tactics/ladder.c

   1 #include <assert.h>
   2 #include <stdio.h>
   3 #include <stdlib.h>
   4
   5 #define QUICK_BOARD_CODE
   6
   7 #define DEBUG
   8 #include "board.h"
   9 #include "debug.h"
  10 #include "mq.h"
  11 #include "tactics/1lib.h"
  12 #include "tactics/ladder.h"
  13
  14
  15 bool
  16 is_border_ladder(struct board *b, coord_t coord, group_t laddered, enum stone lcolor)
  17 {
  18         if (can_countercapture(b, lcolor, laddered, lcolor, NULL, 0))
  19                 return false;
  20
  21         int x = coord_x(coord, b), y = coord_y(coord, b);
  22
  23         if (DEBUGL(5))
  24                 fprintf(stderr, "border ladder\n");
  25         /* Direction along border; xd is horiz. border, yd vertical. */
  26         int xd = 0, yd = 0;
  27         if (board_atxy(b, x + 1, y) == S_OFFBOARD || board_atxy(b, x - 1, y) == S_OFFBOARD)
  28                 yd = 1;
  29         else
  30                 xd = 1;
  31         /* Direction from the border; -1 is above/left, 1 is below/right. */
  32         int dd = (board_atxy(b, x + yd, y + xd) == S_OFFBOARD) ? 1 : -1;
  33         if (DEBUGL(6))
  34                 fprintf(stderr, "xd %d yd %d dd %d\n", xd, yd, dd);
  35         /* | ? ?
  36          * | . O #
  37          * | c X #
  38          * | . O #
  39          * | ? ?   */
  40         /* This is normally caught, unless we have friends both above
  41          * and below... */
  42         if (board_atxy(b, x + xd * 2, y + yd * 2) == lcolor
  43             && board_atxy(b, x - xd * 2, y - yd * 2) == lcolor)
  44                 return false;
  45
  46         /* ...or can't block where we need because of shortage
  47          * of liberties. */
  48         group_t g1 = group_atxy(b, x + xd - yd * dd, y + yd - xd * dd);
  49         int libs1 = board_group_info(b, g1).libs;
  50         group_t g2 = group_atxy(b, x - xd - yd * dd, y - yd - xd * dd);
  51         int libs2 = board_group_info(b, g2).libs;
  52         if (DEBUGL(6))
  53                 fprintf(stderr, "libs1 %d libs2 %d\n", libs1, libs2);
  54         /* Already in atari? */
  55         if (libs1 < 2 || libs2 < 2)
  56                 return false;
  57         /* Would be self-atari? */
  58         if (libs1 < 3 && (board_atxy(b, x + xd * 2, y + yd * 2) != S_NONE
  59                           || coord_is_adjecent(board_group_info(b, g1).lib[0], board_group_info(b, g1).lib[1], b)))
  60                 return false;
  61         if (libs2 < 3 && (board_atxy(b, x - xd * 2, y - yd * 2) != S_NONE
  62                           || coord_is_adjecent(board_group_info(b, g2).lib[0], board_group_info(b, g2).lib[1], b)))
  63                 return false;
  64         return true;
  65 }
  66
  67
  68 /* This is a rather expensive ladder reader. It can read out any sequences
  69  * where laddered group should be kept at two liberties. The recursion
  70  * always makes a "to-be-laddered" move and then considers the chaser's
  71  * two alternatives (usually, one of them is trivially refutable). The
  72  * function returns true if there is a branch that ends up with laddered
  73  * group captured, false if not (i.e. for each branch, laddered group can
  74  * gain three liberties). */
  75
  76 static bool
  77 middle_ladder_walk(struct board *b, struct board *bset, group_t laddered, coord_t nextmove, enum stone lcolor)
  78 {
  79         assert(board_group_info(b, laddered).libs == 1);
  80
  81         /* First, escape. */
  82         if (DEBUGL(6))
  83                 fprintf(stderr, "  ladder escape %s\n", coord2sstr(nextmove, b));
  84         struct move m = { nextmove, lcolor };
  85         int res = board_play(b, &m);
  86         laddered = group_at(b, laddered);
  87         assert(res >= 0);
  88         if (DEBUGL(8)) {
  89                 board_print(b, stderr);
  90                 fprintf(stderr, "%s c %d\n", coord2sstr(laddered, b), board_group_info(b, laddered).libs);
  91         }
  92
  93         if (board_group_info(b, laddered).libs == 1) {
  94                 if (DEBUGL(6))
  95                         fprintf(stderr, "* we can capture now\n");
  96                 return true;
  97         }
  98         if (board_group_info(b, laddered).libs > 2) {
  99                 if (DEBUGL(6))
 100                         fprintf(stderr, "* we are free now\n");
 101                 return false;
 102         }
 103
 104         foreach_neighbor(b, m.coord, {
 105                 if (board_at(b, c) == stone_other(lcolor) && board_group_info(b, group_at(b, c)).libs == 1) {
 106                         /* We can capture one of the ladder stones
 107                          * anytime later. */
 108                         /* XXX: If we were very lucky, capturing
 109                          * this stone will not help us escape.
 110                          * That should be pretty rate. */
 111                         if (DEBUGL(6))
 112                                 fprintf(stderr, "* can capture chaser\n");
 113                         return false;
 114                 }
 115         });
 116
 117         /* Now, consider alternatives. */
 118         int liblist[2], libs = 0;
 119         for (int i = 0; i < 2; i++) {
 120                 coord_t ataristone = board_group_info(b, laddered).lib[i];
 121                 coord_t escape = board_group_info(b, laddered).lib[1 - i];
 122                 if (immediate_liberty_count(b, escape) > 2 + coord_is_adjecent(ataristone, escape, b)) {
 123                         /* Too much free space, ignore. */
 124                         continue;
 125                 }
 126                 liblist[libs++] = i;
 127         }
 128
 129         /* Try out the alternatives. */
 130         bool is_ladder = false;
 131         for (int i = 0; !is_ladder && i < libs; i++) {
 132                 struct board *b2 = b;
 133                 if (i != libs - 1) {
 134                         b2 = bset++;
 135                         board_copy(b2, b);
 136                 }
 137
 138                 coord_t ataristone = board_group_info(b2, laddered).lib[liblist[i]];
 139                 // coord_t escape = board_group_info(b2, laddered).lib[1 - liblist[i]];
 140                 struct move m = { ataristone, stone_other(lcolor) };
 141                 int res = board_play(b2, &m);
 142                 /* If we just played self-atari, abandon ship. */
 143                 /* XXX: If we were very lucky, capturing this stone will
 144                  * not help us escape. That should be pretty rate. */
 145                 if (DEBUGL(6))
 146                         fprintf(stderr, "(%d=%d) ladder atari %s (%d libs)\n", i, res, coord2sstr(ataristone, b2), board_group_info(b2, group_at(b2, ataristone)).libs);
 147                 if (res >= 0 && board_group_info(b2, group_at(b2, ataristone)).libs > 1)
 148                         is_ladder = middle_ladder_walk(b2, bset, laddered, board_group_info(b2, laddered).lib[0], lcolor);
 149
 150                 if (i != libs - 1) {
 151                         board_done_noalloc(b2);
 152                 }
 153         }
 154         if (DEBUGL(6))
 155                 fprintf(stderr, "propagating %d\n", is_ladder);
 156         return is_ladder;
 157 }
 158
 159 bool
 160 is_middle_ladder(struct board *b, coord_t coord, group_t laddered, enum stone lcolor)
 161 {
 162         /* TODO: Remove the redundant parameters. */
 163         assert(board_group_info(b, laddered).libs == 1);
 164         assert(board_group_info(b, laddered).lib[0] == coord);
 165         assert(board_at(b, laddered) == lcolor);
 166
 167         /* If we can move into empty space or do not have enough space
 168          * to escape, this is obviously not a ladder. */
 169         if (immediate_liberty_count(b, coord) != 2) {
 170                 if (DEBUGL(5))
 171                         fprintf(stderr, "no ladder, wrong free space\n");
 172                 return false;
 173         }
 174
 175         /* A fair chance for a ladder. Group in atari, with some but limited
 176          * space to escape. Time for the expensive stuff - set up a temporary
 177          * board and start selective 2-liberty search. */
 178
 179         struct board *bset = malloc2(BOARD_MAX_SIZE * 2 * sizeof(struct board));
 180
 181         struct move_queue ccq = { .moves = 0 };
 182         if (can_countercapture(b, lcolor, laddered, lcolor, &ccq, 0)) {
 183                 /* We could escape by countercapturing a group.
 184                  * Investigate. */
 185                 assert(ccq.moves > 0);
 186                 for (unsigned int i = 0; i < ccq.moves; i++) {
 187                         struct board b2;
 188                         board_copy(&b2, b);
 189                         bool is_ladder = middle_ladder_walk(&b2, bset, laddered, ccq.move[i], lcolor);
 190                         board_done_noalloc(&b2);
 191                         if (!is_ladder) {
 192                                 free(bset);
 193                                 return false;
 194                         }
 195                 }
 196         }
 197
 198         struct board b2;
 199         board_copy(&b2, b);
 200         bool is_ladder = middle_ladder_walk(&b2, bset, laddered, board_group_info(&b2, laddered).lib[0], lcolor);
 201         board_done_noalloc(&b2);
 202         free(bset);
 203         return is_ladder;
 204 }
 205
 206 bool
 207 wouldbe_ladder(struct board *b, group_t group, coord_t escapelib, coord_t chaselib, enum stone lcolor)
 208 {
 209         assert(board_group_info(b, group).libs == 2);
 210         assert(board_at(b, group) == lcolor);
 211
 212         if (DEBUGL(6))
 213                 fprintf(stderr, "would-be ladder check - does %s %s play out chasing move %s?\n",
 214                         stone2str(lcolor), coord2sstr(escapelib, b), coord2sstr(chaselib, b));
 215
 216         if (!coord_is_8adjecent(escapelib, chaselib, b)) {
 217                 if (DEBUGL(5))
 218                         fprintf(stderr, "cannot determine ladder for remote simulated stone\n");
 219                 return false;
 220         }
 221
 222         if (neighbor_count_at(b, chaselib, lcolor) != 1 || immediate_liberty_count(b, chaselib) != 2) {
 223                 if (DEBUGL(5))
 224                         fprintf(stderr, "overly trivial for a ladder\n");
 225                 return false;
 226         }
 227
 228         bool is_ladder = false;
 229         struct board *bset = malloc2(BOARD_MAX_SIZE * 2 * sizeof(struct board));
 230         struct board b2;
 231         board_copy(&b2, b);
 232
 233         struct move m = { chaselib, stone_other(lcolor) };
 234         int res = board_play(&b2, &m);
 235         if (res >= 0)
 236                 is_ladder = middle_ladder_walk(&b2, bset, group, board_group_info(&b2, group).lib[0], lcolor);
 237
 238         board_done_noalloc(&b2);
 239         free(bset);
 240         return is_ladder;
 241 }