879 | @end itemize |
| 880 | @end itemize |
| 881 | |
| 882 | @node Superstrings |
| 883 | @section Superstrings |
| 884 | |
| 885 | A @emph{superstring} is an extended string, where the extensions are |
| 886 | through the following kinds of connections: |
| 887 | |
| 888 | @enumerate |
| 889 | @item Solid connections (just like ordinary string). |
| 890 | @example |
| 891 | OO |
| 892 | @end example |
| 893 | @item Diagonal connection or one space jump through an intersection |
| 894 | where an opponent move would be suicide or self-atari. |
| 895 | @example |
| 896 | @group |
| 897 | ... |
| 898 | O.O |
| 899 | XOX |
| 900 | X.X |
| 901 | @end group |
| 902 | @end example |
| 903 | @item Bamboo joint. |
| 904 | @example |
| 905 | @group |
| 906 | OO |
| 907 | .. |
| 908 | OO |
| 909 | @end group |
| 910 | @end example |
| 911 | @item Diagonal connection where both adjacent intersections are empty. |
| 912 | @example |
| 913 | @group |
| 914 | .O |
| 915 | O. |
| 916 | @end group |
| 917 | @end example |
| 918 | @item Connection through adjacent or diagonal tactically captured stones. |
| 919 | Connections of this type are omitted when the superstring code is |
| 920 | called from @file{reading.c}, but included when the superstring code is |
| 921 | called from @file{owl.c}. |
| 922 | @end enumerate |
| 923 | |
| 924 | Like a dragon, a superstring is an amalgamation of strings, but it is |
| 925 | a much tighter organization of stones than a dragon, and its purpose |
| 926 | is different. Superstrings are encountered already in the tactical |
| 927 | reading because sometimes attacking or defending an element of the |
| 928 | superstring is the best way to attack or defend a string. This is |
| 929 | in contrast with dragons, which are ignored during tactical reading. |
| 930 | |
| 931 | @node Debugging |
| 932 | @section Debugging the reading code |
| 933 | |
| 934 | @cindex How to debug the reading code |
| 935 | @cindex Debugging the reading code |
| 936 | @cindex Reading code debugging tools |
| 937 | |
| 938 | The reading code searches for a path through the move tree to |
| 939 | determine whether a string can be captured. We have a tool for |
| 940 | investigating this with the @option{--decidestring} option. This may |
| 941 | be run with or without an output file. |
| 942 | |
| 943 | Simply running |
| 944 | |
| 945 | @example |
| 946 | |
| 947 | @command{gnugo -t -l [input file name] -L [movenumber] --decidestring [location]} |
| 948 | |
| 949 | @end example |
| 950 | |
| 951 | @noindent |
| 952 | will run @code{attack()} to determine whether the string can be captured. |
| 953 | If it can, it will also run @code{find_defense()} to determine whether or |
| 954 | not it can be defended. It will give a count of the number of |
| 955 | variations read. The @option{-t} is necessary, or else GNU Go will not |
| 956 | report its findings. |
| 957 | |
| 958 | If we add @option{-o @var{output file}} GNU Go will produce |
| 959 | an output file with all variations considered. The variations are |
| 960 | numbered in comments. |
| 961 | |
| 962 | This file of variations is not very useful without a way of |
| 963 | navigating the source code. This is provided with the GDB |
| 964 | source file, listed at the end. You can source this from GDB, |
| 965 | or just make it your GDB init file. |
| 966 | |
| 967 | @cindex GDB |
| 968 | |
| 969 | If you are using GDB to debug GNU Go you may find it less |
| 970 | confusing to compile without optimization. The optimization |
| 971 | sometimes changes the order in which program steps are |
| 972 | executed. For example, to compile @file{reading.c} without optimization, |
| 973 | edit @file{engine/Makefile} to remove the string @code{-O2} from |
| 974 | the file, touch @file{engine/reading.c} and make. Note that the |
| 975 | Makefile is automatically generated and may get overwritten |
| 976 | later. |
| 977 | |
| 978 | If in the course of reading you need to analyze a result where |
| 979 | a function gets its value by returning a cached position from |
| 980 | the hashing code, rerun the example with the hashing turned off |
| 981 | by the command line option @option{--hash 0}. You should get the same |
| 982 | result. (If you do not, please send us a bug report.) Don't |
| 983 | run @option{--hash 0} unless you have a good reason to, since it |
| 984 | increases the number of variations. |
| 985 | |
| 986 | With the source file given at the end of this document loaded, |
| 987 | we can now navigate the variations. It is a good idea to use |
| 988 | cgoban with a small @option{-fontHeight}, so that the |
| 989 | variation window takes in a big picture. (You can resize the |
| 990 | board.) |
| 991 | |
| 992 | Suppose after perusing this file, we find that variation 17 is |
| 993 | interesting and we would like to find out exactly what is |
| 994 | going on here. |
| 995 | |
| 996 | The macro 'jt n' will jump to the n-th variation. |
| 997 | |
| 998 | @example |
| 999 | |
| 1000 | (gdb) set args -l [filename] -L [move number] --decidestring [location] |
| 1001 | (gdb) tbreak main |
| 1002 | (gdb) run |
| 1003 | (gdb) jt 17 |
| 1004 | |
| 1005 | @end example |
| 1006 | |
| 1007 | @noindent |
| 1008 | will then jump to the location in question. |
| 1009 | |
| 1010 | Actually the attack variations and defense variations are numbered |
| 1011 | separately. (But @code{find_defense()} is only run if @code{attack()} succeeds, |
| 1012 | so the defense variations may or may not exist.) It is redundant to |
| 1013 | have to tbreak main each time. So there are two macros avar and dvar. |
| 1014 | |
| 1015 | @example |
| 1016 | |
| 1017 | (gdb) avar 17 |
| 1018 | |
| 1019 | @end example |
| 1020 | |
| 1021 | @noindent |
| 1022 | restarts the program, and jumps to the 17-th attack variation. |
| 1023 | |
| 1024 | @example |
| 1025 | |
| 1026 | (gdb) dvar 17 |
| 1027 | |
| 1028 | @end example |
| 1029 | |
| 1030 | @noindent |
| 1031 | jumps to the 17-th defense variation. Both variation sets are |
| 1032 | found in the same sgf file, though they are numbered separately. |
| 1033 | |
| 1034 | Other commands defined in this file: |
| 1035 | |
| 1036 | @example |
| 1037 | |
| 1038 | @cindex GNU Go's GDB commands |
| 1039 | |
| 1040 | @command{dump} will print the move stack. |
| 1041 | @command{nv} moves to the next variation |
| 1042 | @command{ascii i j} converts (i,j) to ascii |
| 1043 | |
| 1044 | ####################################################### |
| 1045 | ############### .gdbinit file ############### |
| 1046 | ####################################################### |
| 1047 | |
| 1048 | # this command displays the stack |
| 1049 | |
| 1050 | define dump |
| 1051 | set dump_stack() |
| 1052 | end |
| 1053 | |
| 1054 | # display the name of the move in ascii |
| 1055 | |
| 1056 | define ascii |
| 1057 | set gprintf("%o%m\n",$arg0,$arg1) |
| 1058 | end |
| 1059 | |
| 1060 | # display the all information about a dragon |
| 1061 | |
| 1062 | define dragon |
| 1063 | set ascii_report_dragon("$arg0") |
| 1064 | end |
| 1065 | |
| 1066 | define worm |
| 1067 | set ascii_report_worm("$arg0") |
| 1068 | end |
| 1069 | |
| 1070 | # move to the next variation |
| 1071 | |
| 1072 | define nv |
| 1073 | tbreak trymove |
| 1074 | continue |
| 1075 | finish |
| 1076 | next |
| 1077 | end |
| 1078 | |
| 1079 | # move forward to a particular variation |
| 1080 | |
| 1081 | define jt |
| 1082 | while (count_variations < $arg0) |
| 1083 | nv |
| 1084 | end |
| 1085 | nv |
| 1086 | dump |
| 1087 | end |
| 1088 | |
| 1089 | # restart, jump to a particular attack variation |
| 1090 | |
| 1091 | define avar |
| 1092 | delete |
| 1093 | tbreak sgffile_decidestring |
| 1094 | run |
| 1095 | tbreak attack |
| 1096 | continue |
| 1097 | jt $arg0 |
| 1098 | end |
| 1099 | |
| 1100 | # restart, jump to a particular defense variation |
| 1101 | |
| 1102 | define dvar |
| 1103 | delete |
| 1104 | tbreak sgffile_decidestring |
| 1105 | run |
| 1106 | tbreak attack |
| 1107 | continue |
| 1108 | finish |
| 1109 | next 3 |
| 1110 | jt $arg0 |
| 1111 | end |
| 1112 | |
| 1113 | @end example |
| 1114 | |
| 1115 | @node Connection Reading |
| 1116 | @section Connection Reading |
| 1117 | |
| 1118 | GNU Go does reading to determine if strings can be connected. The algorithms |
| 1119 | for this are in @file{readconnect.c}. As with the reading code, the connection |
| 1120 | code is not pattern based. |
| 1121 | |
| 1122 | The connection code is invoked by the engine through the functions: |
| 1123 | |
| 1124 | @itemize |
| 1125 | @item @code{int string_connect(int str1, int str2, int *move)} |
| 1126 | @findex string_connect |
| 1127 | @quotation |
| 1128 | Returns @code{WIN} if @code{str1} and @code{str2} can be connected. |
| 1129 | @end quotation |
| 1130 | @item @code{int disconnect(int str1, int str2, int *move)} |
| 1131 | @findex disconnect |
| 1132 | @quotation |
| 1133 | Returns @code{WIN} if @code{str1} and @code{str2} can be disconnected. |
| 1134 | @end quotation |
| 1135 | @end itemize |
| 1136 | |
| 1137 | To see the connection code in action, you may try the |
| 1138 | following example. |
| 1139 | |
| 1140 | @example |
| 1141 | gnugo --quiet -l connection3.sgf --decide-connection M3/N7 -o vars.sgf |
| 1142 | @end example |
| 1143 | |
| 1144 | (The file @file{connection3.sgf} is in @file{regression/games}.) |
| 1145 | Examine the sgf file produced by this to see what kind of reading |
| 1146 | is done by the functions @code{string_connect()} and |
| 1147 | @code{string_disconnect()}, which are called by the function |
| 1148 | @code{decide_connection}. |
| 1149 | |
| 1150 | One use of the connection code is used is through the autohelper macros |
| 1151 | @code{oplay_connect}, @code{xplay_connect}, @code{oplay_disconnect} and |
| 1152 | @code{xplay_disconnect} which are used in the connection databases. |
| 1153 | |