I've dithered a bit in the past about whether or not it's allowable

[sgt/puzzles] / pegs.c

/*
 * pegs.c: the classic Peg Solitaire game.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <ctype.h>
#include <math.h>

#include "puzzles.h"
#include "tree234.h"

#define GRID_HOLE 0
#define GRID_PEG  1
#define GRID_OBST 2

enum {
    COL_BACKGROUND,
    COL_HIGHLIGHT,
    COL_LOWLIGHT,
    COL_PEG,
    NCOLOURS
};

/*
 * Grid shapes. I do some macro ickery here to ensure that my enum
 * and the various forms of my name list always match up.
 */
#define TYPELIST(A) \
    A(CROSS,Cross,cross) \
    A(OCTAGON,Octagon,octagon) \
    A(RANDOM,Random,random)
#define ENUM(upper,title,lower) TYPE_ ## upper,
#define TITLE(upper,title,lower) #title,
#define LOWER(upper,title,lower) #lower,
#define CONFIG(upper,title,lower) ":" #title

enum { TYPELIST(ENUM) TYPECOUNT };
static char const *const pegs_titletypes[] = { TYPELIST(TITLE) };
static char const *const pegs_lowertypes[] = { TYPELIST(LOWER) };
#define TYPECONFIG TYPELIST(CONFIG)

#define FLASH_FRAME 0.13F

struct game_params {
    int w, h;
    int type;
};

struct game_state {
    int w, h;
    int completed;
    unsigned char *grid;
};

static game_params *default_params(void)
{
    game_params *ret = snew(game_params);

    ret->w = ret->h = 7;
    ret->type = TYPE_CROSS;

    return ret;
}

static const struct game_params pegs_presets[] = {
    {7, 7, TYPE_CROSS},
    {7, 7, TYPE_OCTAGON},
    {5, 5, TYPE_RANDOM},
    {7, 7, TYPE_RANDOM},
    {9, 9, TYPE_RANDOM},
};

static int game_fetch_preset(int i, char **name, game_params **params)
{
    game_params *ret;
    char str[80];

    if (i < 0 || i >= lenof(pegs_presets))
        return FALSE;

    ret = snew(game_params);
    *ret = pegs_presets[i];

    strcpy(str, pegs_titletypes[ret->type]);
    if (ret->type == TYPE_RANDOM)
        sprintf(str + strlen(str), " %dx%d", ret->w, ret->h);

    *name = dupstr(str);
    *params = ret;
    return TRUE;
}

static void free_params(game_params *params)
{
    sfree(params);
}

static game_params *dup_params(game_params *params)
{
    game_params *ret = snew(game_params);
    *ret = *params;                    /* structure copy */
    return ret;
}

static void decode_params(game_params *params, char const *string)
{
    char const *p = string;
    int i;

    params->w = atoi(p);
    while (*p && isdigit((unsigned char)*p)) p++;
    if (*p == 'x') {
        p++;
        params->h = atoi(p);
        while (*p && isdigit((unsigned char)*p)) p++;
    } else {
        params->h = params->w;
    }

    for (i = 0; i < lenof(pegs_lowertypes); i++)
        if (!strcmp(p, pegs_lowertypes[i]))
            params->type = i;
}

static char *encode_params(game_params *params, int full)
{
    char str[80];

    sprintf(str, "%dx%d", params->w, params->h);
    if (full) {
        assert(params->type >= 0 && params->type < lenof(pegs_lowertypes));
        strcat(str, pegs_lowertypes[params->type]);
    }
    return dupstr(str);
}

static config_item *game_configure(game_params *params)
{
    config_item *ret = snewn(4, config_item);
    char buf[80];

    ret[0].name = "Width";
    ret[0].type = C_STRING;
    sprintf(buf, "%d", params->w);
    ret[0].sval = dupstr(buf);
    ret[0].ival = 0;

    ret[1].name = "Height";
    ret[1].type = C_STRING;
    sprintf(buf, "%d", params->h);
    ret[1].sval = dupstr(buf);
    ret[1].ival = 0;

    ret[2].name = "Board type";
    ret[2].type = C_CHOICES;
    ret[2].sval = TYPECONFIG;
    ret[2].ival = params->type;

    ret[3].name = NULL;
    ret[3].type = C_END;
    ret[3].sval = NULL;
    ret[3].ival = 0;

    return ret;
}

static game_params *custom_params(config_item *cfg)
{
    game_params *ret = snew(game_params);

    ret->w = atoi(cfg[0].sval);
    ret->h = atoi(cfg[1].sval);
    ret->type = cfg[2].ival;

    return ret;
}

static char *validate_params(game_params *params, int full)
{
    if (full && (params->w <= 3 || params->h <= 3))
        return "Width and height must both be greater than three";

    /*
     * It might be possible to implement generalisations of Cross
     * and Octagon, but only if I can find a proof that they're all
     * soluble. For the moment, therefore, I'm going to disallow
     * them at any size other than the standard one.
     */
    if (full && (params->type == TYPE_CROSS || params->type == TYPE_OCTAGON)) {
        if (params->w != 7 || params->h != 7)
            return "This board type is only supported at 7x7";
    }
    return NULL;
}

/* ----------------------------------------------------------------------
 * Beginning of code to generate random Peg Solitaire boards.
 * 
 * This procedure is done with no aesthetic judgment, no effort at
 * symmetry, no difficulty grading and generally no finesse
 * whatsoever. We simply begin with an empty board containing a
 * single peg, and repeatedly make random reverse moves until it's
 * plausibly full. This typically yields a scrappy haphazard mess
 * with several holes, an uneven shape, and no redeeming features
 * except guaranteed solubility.
 *
 * My only concessions to sophistication are (a) to repeat the
 * generation process until I at least get a grid that touches
 * every edge of the specified board size, and (b) to try when
 * selecting moves to reuse existing space rather than expanding
 * into new space (so that non-rectangular board shape becomes a
 * factor during play).
 */

struct move {
    /*
     * x,y are the start point of the move during generation (hence
     * its endpoint during normal play).
     * 
     * dx,dy are the direction of the move during generation.
     * Absolute value 1. Hence, for example, x=3,y=5,dx=1,dy=0
     * means that the move during generation starts at (3,5) and
     * ends at (5,5), and vice versa during normal play.
     */
    int x, y, dx, dy;
    /*
     * cost is 0, 1 or 2, depending on how many GRID_OBSTs we must
     * turn into GRID_HOLEs to play this move.
     */
    int cost;
};

static int movecmp(void *av, void *bv)
{
    struct move *a = (struct move *)av;
    struct move *b = (struct move *)bv;

    if (a->y < b->y)
        return -1;
    else if (a->y > b->y)
        return +1;

    if (a->x < b->x)
        return -1;
    else if (a->x > b->x)
        return +1;

    if (a->dy < b->dy)
        return -1;
    else if (a->dy > b->dy)
        return +1;

    if (a->dx < b->dx)
        return -1;
    else if (a->dx > b->dx)
        return +1;

    return 0;
}

static int movecmpcost(void *av, void *bv)
{
    struct move *a = (struct move *)av;
    struct move *b = (struct move *)bv;

    if (a->cost < b->cost)
        return -1;
    else if (a->cost > b->cost)
        return +1;

    return movecmp(av, bv);
}

struct movetrees {
    tree234 *bymove, *bycost;
};

static void update_moves(unsigned char *grid, int w, int h, int x, int y,
                         struct movetrees *trees)
{
    struct move move;
    int dir, pos;

    /*
     * There are twelve moves that can include (x,y): three in each
     * of four directions. Check each one to see if it's possible.
     */
    for (dir = 0; dir < 4; dir++) {
        int dx, dy;

        if (dir & 1)
            dx = 0, dy = dir - 2;
        else
            dy = 0, dx = dir - 1;

        assert(abs(dx) + abs(dy) == 1);

        for (pos = 0; pos < 3; pos++) {
            int v1, v2, v3;

            move.dx = dx;
            move.dy = dy;
            move.x = x - pos*dx;
            move.y = y - pos*dy;

            if (move.x < 0 || move.x >= w || move.y < 0 || move.y >= h)
                continue;              /* completely invalid move */
            if (move.x+2*move.dx < 0 || move.x+2*move.dx >= w ||
                move.y+2*move.dy < 0 || move.y+2*move.dy >= h)
                continue;              /* completely invalid move */

            v1 = grid[move.y * w + move.x];
            v2 = grid[(move.y+move.dy) * w + (move.x+move.dx)];
            v3 = grid[(move.y+2*move.dy)*w + (move.x+2*move.dx)];
            if (v1 == GRID_PEG && v2 != GRID_PEG && v3 != GRID_PEG) {
                struct move *m;

                move.cost = (v2 == GRID_OBST) + (v3 == GRID_OBST);

                /*
                 * This move is possible. See if it's already in
                 * the tree.
                 */
                m = find234(trees->bymove, &move, NULL);
                if (m && m->cost != move.cost) {
                    /*
                     * It's in the tree but listed with the wrong
                     * cost. Remove the old version.
                     */
#ifdef GENERATION_DIAGNOSTICS
                    printf("correcting %d%+d,%d%+d at cost %d\n",
                           m->x, m->dx, m->y, m->dy, m->cost);
#endif
                    del234(trees->bymove, m);
                    del234(trees->bycost, m);
                    sfree(m);
                    m = NULL;
                }
                if (!m) {
                    struct move *m, *m2;
                    m = snew(struct move);
                    *m = move;
                    m2 = add234(trees->bymove, m);
                    m2 = add234(trees->bycost, m);
                    assert(m2 == m);
#ifdef GENERATION_DIAGNOSTICS
                    printf("adding %d%+d,%d%+d at cost %d\n",
                           move.x, move.dx, move.y, move.dy, move.cost);
#endif
                } else {
#ifdef GENERATION_DIAGNOSTICS
                    printf("not adding %d%+d,%d%+d at cost %d\n",
                           move.x, move.dx, move.y, move.dy, move.cost);
#endif
                }
            } else {
                /*
                 * This move is impossible. If it is already in the
                 * tree, delete it.
                 * 
                 * (We make use here of the fact that del234
                 * doesn't have to be passed a pointer to the
                 * _actual_ element it's deleting: it merely needs
                 * one that compares equal to it, and it will
                 * return the one it deletes.)
                 */
                struct move *m = del234(trees->bymove, &move);
#ifdef GENERATION_DIAGNOSTICS
                printf("%sdeleting %d%+d,%d%+d\n", m ? "" : "not ",
                       move.x, move.dx, move.y, move.dy);
#endif
                if (m) {
                    del234(trees->bycost, m);
                    sfree(m);
                }
            }
        }
    }
}

static void pegs_genmoves(unsigned char *grid, int w, int h, random_state *rs)
{
    struct movetrees atrees, *trees = &atrees;
    struct move *m;
    int x, y, i, nmoves;

    trees->bymove = newtree234(movecmp);
    trees->bycost = newtree234(movecmpcost);

    for (y = 0; y < h; y++)
        for (x = 0; x < w; x++)
            if (grid[y*w+x] == GRID_PEG)
                update_moves(grid, w, h, x, y, trees);

    nmoves = 0;

    while (1) {
        int limit, maxcost, index;
        struct move mtmp, move, *m;

        /*
         * See how many moves we can make at zero cost. Make one,
         * if possible. Failing that, make a one-cost move, and
         * then a two-cost one.
         * 
         * After filling at least half the input grid, we no longer
         * accept cost-2 moves: if that's our only option, we give
         * up and finish.
         */
        mtmp.y = h+1;
        maxcost = (nmoves < w*h/2 ? 2 : 1);
        m = NULL;                      /* placate optimiser */
        for (mtmp.cost = 0; mtmp.cost <= maxcost; mtmp.cost++) {
            limit = -1;
            m = findrelpos234(trees->bycost, &mtmp, NULL, REL234_LT, &limit);
#ifdef GENERATION_DIAGNOSTICS
            printf("%d moves available with cost %d\n", limit+1, mtmp.cost);
#endif
            if (m)
                break;
        }
        if (!m)
            break;

        index = random_upto(rs, limit+1);
        move = *(struct move *)index234(trees->bycost, index);

#ifdef GENERATION_DIAGNOSTICS
        printf("selecting move %d%+d,%d%+d at cost %d\n",
               move.x, move.dx, move.y, move.dy, move.cost);
#endif

        grid[move.y * w + move.x] = GRID_HOLE;
        grid[(move.y+move.dy) * w + (move.x+move.dx)] = GRID_PEG;
        grid[(move.y+2*move.dy)*w + (move.x+2*move.dx)] = GRID_PEG;

        for (i = 0; i <= 2; i++) {
            int tx = move.x + i*move.dx;
            int ty = move.y + i*move.dy;
            update_moves(grid, w, h, tx, ty, trees);
        }

        nmoves++;
    }

    while ((m = delpos234(trees->bymove, 0)) != NULL) {
        del234(trees->bycost, m);
        sfree(m);
    }
    freetree234(trees->bymove);
    freetree234(trees->bycost);
}

static void pegs_generate(unsigned char *grid, int w, int h, random_state *rs)
{
    while (1) {
        int x, y, extremes;

        memset(grid, GRID_OBST, w*h);
        grid[(h/2) * w + (w/2)] = GRID_PEG;
#ifdef GENERATION_DIAGNOSTICS
        printf("beginning move selection\n");
#endif
        pegs_genmoves(grid, w, h, rs);
#ifdef GENERATION_DIAGNOSTICS
        printf("finished move selection\n");
#endif

        extremes = 0;
        for (y = 0; y < h; y++) {
            if (grid[y*w+0] != GRID_OBST)
                extremes |= 1;
            if (grid[y*w+w-1] != GRID_OBST)
                extremes |= 2;
        }
        for (x = 0; x < w; x++) {
            if (grid[0*w+x] != GRID_OBST)
                extremes |= 4;
            if (grid[(h-1)*w+x] != GRID_OBST)
                extremes |= 8;
        }

        if (extremes == 15)
            break;
#ifdef GENERATION_DIAGNOSTICS
        printf("insufficient extent; trying again\n");
#endif
    }
#ifdef GENERATION_DIAGNOSTICS
    fflush(stdout);
#endif
}

/* ----------------------------------------------------------------------
 * End of board generation code. Now for the client code which uses
 * it as part of the puzzle.
 */

static char *new_game_desc(game_params *params, random_state *rs,
                           char **aux, int interactive)
{
    int w = params->w, h = params->h;
    unsigned char *grid;
    char *ret;
    int i;

    grid = snewn(w*h, unsigned char);
    if (params->type == TYPE_RANDOM) {
        pegs_generate(grid, w, h, rs);
    } else {
        int x, y, cx, cy, v;

        for (y = 0; y < h; y++)
            for (x = 0; x < w; x++) {
                v = GRID_OBST;         /* placate optimiser */
                switch (params->type) {
                  case TYPE_CROSS:
                    cx = abs(x - w/2);
                    cy = abs(y - h/2);
                    if (cx == 0 && cy == 0)
                        v = GRID_HOLE;
                    else if (cx > 1 && cy > 1)
                        v = GRID_OBST;
                    else
                        v = GRID_PEG;
                    break;
                  case TYPE_OCTAGON:
                    cx = abs(x - w/2);
                    cy = abs(y - h/2);
                    if (cx + cy > 1 + max(w,h)/2)
                        v = GRID_OBST;
                    else
                        v = GRID_PEG;
                    break;
                }
                grid[y*w+x] = v;
            }

        if (params->type == TYPE_OCTAGON) {
            /*
             * The octagonal (European) solitaire layout is
             * actually _insoluble_ with the starting hole at the
             * centre. Here's a proof:
             * 
             * Colour the squares of the board diagonally in
             * stripes of three different colours, which I'll call
             * A, B and C. So the board looks like this:
             * 
             *     A B C
             *   A B C A B
             * A B C A B C A
             * B C A B C A B
             * C A B C A B C
             *   B C A B C
             *     A B C
             * 
             * Suppose we keep running track of the number of pegs
             * occuping each colour of square. This colouring has
             * the property that any valid move whatsoever changes
             * all three of those counts by one (two of them go
             * down and one goes up), which means that the _parity_
             * of every count flips on every move.
             * 
             * If the centre square starts off unoccupied, then
             * there are twelve pegs on each colour and all three
             * counts start off even; therefore, after 35 moves all
             * three counts would have to be odd, which isn't
             * possible if there's only one peg left. []
             * 
             * This proof works just as well if the starting hole
             * is _any_ of the thirteen positions labelled B. Also,
             * we can stripe the board in the opposite direction
             * and rule out any square labelled B in that colouring
             * as well. This leaves:
             * 
             *     Y n Y
             *   n n Y n n
             * Y n n Y n n Y
             * n Y Y n Y Y n
             * Y n n Y n n Y
             *   n n Y n n
             *     Y n Y
             * 
             * where the ns are squares we've proved insoluble, and
             * the Ys are the ones remaining.
             * 
             * That doesn't prove all those starting positions to
             * be soluble, of course; they're merely the ones we
             * _haven't_ proved to be impossible. Nevertheless, it
             * turns out that they are all soluble, so when the
             * user requests an Octagon board the simplest thing is
             * to pick one of these at random.
             * 
             * Rather than picking equiprobably from those twelve
             * positions, we'll pick equiprobably from the three
             * equivalence classes
             */
            switch (random_upto(rs, 3)) {
              case 0:
                /* Remove a random corner piece. */
                {
                    int dx, dy;

                    dx = random_upto(rs, 2) * 2 - 1;   /* +1 or -1 */
                    dy = random_upto(rs, 2) * 2 - 1;   /* +1 or -1 */
                    if (random_upto(rs, 2))
                        dy *= 3;
                    else
                        dx *= 3;
                    grid[(3+dy)*w+(3+dx)] = GRID_HOLE;
                }
                break;
              case 1:
                /* Remove a random piece two from the centre. */
                {
                    int dx, dy;
                    dx = 2 * (random_upto(rs, 2) * 2 - 1);
                    if (random_upto(rs, 2))
                        dy = 0;
                    else
                        dy = dx, dx = 0;
                    grid[(3+dy)*w+(3+dx)] = GRID_HOLE;
                }
                break;
              default /* case 2 */:
                /* Remove a random piece one from the centre. */
                {
                    int dx, dy;
                    dx = random_upto(rs, 2) * 2 - 1;
                    if (random_upto(rs, 2))
                        dy = 0;
                    else
                        dy = dx, dx = 0;
                    grid[(3+dy)*w+(3+dx)] = GRID_HOLE;
                }
                break;
            }
        }
    }

    /*
     * Encode a game description which is simply a long list of P
     * for peg, H for hole or O for obstacle.
     */
    ret = snewn(w*h+1, char);
    for (i = 0; i < w*h; i++)
        ret[i] = (grid[i] == GRID_PEG ? 'P' :
                  grid[i] == GRID_HOLE ? 'H' : 'O');
    ret[w*h] = '\0';

    sfree(grid);

    return ret;
}

static char *validate_desc(game_params *params, char *desc)
{
    int len = params->w * params->h;

    if (len != strlen(desc))
        return "Game description is wrong length";
    if (len != strspn(desc, "PHO"))
        return "Invalid character in game description";

    return NULL;
}

static game_state *new_game(midend *me, game_params *params, char *desc)
{
    int w = params->w, h = params->h;
    game_state *state = snew(game_state);
    int i;

    state->w = w;
    state->h = h;
    state->completed = 0;
    state->grid = snewn(w*h, unsigned char);
    for (i = 0; i < w*h; i++)
        state->grid[i] = (desc[i] == 'P' ? GRID_PEG :
                          desc[i] == 'H' ? GRID_HOLE : GRID_OBST);

    return state;
}

static game_state *dup_game(game_state *state)
{
    int w = state->w, h = state->h;
    game_state *ret = snew(game_state);

    ret->w = state->w;
    ret->h = state->h;
    ret->completed = state->completed;
    ret->grid = snewn(w*h, unsigned char);
    memcpy(ret->grid, state->grid, w*h);

    return ret;
}

static void free_game(game_state *state)
{
    sfree(state->grid);
    sfree(state);
}

static char *solve_game(game_state *state, game_state *currstate,
                        char *aux, char **error)
{
    return NULL;
}

static char *game_text_format(game_state *state)
{
    int w = state->w, h = state->h;
    int x, y;
    char *ret;

    ret = snewn((w+1)*h + 1, char);

    for (y = 0; y < h; y++) {
        for (x = 0; x < w; x++)
            ret[y*(w+1)+x] = (state->grid[y*w+x] == GRID_HOLE ? '-' :
                              state->grid[y*w+x] == GRID_PEG ? '*' : ' ');
        ret[y*(w+1)+w] = '\n';
    }
    ret[h*(w+1)] = '\0';

    return ret;
}

struct game_ui {
    int dragging;                      /* boolean: is a drag in progress? */
    int sx, sy;                        /* grid coords of drag start cell */
    int dx, dy;                        /* pixel coords of current drag posn */
};

static game_ui *new_ui(game_state *state)
{
    game_ui *ui = snew(game_ui);

    ui->sx = ui->sy = ui->dx = ui->dy = 0;
    ui->dragging = FALSE;

    return ui;
}

static void free_ui(game_ui *ui)
{
    sfree(ui);
}

static char *encode_ui(game_ui *ui)
{
    return NULL;
}

static void decode_ui(game_ui *ui, char *encoding)
{
}

static void game_changed_state(game_ui *ui, game_state *oldstate,
                               game_state *newstate)
{
    /*
     * Cancel a drag, in case the source square has become
     * unoccupied.
     */
    ui->dragging = FALSE;
}

#define PREFERRED_TILE_SIZE 33
#define TILESIZE (ds->tilesize)
#define BORDER (TILESIZE / 2)

#define HIGHLIGHT_WIDTH (TILESIZE / 16)

#define COORD(x)     ( BORDER + (x) * TILESIZE )
#define FROMCOORD(x) ( ((x) + TILESIZE - BORDER) / TILESIZE - 1 )

struct game_drawstate {
    int tilesize;
    blitter *drag_background;
    int dragging, dragx, dragy;
    int w, h;
    unsigned char *grid;
    int started;
    int bgcolour;
};

static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
                            int x, int y, int button)
{
    int w = state->w, h = state->h;

    if (button == LEFT_BUTTON) {
        int tx, ty;

        /*
         * Left button down: we attempt to start a drag.
         */
        
        /*
         * There certainly shouldn't be a current drag in progress,
         * unless the midend failed to send us button events in
         * order; it has a responsibility to always get that right,
         * so we can legitimately punish it by failing an
         * assertion.
         */
        assert(!ui->dragging);

        tx = FROMCOORD(x);
        ty = FROMCOORD(y);
        if (tx >= 0 && tx < w && ty >= 0 && ty < h &&
            state->grid[ty*w+tx] == GRID_PEG) {
            ui->dragging = TRUE;
            ui->sx = tx;
            ui->sy = ty;
            ui->dx = x;
            ui->dy = y;
            return "";                 /* ui modified */
        }
    } else if (button == LEFT_DRAG && ui->dragging) {
        /*
         * Mouse moved; just move the peg being dragged.
         */
        ui->dx = x;
        ui->dy = y;
        return "";                     /* ui modified */
    } else if (button == LEFT_RELEASE && ui->dragging) {
        char buf[80];
        int tx, ty, dx, dy;

        /*
         * Button released. Identify the target square of the drag,
         * see if it represents a valid move, and if so make it.
         */
        ui->dragging = FALSE;          /* cancel the drag no matter what */
        tx = FROMCOORD(x);
        ty = FROMCOORD(y);
        if (tx < 0 || tx >= w || ty < 0 || ty >= h)
            return "";                 /* target out of range */
        dx = tx - ui->sx;
        dy = ty - ui->sy;
        if (max(abs(dx),abs(dy)) != 2 || min(abs(dx),abs(dy)) != 0)
            return "";                 /* move length was wrong */
        dx /= 2;
        dy /= 2;

        if (state->grid[ty*w+tx] != GRID_HOLE ||
            state->grid[(ty-dy)*w+(tx-dx)] != GRID_PEG ||
            state->grid[ui->sy*w+ui->sx] != GRID_PEG)
            return "";                 /* grid contents were invalid */

        /*
         * We have a valid move. Encode it simply as source and
         * destination coordinate pairs.
         */
        sprintf(buf, "%d,%d-%d,%d", ui->sx, ui->sy, tx, ty);
        return dupstr(buf);
    }
    return NULL;
}

static game_state *execute_move(game_state *state, char *move)
{
    int w = state->w, h = state->h;
    int sx, sy, tx, ty;
    game_state *ret;

    if (sscanf(move, "%d,%d-%d,%d", &sx, &sy, &tx, &ty) == 4) {
        int mx, my, dx, dy;

        if (sx < 0 || sx >= w || sy < 0 || sy >= h)
            return NULL;               /* source out of range */
        if (tx < 0 || tx >= w || ty < 0 || ty >= h)
            return NULL;               /* target out of range */

        dx = tx - sx;
        dy = ty - sy;
        if (max(abs(dx),abs(dy)) != 2 || min(abs(dx),abs(dy)) != 0)
            return NULL;               /* move length was wrong */
        mx = sx + dx/2;
        my = sy + dy/2;

        if (state->grid[sy*w+sx] != GRID_PEG ||
            state->grid[my*w+mx] != GRID_PEG ||
            state->grid[ty*w+tx] != GRID_HOLE)
            return NULL;               /* grid contents were invalid */

        ret = dup_game(state);
        ret->grid[sy*w+sx] = GRID_HOLE;
        ret->grid[my*w+mx] = GRID_HOLE;
        ret->grid[ty*w+tx] = GRID_PEG;

        /*
         * Opinion varies on whether getting to a single peg counts as
         * completing the game, or whether that peg has to be at a
         * specific location (central in the classic cross game, for
         * instance). For now we take the former, rather lax position.
         */
        if (!ret->completed) {
            int count = 0, i;
            for (i = 0; i < w*h; i++)
                if (ret->grid[i] == GRID_PEG)
                    count++;
            if (count == 1)
                ret->completed = 1;
        }

        return ret;
    }
    return NULL;
}

/* ----------------------------------------------------------------------
 * Drawing routines.
 */

static void game_compute_size(game_params *params, int tilesize,
                              int *x, int *y)
{
    /* Ick: fake up `ds->tilesize' for macro expansion purposes */
    struct { int tilesize; } ads, *ds = &ads;
    ads.tilesize = tilesize;

    *x = TILESIZE * params->w + 2 * BORDER;
    *y = TILESIZE * params->h + 2 * BORDER;
}

static void game_set_size(drawing *dr, game_drawstate *ds,
                          game_params *params, int tilesize)
{
    ds->tilesize = tilesize;

    assert(TILESIZE > 0);

    assert(!ds->drag_background);      /* set_size is never called twice */
    ds->drag_background = blitter_new(dr, TILESIZE, TILESIZE);
}

static float *game_colours(frontend *fe, game_state *state, int *ncolours)
{
    float *ret = snewn(3 * NCOLOURS, float);

    game_mkhighlight(fe, ret, COL_BACKGROUND, COL_HIGHLIGHT, COL_LOWLIGHT);

    ret[COL_PEG * 3 + 0] = 0.0F;
    ret[COL_PEG * 3 + 1] = 0.0F;
    ret[COL_PEG * 3 + 2] = 1.0F;

    *ncolours = NCOLOURS;
    return ret;
}

static game_drawstate *game_new_drawstate(drawing *dr, game_state *state)
{
    int w = state->w, h = state->h;
    struct game_drawstate *ds = snew(struct game_drawstate);

    ds->tilesize = 0;                  /* not decided yet */

    /* We can't allocate the blitter rectangle for the drag background
     * until we know what size to make it. */
    ds->drag_background = NULL;
    ds->dragging = FALSE;

    ds->w = w;
    ds->h = h;
    ds->grid = snewn(w*h, unsigned char);
    memset(ds->grid, 255, w*h);

    ds->started = FALSE;
    ds->bgcolour = -1;

    return ds;
}

static void game_free_drawstate(drawing *dr, game_drawstate *ds)
{
    if (ds->drag_background)
        blitter_free(dr, ds->drag_background);
    sfree(ds->grid);
    sfree(ds);
}

static void draw_tile(drawing *dr, game_drawstate *ds,
                      int x, int y, int v, int bgcolour)
{
    if (bgcolour >= 0) {
        draw_rect(dr, x, y, TILESIZE, TILESIZE, bgcolour);
    }

    if (v == GRID_HOLE) {
        draw_circle(dr, x+TILESIZE/2, y+TILESIZE/2, TILESIZE/4,
                    COL_LOWLIGHT, COL_LOWLIGHT);
    } else if (v == GRID_PEG) {
        draw_circle(dr, x+TILESIZE/2, y+TILESIZE/2, TILESIZE/3,
                    COL_PEG, COL_PEG);
    }

    draw_update(dr, x, y, TILESIZE, TILESIZE);
}

static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
                        game_state *state, int dir, game_ui *ui,
                        float animtime, float flashtime)
{
    int w = state->w, h = state->h;
    int x, y;
    int bgcolour;

    if (flashtime > 0) {
        int frame = (int)(flashtime / FLASH_FRAME);
        bgcolour = (frame % 2 ? COL_LOWLIGHT : COL_HIGHLIGHT);
    } else
        bgcolour = COL_BACKGROUND;

    /*
     * Erase the sprite currently being dragged, if any.
     */
    if (ds->dragging) {
        assert(ds->drag_background);
        blitter_load(dr, ds->drag_background, ds->dragx, ds->dragy);
        draw_update(dr, ds->dragx, ds->dragy, TILESIZE, TILESIZE);
        ds->dragging = FALSE;
    }

    if (!ds->started) {
        draw_rect(dr, 0, 0,
                  TILESIZE * state->w + 2 * BORDER,
                  TILESIZE * state->h + 2 * BORDER, COL_BACKGROUND);

        /*
         * Draw relief marks around all the squares that aren't
         * GRID_OBST.
         */
        for (y = 0; y < h; y++)
            for (x = 0; x < w; x++)
                if (state->grid[y*w+x] != GRID_OBST) {
                    /*
                     * First pass: draw the full relief square.
                     */
                    int coords[6];
                    coords[0] = COORD(x+1) + HIGHLIGHT_WIDTH - 1;
                    coords[1] = COORD(y) - HIGHLIGHT_WIDTH;
                    coords[2] = COORD(x) - HIGHLIGHT_WIDTH;
                    coords[3] = COORD(y+1) + HIGHLIGHT_WIDTH - 1;
                    coords[4] = COORD(x) - HIGHLIGHT_WIDTH;
                    coords[5] = COORD(y) - HIGHLIGHT_WIDTH;
                    draw_polygon(dr, coords, 3, COL_HIGHLIGHT, COL_HIGHLIGHT);
                    coords[4] = COORD(x+1) + HIGHLIGHT_WIDTH - 1;
                    coords[5] = COORD(y+1) + HIGHLIGHT_WIDTH - 1;
                    draw_polygon(dr, coords, 3, COL_LOWLIGHT, COL_LOWLIGHT);
                }
        for (y = 0; y < h; y++)
            for (x = 0; x < w; x++)
                if (state->grid[y*w+x] != GRID_OBST) {
                    /*
                     * Second pass: draw everything but the two
                     * diagonal corners.
                     */
                    draw_rect(dr, COORD(x) - HIGHLIGHT_WIDTH,
                              COORD(y) - HIGHLIGHT_WIDTH,
                              TILESIZE + HIGHLIGHT_WIDTH,
                              TILESIZE + HIGHLIGHT_WIDTH, COL_HIGHLIGHT);
                    draw_rect(dr, COORD(x),
                              COORD(y),
                              TILESIZE + HIGHLIGHT_WIDTH,
                              TILESIZE + HIGHLIGHT_WIDTH, COL_LOWLIGHT);
                }
        for (y = 0; y < h; y++)
            for (x = 0; x < w; x++)
                if (state->grid[y*w+x] != GRID_OBST) {
                    /*
                     * Third pass: draw a trapezium on each edge.
                     */
                    int coords[8];
                    int dx, dy, s, sn, c;

                    for (dx = 0; dx < 2; dx++) {
                        dy = 1 - dx;
                        for (s = 0; s < 2; s++) {
                            sn = 2*s - 1;
                            c = s ? COL_LOWLIGHT : COL_HIGHLIGHT;

                            coords[0] = COORD(x) + (s*dx)*(TILESIZE-1);
                            coords[1] = COORD(y) + (s*dy)*(TILESIZE-1);
                            coords[2] = COORD(x) + (s*dx+dy)*(TILESIZE-1);
                            coords[3] = COORD(y) + (s*dy+dx)*(TILESIZE-1);
                            coords[4] = coords[2] - HIGHLIGHT_WIDTH * (dy-sn*dx);
                            coords[5] = coords[3] - HIGHLIGHT_WIDTH * (dx-sn*dy);
                            coords[6] = coords[0] + HIGHLIGHT_WIDTH * (dy+sn*dx);
                            coords[7] = coords[1] + HIGHLIGHT_WIDTH * (dx+sn*dy);
                            draw_polygon(dr, coords, 4, c, c);
                        }
                    }
                }
        for (y = 0; y < h; y++)
            for (x = 0; x < w; x++)
                if (state->grid[y*w+x] != GRID_OBST) {
                    /*
                     * Second pass: draw everything but the two
                     * diagonal corners.
                     */
                    draw_rect(dr, COORD(x),
                              COORD(y),
                              TILESIZE,
                              TILESIZE, COL_BACKGROUND);
                }

        ds->started = TRUE;

        draw_update(dr, 0, 0,
                    TILESIZE * state->w + 2 * BORDER,
                    TILESIZE * state->h + 2 * BORDER);
    }

    /*
     * Loop over the grid redrawing anything that looks as if it
     * needs it.
     */
    for (y = 0; y < h; y++)
        for (x = 0; x < w; x++) {
            int v;

            v = state->grid[y*w+x];
            /*
             * Blank the source of a drag so it looks as if the
             * user picked the peg up physically.
             */
            if (ui->dragging && ui->sx == x && ui->sy == y && v == GRID_PEG)
                v = GRID_HOLE;
            if (v != GRID_OBST &&
                (bgcolour != ds->bgcolour || /* always redraw when flashing */
                 v != ds->grid[y*w+x])) {
                draw_tile(dr, ds, COORD(x), COORD(y), v, bgcolour);
            }
        }

    /*
     * Draw the dragging sprite if any.
     */
    if (ui->dragging) {
        ds->dragging = TRUE;
        ds->dragx = ui->dx - TILESIZE/2;
        ds->dragy = ui->dy - TILESIZE/2;
        blitter_save(dr, ds->drag_background, ds->dragx, ds->dragy);
        draw_tile(dr, ds, ds->dragx, ds->dragy, GRID_PEG, -1);
    }

    ds->bgcolour = bgcolour;
}

static float game_anim_length(game_state *oldstate, game_state *newstate,
                              int dir, game_ui *ui)
{
    return 0.0F;
}

static float game_flash_length(game_state *oldstate, game_state *newstate,
                               int dir, game_ui *ui)
{
    if (!oldstate->completed && newstate->completed)
        return 2 * FLASH_FRAME;
    else
        return 0.0F;
}

static int game_wants_statusbar(void)
{
    return FALSE;
}

static int game_timing_state(game_state *state, game_ui *ui)
{
    return TRUE;
}

static void game_print_size(game_params *params, float *x, float *y)
{
}

static void game_print(drawing *dr, game_state *state, int tilesize)
{
}

#ifdef COMBINED
#define thegame pegs
#endif

const struct game thegame = {
    "Pegs", "games.pegs",
    default_params,
    game_fetch_preset,
    decode_params,
    encode_params,
    free_params,
    dup_params,
    TRUE, game_configure, custom_params,
    validate_params,
    new_game_desc,
    validate_desc,
    new_game,
    dup_game,
    free_game,
    FALSE, solve_game,
    TRUE, game_text_format,
    new_ui,
    free_ui,
    encode_ui,
    decode_ui,
    game_changed_state,
    interpret_move,
    execute_move,
    PREFERRED_TILE_SIZE, game_compute_size, game_set_size,
    game_colours,
    game_new_drawstate,
    game_free_drawstate,
    game_redraw,
    game_anim_length,
    game_flash_length,
    FALSE, FALSE, game_print_size, game_print,
    game_wants_statusbar,
    FALSE, game_timing_state,
    0,                                 /* mouse_priorities */
};