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