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 "tactics/ladder.h"
   9
  10
  11 bool
  12 is_border_ladder(struct board *b, coord_t coord, enum stone lcolor)
  13 {
  14         int x = coord_x(coord, b), y = coord_y(coord, b);
  15
  16         if (DEBUGL(5))
  17                 fprintf(stderr, "border ladder\n");
  18         /* Direction along border; xd is horiz. border, yd vertical. */
  19         int xd = 0, yd = 0;
  20         if (board_atxy(b, x + 1, y) == S_OFFBOARD || board_atxy(b, x - 1, y) == S_OFFBOARD)
  21                 yd = 1;
  22         else
  23                 xd = 1;
  24         /* Direction from the border; -1 is above/left, 1 is below/right. */
  25         int dd = (board_atxy(b, x + yd, y + xd) == S_OFFBOARD) ? 1 : -1;
  26         if (DEBUGL(6))
  27                 fprintf(stderr, "xd %d yd %d dd %d\n", xd, yd, dd);
  28         /* | ? ?
  29          * | . O #
  30          * | c X #
  31          * | . O #
  32          * | ? ?   */
  33         /* This is normally caught, unless we have friends both above
  34          * and below... */
  35         if (board_atxy(b, x + xd * 2, y + yd * 2) == lcolor
  36             && board_atxy(b, x - xd * 2, y - yd * 2) == lcolor)
  37                 return false;
  38         /* ...or can't block where we need because of shortage
  39          * of liberties. */
  40         int libs1 = board_group_info(b, group_atxy(b, x + xd - yd * dd, y + yd - xd * dd)).libs;
  41         int libs2 = board_group_info(b, group_atxy(b, x - xd - yd * dd, y - yd - xd * dd)).libs;
  42         if (DEBUGL(6))
  43                 fprintf(stderr, "libs1 %d libs2 %d\n", libs1, libs2);
  44         if (libs1 < 2 || libs2 < 2)
  45                 return false;
  46         if (board_atxy(b, x + xd * 2, y + yd * 2) == lcolor && libs1 < 3)
  47                 return false;
  48         if (board_atxy(b, x - xd * 2, y - yd * 2) == lcolor && libs2 < 3)
  49                 return false;
  50         return true;
  51 }
  52
  53
  54 /* This is a rather expensive ladder reader. It can read out any sequences
  55  * where laddered group should be kept at two liberties. The recursion
  56  * always makes a "to-be-laddered" move and then considers the chaser's
  57  * two alternatives (usually, one of them is trivially refutable). The
  58  * function returns true if there is a branch that ends up with laddered
  59  * group captured, false if not (i.e. for each branch, laddered group can
  60  * gain three liberties). */
  61
  62 static bool
  63 middle_ladder_walk(struct board *b, group_t laddered, coord_t nextmove, enum stone lcolor)
  64 {
  65         assert(board_group_info(b, laddered).libs == 1);
  66
  67         /* First, escape. */
  68         if (DEBUGL(6))
  69                 fprintf(stderr, "  ladder escape %s\n", coord2sstr(nextmove, b));
  70         struct move m = { nextmove, lcolor };
  71         int res = board_play(b, &m);
  72         laddered = group_at(b, laddered);
  73         assert(res >= 0);
  74         if (DEBUGL(8)) {
  75                 board_print(b, stderr);
  76                 fprintf(stderr, "%s c %d\n", coord2sstr(laddered, b), board_group_info(b, laddered).libs);
  77         }
  78
  79         if (board_group_info(b, laddered).libs == 1) {
  80                 if (DEBUGL(6))
  81                         fprintf(stderr, "* we can capture now\n");
  82                 return true;
  83         }
  84         if (board_group_info(b, laddered).libs > 2) {
  85                 if (DEBUGL(6))
  86                         fprintf(stderr, "* we are free now\n");
  87                 return false;
  88         }
  89
  90         foreach_neighbor(b, m.coord, {
  91                 if (board_at(b, c) == stone_other(lcolor) && board_group_info(b, group_at(b, c)).libs == 1) {
  92                         /* We can capture one of the ladder stones
  93                          * anytime later. */
  94                         /* XXX: If we were very lucky, capturing
  95                          * this stone will not help us escape.
  96                          * That should be pretty rate. */
  97                         if (DEBUGL(6))
  98                                 fprintf(stderr, "* can capture chaser\n");
  99                         return false;
 100                 }
 101         });
 102
 103         /* Now, consider alternatives. */
 104         int liblist[2], libs = 0;
 105         for (int i = 0; i < 2; i++) {
 106                 coord_t ataristone = board_group_info(b, laddered).lib[i];
 107                 coord_t escape = board_group_info(b, laddered).lib[1 - i];
 108                 if (immediate_liberty_count(b, escape) > 2 + coord_is_adjecent(ataristone, escape, b)) {
 109                         /* Too much free space, ignore. */
 110                         continue;
 111                 }
 112                 liblist[libs++] = i;
 113         }
 114
 115         /* Try out the alternatives. */
 116         bool is_ladder = false;
 117         for (int i = 0; !is_ladder && i < libs; i++) {
 118                 struct board *b2 = b;
 119                 struct board b2s;
 120                 if (i != libs - 1) {
 121                         board_copy(&b2s, b);
 122                         b2 = &b2s;
 123                 }
 124
 125                 coord_t ataristone = board_group_info(b2, laddered).lib[liblist[i]];
 126                 // coord_t escape = board_group_info(b2, laddered).lib[1 - liblist[i]];
 127                 struct move m = { ataristone, stone_other(lcolor) };
 128                 int res = board_play(b2, &m);
 129                 /* If we just played self-atari, abandon ship. */
 130                 /* XXX: If we were very lucky, capturing this stone will
 131                  * not help us escape. That should be pretty rate. */
 132                 if (DEBUGL(6))
 133                         fprintf(stderr, "(%d=%d) ladder atari %s (%d libs)\n", i, res, coord2sstr(ataristone, b2), board_group_info(b2, group_at(b2, ataristone)).libs);
 134                 if (res >= 0 && board_group_info(b2, group_at(b2, ataristone)).libs > 1)
 135                         is_ladder = middle_ladder_walk(b2, laddered, board_group_info(b2, laddered).lib[0], lcolor);
 136
 137                 if (i != libs - 1) {
 138                         board_done_noalloc(&b2s);
 139                 }
 140         }
 141         if (DEBUGL(6))
 142                 fprintf(stderr, "propagating %d\n", is_ladder);
 143         return is_ladder;
 144 }
 145
 146 bool
 147 is_middle_ladder(struct board *b, coord_t coord, group_t laddered, enum stone lcolor)
 148 {
 149         /* TODO: Remove the redundant parameters. */
 150         assert(board_group_info(b, laddered).libs == 1);
 151         assert(board_group_info(b, laddered).lib[0] == coord);
 152         assert(board_at(b, laddered) == lcolor);
 153
 154         /* If we can move into empty space or do not have enough space
 155          * to escape, this is obviously not a ladder. */
 156         if (immediate_liberty_count(b, coord) != 2) {
 157                 if (DEBUGL(5))
 158                         fprintf(stderr, "no ladder, wrong free space\n");
 159                 return false;
 160         }
 161
 162         /* A fair chance for a ladder. Group in atari, with some but limited
 163          * space to escape. Time for the expensive stuff - set up a temporary
 164          * board and start selective 2-liberty search. */
 165         struct board b2;
 166         board_copy(&b2, b);
 167         bool is_ladder = middle_ladder_walk(&b2, laddered, board_group_info(&b2, laddered).lib[0], lcolor);
 168         board_done_noalloc(&b2);
 169         return is_ladder;
 170 }
 171
 172 bool
 173 wouldbe_ladder(struct board *b, group_t group, coord_t escapelib, coord_t chaselib, enum stone lcolor)
 174 {
 175         assert(board_group_info(b, group).libs == 2);
 176         assert(board_at(b, group) == lcolor);
 177
 178         if (DEBUGL(6))
 179                 fprintf(stderr, "would-be ladder check - does %s %s play out chasing move %s?\n",
 180                         stone2str(lcolor), coord2sstr(escapelib, b), coord2sstr(chaselib, b));
 181
 182         if (!coord_is_8adjecent(escapelib, chaselib, b)) {
 183                 if (DEBUGL(5))
 184                         fprintf(stderr, "cannot determine ladder for remote simulated stone\n");
 185                 return false;
 186         }
 187
 188         if (neighbor_count_at(b, chaselib, lcolor) != 1 || immediate_liberty_count(b, chaselib) != 2) {
 189                 if (DEBUGL(5))
 190                         fprintf(stderr, "overly trivial for a ladder\n");
 191                 return false;
 192         }
 193
 194         bool is_ladder = false;
 195         struct board b2;
 196         board_copy(&b2, b);
 197
 198         struct move m = { chaselib, stone_other(lcolor) };
 199         int res = board_play(&b2, &m);
 200         if (res >= 0)
 201                 is_ladder = middle_ladder_walk(&b2, group, board_group_info(&b2, group).lib[0], lcolor);
 202
 203         board_done_noalloc(&b2);
 204         return is_ladder;
 205 }