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         /* ...or can't block where we need because of shortage
  41          * of liberties. */
  42         int libs1 = board_group_info(b, group_atxy(b, x + xd - yd * dd, y + yd - xd * dd)).libs;
  43         int libs2 = board_group_info(b, group_atxy(b, x - xd - yd * dd, y - yd - xd * dd)).libs;
  44         if (DEBUGL(6))
  45                 fprintf(stderr, "libs1 %d libs2 %d\n", libs1, libs2);
  46         if (libs1 < 2 || libs2 < 2)
  47                 return false;
  48         if (board_atxy(b, x + xd * 2, y + yd * 2) == lcolor && libs1 < 3)
  49                 return false;
  50         if (board_atxy(b, x - xd * 2, y - yd * 2) == lcolor && libs2 < 3)
  51                 return false;
  52         return true;
  53 }
  54
  55
  56 /* This is a rather expensive ladder reader. It can read out any sequences
  57  * where laddered group should be kept at two liberties. The recursion
  58  * always makes a "to-be-laddered" move and then considers the chaser's
  59  * two alternatives (usually, one of them is trivially refutable). The
  60  * function returns true if there is a branch that ends up with laddered
  61  * group captured, false if not (i.e. for each branch, laddered group can
  62  * gain three liberties). */
  63
  64 static bool
  65 middle_ladder_walk(struct board *b, group_t laddered, coord_t nextmove, enum stone lcolor)
  66 {
  67         assert(board_group_info(b, laddered).libs == 1);
  68
  69         /* First, escape. */
  70         if (DEBUGL(6))
  71                 fprintf(stderr, "  ladder escape %s\n", coord2sstr(nextmove, b));
  72         struct move m = { nextmove, lcolor };
  73         int res = board_play(b, &m);
  74         laddered = group_at(b, laddered);
  75         assert(res >= 0);
  76         if (DEBUGL(8)) {
  77                 board_print(b, stderr);
  78                 fprintf(stderr, "%s c %d\n", coord2sstr(laddered, b), board_group_info(b, laddered).libs);
  79         }
  80
  81         if (board_group_info(b, laddered).libs == 1) {
  82                 if (DEBUGL(6))
  83                         fprintf(stderr, "* we can capture now\n");
  84                 return true;
  85         }
  86         if (board_group_info(b, laddered).libs > 2) {
  87                 if (DEBUGL(6))
  88                         fprintf(stderr, "* we are free now\n");
  89                 return false;
  90         }
  91
  92         foreach_neighbor(b, m.coord, {
  93                 if (board_at(b, c) == stone_other(lcolor) && board_group_info(b, group_at(b, c)).libs == 1) {
  94                         /* We can capture one of the ladder stones
  95                          * anytime later. */
  96                         /* XXX: If we were very lucky, capturing
  97                          * this stone will not help us escape.
  98                          * That should be pretty rate. */
  99                         if (DEBUGL(6))
 100                                 fprintf(stderr, "* can capture chaser\n");
 101                         return false;
 102                 }
 103         });
 104
 105         /* Now, consider alternatives. */
 106         int liblist[2], libs = 0;
 107         for (int i = 0; i < 2; i++) {
 108                 coord_t ataristone = board_group_info(b, laddered).lib[i];
 109                 coord_t escape = board_group_info(b, laddered).lib[1 - i];
 110                 if (immediate_liberty_count(b, escape) > 2 + coord_is_adjecent(ataristone, escape, b)) {
 111                         /* Too much free space, ignore. */
 112                         continue;
 113                 }
 114                 liblist[libs++] = i;
 115         }
 116
 117         /* Try out the alternatives. */
 118         bool is_ladder = false;
 119         for (int i = 0; !is_ladder && i < libs; i++) {
 120                 struct board *b2 = b;
 121                 struct board b2s;
 122                 if (i != libs - 1) {
 123                         board_copy(&b2s, b);
 124                         b2 = &b2s;
 125                 }
 126
 127                 coord_t ataristone = board_group_info(b2, laddered).lib[liblist[i]];
 128                 // coord_t escape = board_group_info(b2, laddered).lib[1 - liblist[i]];
 129                 struct move m = { ataristone, stone_other(lcolor) };
 130                 int res = board_play(b2, &m);
 131                 /* If we just played self-atari, abandon ship. */
 132                 /* XXX: If we were very lucky, capturing this stone will
 133                  * not help us escape. That should be pretty rate. */
 134                 if (DEBUGL(6))
 135                         fprintf(stderr, "(%d=%d) ladder atari %s (%d libs)\n", i, res, coord2sstr(ataristone, b2), board_group_info(b2, group_at(b2, ataristone)).libs);
 136                 if (res >= 0 && board_group_info(b2, group_at(b2, ataristone)).libs > 1)
 137                         is_ladder = middle_ladder_walk(b2, laddered, board_group_info(b2, laddered).lib[0], lcolor);
 138
 139                 if (i != libs - 1) {
 140                         board_done_noalloc(&b2s);
 141                 }
 142         }
 143         if (DEBUGL(6))
 144                 fprintf(stderr, "propagating %d\n", is_ladder);
 145         return is_ladder;
 146 }
 147
 148 bool
 149 is_middle_ladder(struct board *b, coord_t coord, group_t laddered, enum stone lcolor)
 150 {
 151         /* TODO: Remove the redundant parameters. */
 152         assert(board_group_info(b, laddered).libs == 1);
 153         assert(board_group_info(b, laddered).lib[0] == coord);
 154         assert(board_at(b, laddered) == lcolor);
 155
 156         /* If we can move into empty space or do not have enough space
 157          * to escape, this is obviously not a ladder. */
 158         if (immediate_liberty_count(b, coord) != 2) {
 159                 if (DEBUGL(5))
 160                         fprintf(stderr, "no ladder, wrong free space\n");
 161                 return false;
 162         }
 163
 164         /* A fair chance for a ladder. Group in atari, with some but limited
 165          * space to escape. Time for the expensive stuff - set up a temporary
 166          * board and start selective 2-liberty search. */
 167
 168         struct move_queue ccq = { .moves = 0 };
 169         if (can_countercapture(b, lcolor, laddered, lcolor, &ccq, 0)) {
 170                 /* We could escape by countercapturing a group.
 171                  * Investigate. */
 172                 assert(ccq.moves > 0);
 173                 for (unsigned int i = 0; i < ccq.moves; i++) {
 174                         struct board b2;
 175                         board_copy(&b2, b);
 176                         bool is_ladder = middle_ladder_walk(&b2, laddered, ccq.move[i], lcolor);
 177                         board_done_noalloc(&b2);
 178                         if (!is_ladder)
 179                                 return false;
 180                 }
 181         }
 182
 183         struct board b2;
 184         board_copy(&b2, b);
 185         bool is_ladder = middle_ladder_walk(&b2, laddered, board_group_info(&b2, laddered).lib[0], lcolor);
 186         board_done_noalloc(&b2);
 187         return is_ladder;
 188 }
 189
 190 bool
 191 wouldbe_ladder(struct board *b, group_t group, coord_t escapelib, coord_t chaselib, enum stone lcolor)
 192 {
 193         assert(board_group_info(b, group).libs == 2);
 194         assert(board_at(b, group) == lcolor);
 195
 196         if (DEBUGL(6))
 197                 fprintf(stderr, "would-be ladder check - does %s %s play out chasing move %s?\n",
 198                         stone2str(lcolor), coord2sstr(escapelib, b), coord2sstr(chaselib, b));
 199
 200         if (!coord_is_8adjecent(escapelib, chaselib, b)) {
 201                 if (DEBUGL(5))
 202                         fprintf(stderr, "cannot determine ladder for remote simulated stone\n");
 203                 return false;
 204         }
 205
 206         if (neighbor_count_at(b, chaselib, lcolor) != 1 || immediate_liberty_count(b, chaselib) != 2) {
 207                 if (DEBUGL(5))
 208                         fprintf(stderr, "overly trivial for a ladder\n");
 209                 return false;
 210         }
 211
 212         bool is_ladder = false;
 213         struct board b2;
 214         board_copy(&b2, b);
 215
 216         struct move m = { chaselib, stone_other(lcolor) };
 217         int res = board_play(&b2, &m);
 218         if (res >= 0)
 219                 is_ladder = middle_ladder_walk(&b2, group, board_group_info(&b2, group).lib[0], lcolor);
 220
 221         board_done_noalloc(&b2);
 222         return is_ladder;
 223 }