can_countercapture(): foolproof api

[pachi.git] / tactics / selfatari.c

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#define QUICK_BOARD_CODE

#define DEBUG
#include "board.h"
#include "debug.h"
#include "mq.h"
#include "random.h"
#include "tactics/1lib.h"
#include "tactics/selfatari.h"
#include "nakade.h"


struct selfatari_state {
        int groupcts[S_MAX];
        group_t groupids[S_MAX][4];
        coord_t groupneis[S_MAX][4];

        /* This is set if this move puts a group out of _all_
         * liberties; we need to watch out for snapback then. */
        bool friend_has_no_libs;
        /* We may have one liberty, but be looking for one more.
         * In that case, @needs_more_lib is id of group
         * already providing one, don't consider it again. */
        group_t needs_more_lib;
        /* ID of the first liberty, providing it again is not
         * interesting. */
        coord_t needs_more_lib_except;
};

static bool
three_liberty_suicide(struct board *b, group_t g, enum stone color, coord_t to, struct selfatari_state *s)
{
        /* If a group has three liberties, by playing on one of
         * them it is possible to kill the group clumsily. Check
         * against that condition: "After our move, the opponent
         * can unconditionally capture the group."
         *
         * Examples:
         *
         * O O O O O O O   X X O O O O O O     v-v- ladder
         * O X X X X X O   . O X X X X X O   . . . O O
         * O X ! . ! X O   . O X ! . ! O .   O X X . O
         * O X X X X X O   # # # # # # # #   O O O O O */

        /* Extract the other two liberties. */
        coord_t other_libs[2];
        bool other_libs_adj[2];
        for (int i = 0, j = 0; i < 3; i++) {
                coord_t lib = board_group_info(b, g).lib[i];
                if (lib != to) {
                        other_libs_adj[j] = coord_is_adjecent(lib, to, b);
                        other_libs[j++] = lib;
                }
        }

        /* Make sure this move is not useful by gaining liberties,
         * splitting the other two liberties (quite possibly splitting
         * 3-eyespace!) or connecting to a different group. */
        if (immediate_liberty_count(b, to) - (other_libs_adj[0] || other_libs_adj[1]) > 0)
                return false;
        assert(!(other_libs_adj[0] && other_libs_adj[1]));
        if (s->groupcts[color] > 1)
                return false;

        /* Playing on the third liberty might be useful if it enables
         * capturing some group (are we doing nakade or semeai?). */
        for (int i = 0; i < s->groupcts[stone_other(color)]; i++)
                if (board_group_info(b, s->groupids[stone_other(color)][i]).libs <= 3)
                        return false;


        /* Okay. This looks like a pretty dangerous situation. The
         * move looks useless, it definitely converts us to a 2-lib
         * group. But we still want to play it e.g. if it takes off
         * liberties of some unconspicous enemy group, and of course
         * also at the game end to leave just single-point eyes. */

        if (DEBUGL(6))
                fprintf(stderr, "3-lib danger\n");

        /* Therefore, the final suicidal test is: (After filling this
         * liberty,) when opponent fills liberty [0], playing liberty
         * [1] will not help the group, or vice versa. */
        bool other_libs_neighbors = coord_is_adjecent(other_libs[0], other_libs[1], b);
        for (int i = 0; i < 2; i++) {
                int null_libs = other_libs_neighbors + other_libs_adj[i];
                if (board_is_one_point_eye(b, other_libs[1 - i], color)) {
                        /* The other liberty is an eye, happily go ahead.
                         * There are of course situations where this will
                         * take off semeai liberties, but without this check,
                         * many terminal endgame plays will be messed up. */
                        return false;
                }
                if (immediate_liberty_count(b, other_libs[i]) - null_libs > 1) {
                        /* Gains liberties. */
                        /* TODO: Check for ladder! */
next_lib:
                        continue;
                }
                foreach_neighbor(b, other_libs[i], {
                        if (board_at(b, c) == color
                            && group_at(b, c) != g
                            && board_group_info(b, group_at(b, c)).libs > 1) {
                                /* Can connect to a friend. */
                                /* TODO: > 2? But maybe the group can capture
                                 * a neighbor! But then better let it do that
                                 * first? */
                                goto next_lib;
                        }
                });
                /* If we can capture a neighbor, better do it now
                 * before wasting a liberty. So no need to check. */
                /* Ok, the last liberty has no way to get out. */
                if (DEBUGL(6))
                        fprintf(stderr, "3-lib dangerous: %s\n", coord2sstr(other_libs[i], b));
                return true;
        }

        return false;
}

static int
examine_friendly_groups(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
{
        for (int i = 0; i < s->groupcts[color]; i++) {
                /* We can escape by connecting to this group if it's
                 * not in atari. */
                group_t g = s->groupids[color][i];

                if (board_group_info(b, g).libs == 1) {
                        if (!s->needs_more_lib)
                                s->friend_has_no_libs = true;
                        // or we already have a friend with 1 lib
                        continue;
                }

                /* Could we self-atari the group here? */
                if (board_group_info(b, g).libs > 2) {
                        if (board_group_info(b, g).libs == 3
                            && three_liberty_suicide(b, g, color, to, s))
                                return true;
                        return false;
                }

                /* We need to have another liberty, and
                 * it must not be the other liberty of
                 * the group. */
                int lib2 = board_group_other_lib(b, g, to);
                /* Maybe we already looked at another
                 * group providing one liberty? */
                if (s->needs_more_lib && s->needs_more_lib != g
                    && s->needs_more_lib_except != lib2)
                        return false;

                /* Can we get the liberty locally? */
                /* Yes if we are route to more liberties... */
                if (s->groupcts[S_NONE] > 1)
                        return false;
                /* ...or one liberty, but not lib2. */
                if (s->groupcts[S_NONE] > 0
                    && !coord_is_adjecent(lib2, to, b))
                        return false;

                /* ...ok, then we can still contribute a liberty
                 * later by capturing something. */
                s->needs_more_lib = g;
                s->needs_more_lib_except = lib2;
                s->friend_has_no_libs = false;
        }

        return -1;
}

static int
examine_enemy_groups(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
{
        /* We may be able to gain a liberty by capturing this group. */
        group_t can_capture = 0;

        /* Examine enemy groups: */
        for (int i = 0; i < s->groupcts[stone_other(color)]; i++) {
                /* We can escape by capturing this group if it's in atari. */
                group_t g = s->groupids[stone_other(color)][i];
                if (board_group_info(b, g).libs > 1)
                        continue;

                /* But we need to get to at least two liberties by this;
                 * we already have one outside liberty, or the group is
                 * more than 1 stone (in that case, capturing is always
                 * nice!). */
                if (s->groupcts[S_NONE] > 0 || !group_is_onestone(b, g))
                        return false;
                /* ...or, it's a ko stone, */
                if (neighbor_count_at(b, g, color) + neighbor_count_at(b, g, S_OFFBOARD) == 3) {
                        /* and we don't have a group to save: then, just taking
                         * single stone means snapback! */
                        if (!s->friend_has_no_libs)
                                return false;
                }
                /* ...or, we already have one indirect liberty provided
                 * by another group. */
                if (s->needs_more_lib || (can_capture && can_capture != g))
                        return false;
                can_capture = g;

        }

        if (DEBUGL(6))
                fprintf(stderr, "no cap group\n");

        if (!s->needs_more_lib && !can_capture && !s->groupcts[S_NONE]) {
                /* We have no hope for more fancy tactics - this move is simply
                 * a suicide, not even a self-atari. */
                if (DEBUGL(6))
                        fprintf(stderr, "suicide\n");
                return true;
        }
        /* XXX: I wonder if it makes sense to continue if we actually
         * just !s->needs_more_lib. */

        return -1;
}

static inline bool
is_neighbor_group(struct board *b, enum stone color, group_t g, struct selfatari_state *s)
{
        for (int i = 0; i < s->groupcts[color]; i++)
                if (g == s->groupids[color][i])
                        return true;
        return false;
}


/* Instead of playing this self-atari, could we have connected/escaped by 
 * playing on the other liberty of a neighboring group ?
 * Or there's a strong enemy group there (only checked if @check_enemy is set)
 */
static inline bool
is_bad_nakade(struct board *b, enum stone color, coord_t to, coord_t lib2,
              bool check_enemy, struct selfatari_state *s)
{
        /* Let's look at neighbors of the other liberty: */
        foreach_neighbor(b, lib2, {
                /* If the other liberty has empty neighbor,
                 * it must be the original liberty; otherwise,
                 * since the whole group has only 2 liberties,
                 * the other liberty may not be internal and
                 * we are nakade'ing eyeless group from outside,
                 * which is stupid. */
                if (board_at(b, c) == S_NONE) {
                        if (c == to)
                                continue;
                        else
                                return true;
                }});


        /* Let's look at neighbors of the other liberty: */
        foreach_neighbor(b, lib2, {
                if (board_at(b, c) != color)
                        continue;

                group_t g2 = group_at(b, c);
                /* Looking for a group we don't know about */
                if (is_neighbor_group(b, color, g2, s))
                        continue;
                
                /* Should connect these groups instead of self-atari
                 * on the other side. */
                if (board_at(b, c) == color)
                        return true;
                

                /* FIXME Do we really need this ? */
                if (!check_enemy)
                        continue;

                /* The neighbor is enemy color. It's ok if this is its
                 * only liberty or it's one of our neighbor groups. */
                if (board_group_info(b, g2).libs == 1)
                        continue;
                if (board_group_info(b, g2).libs == 2
                    && (board_group_info(b, g2).lib[0] == to
                        || board_group_info(b, g2).lib[1] == to))
                        continue;

                /* Stronger enemy group. No nakade. */
                return true;
        });
        return false;
}

/* Instead of playing this self-atari, could we have connected/escaped by 
 * playing on the other liberty of a neighboring group ? */
static inline bool
can_escape_instead(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
{
        for (int i = 0; i < s->groupcts[color]; i++) {
                group_t g = s->groupids[color][i];
                if (board_group_info(b, g).libs != 2)
                        continue;
                coord_t other = board_group_other_lib(b, g, to);
                
                /* Let's look at the other liberty of that group. */
                if (immediate_liberty_count(b, other) >= 2 ||      /* Can escape ! */
                    is_bad_nakade(b, color, to, other, false, s))  /* Should connect instead */
                        return true;
        }
        return false;
}

static inline bool
unreachable_lib_from_neighbors(struct board *b, enum stone color, coord_t to, struct selfatari_state *s,
                               coord_t lib)
{
        for (int i = 0; i < s->groupcts[color]; i++) {
                group_t g = s->groupids[color][i];
                for (int j = 0; j < board_group_info(b, g).libs; j++)
                        if (board_group_info(b, g).lib[j] == lib)
                                return false;
        }
        return true;
}

/* This only looks at existing empty spots, not captures */
static inline bool
capture_would_make_extra_eye(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
{
        foreach_neighbor(b, to, {
                if (board_at(b, c) == S_NONE)
                        if (unreachable_lib_from_neighbors(b, color, to, s, c))
                                return true;
        });
        return false;
}

static bool
nakade_making_dead_shape(struct board *b, enum stone color, coord_t to, struct selfatari_state *s,
                         bool atariing_group, int stones)
{
        int cap_would_make_eye = false;
        
        if (!stones)
                return false;
        assert(stones != 1);
        assert(stones <= 5);
        
        /* If not atariing surrounding group it's a good move if:
         *   - Shape after capturing us is dead   AND 
         *     - Oponent gets extra eye if he plays first OR
         *     - would create living shape
         *
         * If atariing surrounding group we only care about dead shape. */
        
        if (!atariing_group)
                cap_would_make_eye = capture_would_make_extra_eye(b, color, to, s);
        /* TODO: If there's so much eye space that even with filling + capture
         *       opponent still makes an extra eye it's a silly move. */
     
        /* Can oponent make living shape if we don't play ?
         * (don't bother killing stuff that's already dead...) */
        if (!atariing_group && !cap_would_make_eye && s->groupcts[color] == 1) 
        {
            int would_live = false;
            int prev_neighbor = neighbor_count_at(b, to, color);

            /* Play opponent color where we want to play */
            with_move(b, to, stone_other(color), {

                /* Had 2 libs ? One more move to capture then */
                if (prev_neighbor == neighbor_count_at(b, to, color)) {
                        group_t standing = -1;      
                        foreach_neighbor(b, to, {
                                group_t g = group_at(b, c);                 
                                if (board_at(b, c) == color) {
                                        assert(board_group_info(b, g).libs == 1);  /* Should be in atari */
                                        standing = g;
                                }
                        });
                        assert(standing != -1);
                        
                        with_move_strict(b, board_group_info(b, standing).lib[0], stone_other(color), {
                                /* Empty now since it's been captured */
                                coord_t empty = group_base(s->groupids[color][0]);
                                would_live = !nakade_dead_shape(b, empty, stone_other(color));
                        });
                }
                else {  /* Empty now since it's been captured */                        
                        coord_t empty = group_base(s->groupids[color][0]);
                        would_live = !nakade_dead_shape(b, empty, stone_other(color));
                }
            });

            if (!would_live)    /* And !cap_would_make_eye here */
                    return false;   /* Bad nakade */
        }
        
        int dead_shape;
        /* Play self-atari */
        with_move_strict(b, to, color, {        
                /* Play capture */
                group_t g = group_at(b, to);
                with_move_strict(b, board_group_info(b, g).lib[0], stone_other(color), {
                                dead_shape = nakade_dead_shape(b, to, stone_other(color));
                });
        });
        return dead_shape;
}


/* More complex throw-in, or in-progress capture from
 * the inside - we are in one of several situations:
 * a O O O O X  b O O O X  c O O O X  d O O O O O
 *   O . X . O    O X . .    O . X .    O . X . O
 *   # # # # #    # # # #    # # # #    # # # # #
 * Throw-ins have been taken care of in check_throwin(),
 * so it's either b or d now:
 * - b is desirable here (since maybe O has no backup two eyes)
 * - d is desirable if putting group in atari (otherwise we
 *   would never capture a single-eyed group). */
#define check_throw_in_or_inside_capture(b, color, to, s, capturing)                    \
        if (s->groupcts[color] == 1 && group_is_onestone(b, s->groupids[color][0])) {   \
                group_t g2 = s->groupids[color][0];                                     \
                assert(board_group_info(b, g2).libs <= 2);                              \
                if (board_group_info(b, g2).libs == 1)                                  \
                        return false;  /*  b */                                         \
                return !capturing;                                                      \
        }


static int
setup_nakade_or_snapback(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
{
        /* There is another possibility - we can self-atari if it is
         * a nakade: we put an enemy group in atari from the inside. */
        /* This branch also allows eyes falsification:
         * O O O . .  (This is different from throw-in to false eye
         * X X O O .  checked below in that there is no X stone at the
         * X . X O .  right of the star point in this diagram.)
         * X X X O O
         * X O * . . */
        /* We also allow to only nakade if the created shape is dead
         * (http://senseis.xmp.net/?Nakade). */
        
        /* This branch also covers snapback, which is kind of special
         * nakade case. ;-) */
        /* FIXME looks like check_throwin() does it actually. */
        
        /* Look at the enemy groups and determine the other contended
         * liberty. We must make sure the liberty:
         * (i) is an internal liberty
         * (ii) filling it to capture our group will not gain safety */
        coord_t lib2 = pass;
        for (int i = 0; i < s->groupcts[stone_other(color)]; i++) {
                group_t g = s->groupids[stone_other(color)][i];
                if (board_group_info(b, g).libs != 2)
                        continue;

                coord_t this_lib2 = board_group_other_lib(b, g, to);
                if (is_pass(lib2)) 
                        lib2 = this_lib2;
                else if (this_lib2 != lib2) {
                        /* If we have two neighboring groups that do
                         * not share the other liberty, this for sure
                         * is not a good nakade. */
                        return -1;
                }
        }
        if (is_pass(lib2)) {
                /* Not putting any group in atari.
                 * Could be creating dead shape though */
                
                /* Before checking if it's a useful nakade
                 * make sure it can't connect out ! */
                if (can_escape_instead(b, color, to, s))
                        return -1;
                
                check_throw_in_or_inside_capture(b, color, to, s, false);
                
                int stones = 0;
                for (int j = 0; j < s->groupcts[color]; j++) {
                        group_t g2 = s->groupids[color][j];
                        stones += group_stone_count(b, g2, 6);
                        if (stones > 5)
                                return true;
                }
                
                return (nakade_making_dead_shape(b, color, to, s, false, stones) ? false : -1);
        }

        /* Let's look at neighbors of the other liberty: */
        if (is_bad_nakade(b, color, to, lib2, true, s) ||
            can_escape_instead(b, color, to, s))
                return -1;
        
        /* Now, we must distinguish between nakade and eye
         * falsification; moreover, we must not falsify an eye
         * by more than two stones. */

        if (s->groupcts[color] < 1)
                return false;  /* Simple throw-in, an easy case */
        
        check_throw_in_or_inside_capture(b, color, to, s, true);
        
        /* We would create more than 2-stone group; in that
         * case, the liberty of our result must be lib2,
         * indicating this really is a nakade. */
        int stones = 0;
        for (int j = 0; j < s->groupcts[color]; j++) {
                group_t g2 = s->groupids[color][j];
                assert(board_group_info(b, g2).libs <= 2);
                if (board_group_info(b, g2).libs == 2) {
                        if (board_group_info(b, g2).lib[0] != lib2
                            && board_group_info(b, g2).lib[1] != lib2)
                                return -1;
                } else
                        assert(board_group_info(b, g2).lib[0] == to);           
                /* See below: */
                stones += group_stone_count(b, g2, 6);
                if (stones > 5)
                        return true;
        }
        
        return !nakade_making_dead_shape(b, color, to, s, true, stones);
}

#if 0
/* Fast but there are issues with this (triangle six is not dead !)
 * We also need to know status if opponent plays first */
static inline int
nakade_making_dead_shape_hack(struct board *b, enum stone color, coord_t to, int lib2,
                              struct selfatari_state *s, int stones)
{
        /* It also remains to be seen whether it is nakade
         * and not seki destruction. To do this properly, we
         * would have to look at the group shape. But we can
         * cheat too! Brett Combs helps to introduce a static
         * rule that should in fact cover *all* cases:
         * 1. Total number of pre-selfatari nakade stones must
         *    be 5 or smaller. (See above for that.)
         * 2. If the selfatari is 8-touching all nakade stones,
         *    it is proper nakade.
         * 3. Otherwise, there must be only a single nakade
         *    group, it must be at least 4-stone and its other
         *    liberty must be 8-touching the same number of
         *    stones as us. */
        int touch8 = neighbor_count_at(b, to, color);
        foreach_diag_neighbor(b, to) {
                if (board_at(b, c) != color) continue;
                /* Consider only internal stones. Otherwise, e.g.
                 * X O . X
                 * X . O X  can make trouble, bottom O is
                 * O X X X  irrelevant. */
                if (board_group_info(b, group_at(b, c)).lib[0] == to
                    || board_group_info(b, group_at(b, c)).lib[1] == to)
                        touch8++;
        } foreach_diag_neighbor_end;
        if (touch8 == stones)
                return true;

        if (s->groupcts[color] > 1)
                return false;
        if (stones == 3)   // 4 stones and self-atari not 8-connected to all of them -> living shape
                return false;
        if (stones < 3)    // always dead shape
                return true;
        
        int ltouch8 = neighbor_count_at(b, lib2, color);
        foreach_diag_neighbor(b, lib2) {
                if (board_at(b, c) != color) continue;
                if (board_group_info(b, group_at(b, c)).lib[0] == to
                    || board_group_info(b, group_at(b, c)).lib[1] == to)
                        ltouch8++;
        } foreach_diag_neighbor_end;
        return ltouch8 == touch8;
}
#endif


static int
check_throwin(struct board *b, enum stone color, coord_t to, struct selfatari_state *s)
{
        /* We can be throwing-in to false eye:
         * X X X O X X X O X X X X X
         * X . * X * O . X * O O . X
         * # # # # # # # # # # # # # */
        /* We cannot sensibly throw-in into a corner. */
        if (neighbor_count_at(b, to, S_OFFBOARD) < 2
            && neighbor_count_at(b, to, stone_other(color))
               + neighbor_count_at(b, to, S_OFFBOARD) == 3
            && board_is_false_eyelike(b, to, stone_other(color))) {
                assert(s->groupcts[color] <= 1);
                /* Single-stone throw-in may be ok... */
                if (s->groupcts[color] == 0) {
                        /* O X .  There is one problem - when it's
                         * . * X  actually not a throw-in!
                         * # # #  */
                        foreach_neighbor(b, to, {
                                if (board_at(b, c) == S_NONE) {
                                        /* Is the empty neighbor an escape path? */
                                        /* (Note that one S_NONE neighbor is already @to.) */
                                        if (neighbor_count_at(b, c, stone_other(color))
                                            + neighbor_count_at(b, c, S_OFFBOARD) < 2)
                                                return -1;
                                }
                        });
                        return false;
                }

                /* Multi-stone throwin...? */
                assert(s->groupcts[color] == 1);
                group_t g = s->groupids[color][0];

                assert(board_group_info(b, g).libs <= 2);
                /* Suicide is definitely NOT ok, no matter what else
                 * we could test. */
                if (board_group_info(b, g).libs == 1)
                        return true;

                /* In that case, we must be connected to at most one stone,
                 * or throwin will not destroy any eyes. */
                if (group_is_onestone(b, g))
                        return false;
        }
        return -1;
}

bool
is_bad_selfatari_slow(struct board *b, enum stone color, coord_t to)
{
        if (DEBUGL(5))
                fprintf(stderr, "sar check %s %s\n", stone2str(color), coord2sstr(to, b));
        /* Assess if we actually gain any liberties by this escape route.
         * Note that this is not 100% as we cannot check whether we are
         * connecting out or just to ourselves. */

        struct selfatari_state s;
        memset(&s, 0, sizeof(s));
        int d;

        foreach_neighbor(b, to, {
                enum stone color = board_at(b, c);
                group_t group = group_at(b, c);
                bool dup = false;
                for (int i = 0; i < s.groupcts[color]; i++)
                        if (s.groupids[color][i] == group) {
                                dup = true;
                                break;
                        }
                if (!dup) {
                        s.groupneis[color][s.groupcts[color]] = c;
                        s.groupids[color][s.groupcts[color]++] = group_at(b, c);
                }
        });

        /* We have shortage of liberties; that's the point. */
        assert(s.groupcts[S_NONE] <= 1);

        d = examine_friendly_groups(b, color, to, &s);
        if (d >= 0)
                return d;
        if (DEBUGL(6))
                fprintf(stderr, "no friendly group\n");

        d = examine_enemy_groups(b, color, to, &s);
        if (d >= 0)
                return d;
        if (DEBUGL(6))
                fprintf(stderr, "no capture\n");

        d = check_throwin(b, color, to, &s);
        if (d >= 0)
                return d;

        if (DEBUGL(6))
                fprintf(stderr, "no throw-in group\n");

        d = setup_nakade_or_snapback(b, color, to, &s);
        if (d >= 0)
                return d;
        if (DEBUGL(6))
                fprintf(stderr, "no nakade group\n");


        /* No way to pull out, no way to connect out. This really
         * is a bad self-atari! */
        return true;
}


coord_t
selfatari_cousin(struct board *b, enum stone color, coord_t coord, group_t *bygroup)
{
        group_t groups[4]; int groups_n = 0;
        int groupsbycolor[4] = {0, 0, 0, 0};
        if (DEBUGL(6))
                fprintf(stderr, "cousin group search: ");
        foreach_neighbor(b, coord, {
                enum stone s = board_at(b, c);
                group_t g = group_at(b, c);
                if (board_group_info(b, g).libs == 2) {
                        groups[groups_n++] = g;
                        groupsbycolor[s]++;
                        if (DEBUGL(6))
                                fprintf(stderr, "%s(%s) ", coord2sstr(c, b), stone2str(s));
                }
        });
        if (DEBUGL(6))
                fprintf(stderr, "\n");

        if (!groups_n)
                return pass;

        int gn;
        if (groupsbycolor[stone_other(color)]) {
                /* Prefer to fill the other liberty of an opponent
                 * group to filling own approach liberties. */
                int gl = fast_random(groups_n);
                for (gn = gl; gn < groups_n; gn++)
                        if (board_at(b, groups[gn]) == stone_other(color))
                                goto found;
                for (gn = 0; gn < gl; gn++)
                        if (board_at(b, groups[gn]) == stone_other(color))
                                goto found;
found:;
        } else {
                gn = fast_random(groups_n);
        }
        int gl = gn;
        for (; gn - gl < groups_n; gn++) {
                int gnm = gn % groups_n;
                group_t group = groups[gnm];

                coord_t lib2;
                /* Can we get liberties by capturing a neighbor? */
                struct move_queue ccq; ccq.moves = 0;
                if (board_at(b, group) == color &&
                    can_countercapture(b, group, &ccq, 0)) {
                        lib2 = mq_pick(&ccq);

                } else {
                        lib2 = board_group_other_lib(b, group, coord);
                        if (board_is_one_point_eye(b, lib2, board_at(b, group)))
                                continue;
                        if (is_bad_selfatari(b, color, lib2))
                                continue;
                }
                if (bygroup)
                        *bygroup = group;
                return lib2;
        }
        return pass;
}