tactics/selfatari.c

   1 #include <assert.h>
   2 #include <stdio.h>
   3 #include <stdlib.h>
   4
   5 #define DEBUG
   6 #include "board.h"
   7 #include "debug.h"
   8 #include "tactics/selfatari.h"
   9
  10
  11 struct selfatari_state {
  12         int groupcts[S_MAX];
  13         group_t groupids[S_MAX][4];
  14
  15         /* This is set if this move puts a group out of _all_
  16          * liberties; we need to watch out for snapback then. */
  17         bool friend_has_no_libs;
  18         /* We may have one liberty, but be looking for one more.
  19          * In that case, @needs_more_lib is id of group
  20          * already providing one, don't consider it again. */
  21         group_t needs_more_lib;
  22         /* ID of the first liberty, providing it again is not
  23          * interesting. */
  24         coord_t needs_more_lib_except;
  25 };
  26
  27 static int
  28 examine_friendly_groups(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
  29 {
  30         for (int i = 0; i < s->groupcts[color]; i++) {
  31                 /* We can escape by connecting to this group if it's
  32                  * not in atari. */
  33                 group_t g = s->groupids[color][i];
  34
  35                 if (board_group_info(b, g).libs == 1) {
  36                         if (!s->needs_more_lib)
  37                                 s->friend_has_no_libs = true;
  38                         // or we already have a friend with 1 lib
  39                         continue;
  40                 }
  41
  42                 /* Could we self-atari the group here? */
  43                 if (board_group_info(b, g).libs > 2)
  44                         return false;
  45
  46                 /* We need to have another liberty, and
  47                  * it must not be the other liberty of
  48                  * the group. */
  49                 int lib2 = board_group_other_lib(b, g, to);
  50                 /* Maybe we already looked at another
  51                  * group providing one liberty? */
  52                 if (s->needs_more_lib && s->needs_more_lib != g
  53                     && s->needs_more_lib_except != lib2)
  54                         return false;
  55
  56                 /* Can we get the liberty locally? */
  57                 /* Yes if we are route to more liberties... */
  58                 if (s->groupcts[S_NONE] > 1)
  59                         return false;
  60                 /* ...or one liberty, but not lib2. */
  61                 if (s->groupcts[S_NONE] > 0
  62                     && !coord_is_adjecent(lib2, to, b))
  63                         return false;
  64
  65                 /* ...ok, then we can still contribute a liberty
  66                  * later by capturing something. */
  67                 s->needs_more_lib = g;
  68                 s->needs_more_lib_except = lib2;
  69                 s->friend_has_no_libs = false;
  70         }
  71
  72         return -1;
  73 }
  74
  75 static int
  76 examine_enemy_groups(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
  77 {
  78         /* We may be able to gain a liberty by capturing this group. */
  79         group_t can_capture = 0;
  80
  81         /* Examine enemy groups: */
  82         for (int i = 0; i < s->groupcts[stone_other(color)]; i++) {
  83                 /* We can escape by capturing this group if it's in atari. */
  84                 group_t g = s->groupids[stone_other(color)][i];
  85                 if (board_group_info(b, g).libs > 1)
  86                         continue;
  87
  88                 /* But we need to get to at least two liberties by this;
  89                  * we already have one outside liberty, or the group is
  90                  * more than 1 stone (in that case, capturing is always
  91                  * nice!). */
  92                 if (s->groupcts[S_NONE] > 0 || !group_is_onestone(b, g))
  93                         return false;
  94                 /* ...or, it's a ko stone, */
  95                 if (neighbor_count_at(b, g, color) + neighbor_count_at(b, g, S_OFFBOARD) == 3) {
  96                         /* and we don't have a group to save: then, just taking
  97                          * single stone means snapback! */
  98                         if (!s->friend_has_no_libs)
  99                                 return false;
 100                 }
 101                 /* ...or, we already have one indirect liberty provided
 102                  * by another group. */
 103                 if (s->needs_more_lib || (can_capture && can_capture != g))
 104                         return false;
 105                 can_capture = g;
 106
 107         }
 108
 109         if (DEBUGL(6))
 110                 fprintf(stderr, "no cap group\n");
 111
 112         if (!s->needs_more_lib && !can_capture && !s->groupcts[S_NONE]) {
 113                 /* We have no hope for more fancy tactics - this move is simply
 114                  * a suicide, not even a self-atari. */
 115                 if (DEBUGL(6))
 116                         fprintf(stderr, "suicide\n");
 117                 return true;
 118         }
 119         /* XXX: I wonder if it makes sense to continue if we actually
 120          * just !s->needs_more_lib. */
 121
 122         return -1;
 123 }
 124
 125 static int
 126 setup_nakade_or_snapback(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
 127 {
 128         /* There is another possibility - we can self-atari if it is
 129          * a nakade: we put an enemy group in atari from the inside. */
 130         /* This branch also allows eyes falsification:
 131          * O O O . .  (This is different from throw-in to false eye
 132          * X X O O .  checked below in that there is no X stone at the
 133          * X . X O .  right of the star point in this diagram.)
 134          * X X X O O
 135          * X O * . . */
 136         /* TODO: Allow to only nakade if the created shape is dead
 137          * (http://senseis.xmp.net/?Nakade). */
 138
 139         /* This branch also covers snapback, which is kind of special
 140          * nakade case. ;-) */
 141         for (int i = 0; i < s->groupcts[stone_other(color)]; i++) {
 142                 group_t g = s->groupids[stone_other(color)][i];
 143                 if (board_group_info(b, g).libs != 2)
 144                         goto next_group;
 145                 /* Simple check not to re-examine the same group. */
 146                 if (i > 0 && s->groupids[stone_other(color)][i] == s->groupids[stone_other(color)][i - 1])
 147                         continue;
 148
 149                 /* We must make sure the other liberty of that group:
 150                  * (i) is an internal liberty
 151                  * (ii) filling it to capture our group will not gain
 152                  * safety */
 153
 154                 /* Let's look at neighbors of the other liberty: */
 155                 int lib2 = board_group_other_lib(b, g, to);
 156                 foreach_neighbor(b, lib2, {
 157                         /* This neighbor of course does not contribute
 158                          * anything to the enemy. */
 159                         if (board_at(b, c) == S_OFFBOARD)
 160                                 continue;
 161
 162                         /* If the other liberty has empty neighbor,
 163                          * it must be the original liberty; otherwise,
 164                          * since the whole group has only 2 liberties,
 165                          * the other liberty may not be internal and
 166                          * we are nakade'ing eyeless group from outside,
 167                          * which is stupid. */
 168                         if (board_at(b, c) == S_NONE) {
 169                                 if (c == to)
 170                                         continue;
 171                                 else
 172                                         goto next_group;
 173                         }
 174
 175                         int g2 = group_at(b, c);
 176                         /* If the neighbor is of our color, it must
 177                          * be also a 2-lib group. If it is more,
 178                          * we CERTAINLY want that liberty to be played
 179                          * first, what if it is an alive group? If it
 180                          * is in atari, we want to extend from it to
 181                          * prevent eye-making capture. However, if it
 182                          * is 2-lib, it is selfatari connecting two
 183                          * nakade'ing groups! */
 184                         /* X X X X  We will not allow play on 'a',
 185                          * X X a X  because 'b' would capture two
 186                          * X O b X  different groups, forming two
 187                          * X X X X  eyes. */
 188                         if (board_at(b, c) == color) {
 189                                 if (board_group_info(b, group_at(b, c)).libs == 2)
 190                                         continue;
 191                                 goto next_group;
 192                         }
 193
 194                         /* The neighbor is enemy color. It's ok if
 195                          * it's still the same group or this is its
 196                          * only liberty. */
 197                         if (g == g2 || board_group_info(b, g2).libs == 1)
 198                                 continue;
 199                         /* Otherwise, it must have the exact same
 200                          * liberties as the original enemy group. */
 201                         if (board_group_info(b, g2).libs == 2
 202                             && (board_group_info(b, g2).lib[0] == to
 203                                 || board_group_info(b, g2).lib[1] == to))
 204                                 continue;
 205
 206                         goto next_group;
 207                 });
 208
 209                 /* Now, we must distinguish between nakade and eye
 210                  * falsification; we must not falsify an eye by more
 211                  * than two stones. */
 212                 if (s->groupcts[color] < 1)
 213                         return false; // simple throw-in
 214                 if (s->groupcts[color] == 1 && group_is_onestone(b, s->groupids[color][0])) {
 215                         /* More complex throw-in - we are in one of
 216                          * three situations:
 217                          * a O O O O X  b O O O X  c O O O X
 218                          *   O . X . O    O X . .    O . X .
 219                          *   # # # # #    # # # #    # # # #
 220                          * b is desirable here (since maybe O has no
 221                          * backup two eyes); a may be desirable, but
 222                          * is tested next in check_throwin(). c is
 223                          * never desirable. */
 224                         group_t g2 = s->groupids[color][0];
 225                         assert(board_group_info(b, g2).libs <= 2);
 226                         if (board_group_info(b, g2).libs == 1)
 227                                 return false; // b
 228                         goto next_group; // a or c
 229                 }
 230
 231                 /* We would create more than 2-stone group; in that
 232                  * case, the liberty of our result must be lib2,
 233                  * indicating this really is a nakade. */
 234                 for (int j = 0; j < s->groupcts[color]; j++) {
 235                         group_t g2 = s->groupids[color][j];
 236                         assert(board_group_info(b, g2).libs <= 2);
 237                         if (board_group_info(b, g2).libs == 2) {
 238                                 if (board_group_info(b, g2).lib[0] != lib2
 239                                     && board_group_info(b, g2).lib[1] != lib2)
 240                                         goto next_group;
 241                         } else {
 242                                 assert(board_group_info(b, g2).lib[0] == to);
 243                         }
 244                 }
 245
 246                 return false;
 247 next_group:
 248                 /* Unless we are dealing with snapback setup, we don't need to look
 249                  * further. */
 250                 if (s->groupcts[color])
 251                         return -1;
 252         }
 253
 254         return -1;
 255 }
 256
 257 static int
 258 check_throwin(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
 259 {
 260         /* We can be throwing-in to false eye:
 261          * X X X O X X X O X X X X X
 262          * X . * X * O . X * O O . X
 263          * # # # # # # # # # # # # # */
 264         /* We cannot sensibly throw-in into a corner. */
 265         if (neighbor_count_at(b, to, S_OFFBOARD) < 2
 266             && neighbor_count_at(b, to, stone_other(color))
 267                + neighbor_count_at(b, to, S_OFFBOARD) == 3
 268             && board_is_false_eyelike(b, to, stone_other(color))) {
 269                 assert(s->groupcts[color] <= 1);
 270                 /* Single-stone throw-in may be ok... */
 271                 if (s->groupcts[color] == 0) {
 272                         /* O X .  There is one problem - when it's
 273                          * . * X  actually not a throw-in!
 274                          * # # #  */
 275                         foreach_neighbor(b, to, {
 276                                 if (board_at(b, c) == S_NONE) {
 277                                         /* Is the empty neighbor an escape path? */
 278                                         /* (Note that one S_NONE neighbor is already @to.) */
 279                                         if (neighbor_count_at(b, c, stone_other(color))
 280                                             + neighbor_count_at(b, c, S_OFFBOARD) < 2)
 281                                                 return -1;
 282                                 }
 283                         });
 284                         return false;
 285                 }
 286
 287                 /* Multi-stone throwin...? */
 288                 assert(s->groupcts[color] == 1);
 289                 group_t g = s->groupids[color][0];
 290
 291                 assert(board_group_info(b, g).libs <= 2);
 292                 /* Suicide is definitely NOT ok, no matter what else
 293                  * we could test. */
 294                 if (board_group_info(b, g).libs == 1)
 295                         return true;
 296
 297                 /* In that case, we must be connected to at most one stone,
 298                  * or throwin will not destroy any eyes. */
 299                 if (group_is_onestone(b, g))
 300                         return false;
 301         }
 302         return -1;
 303 }
 304
 305 bool
 306 is_bad_selfatari_slow(struct board *b, enum stone color, coord_t to)
 307 {
 308         if (DEBUGL(5))
 309                 fprintf(stderr, "sar check %s %s\n", stone2str(color), coord2sstr(to, b));
 310         /* Assess if we actually gain any liberties by this escape route.
 311          * Note that this is not 100% as we cannot check whether we are
 312          * connecting out or just to ourselves. */
 313
 314         struct selfatari_state s;
 315         memset(&s, 0, sizeof(s));
 316         int d;
 317
 318         foreach_neighbor(b, to, {
 319                 enum stone color = board_at(b, c);
 320                 s.groupids[color][s.groupcts[color]++] = group_at(b, c);
 321         });
 322
 323         /* We have shortage of liberties; that's the point. */
 324         assert(s.groupcts[S_NONE] <= 1);
 325
 326         d = examine_friendly_groups(b, color, to, &s);
 327         if (d >= 0)
 328                 return d;
 329
 330         if (DEBUGL(6))
 331                 fprintf(stderr, "no friendly group\n");
 332
 333         d = examine_enemy_groups(b, color, to, &s);
 334         if (d >= 0)
 335                 return d;
 336
 337         if (DEBUGL(6))
 338                 fprintf(stderr, "no escape\n");
 339
 340         d = setup_nakade_or_snapback(b, color, to, &s);
 341         if (d >= 0)
 342                 return d;
 343
 344         if (DEBUGL(6))
 345                 fprintf(stderr, "no nakade group\n");
 346
 347         d = check_throwin(b, color, to, &s);
 348         if (d >= 0)
 349                 return d;
 350
 351         if (DEBUGL(6))
 352                 fprintf(stderr, "no throw-in group\n");
 353
 354         /* No way to pull out, no way to connect out. This really
 355          * is a bad self-atari! */
 356         return true;
 357 }