Another function pair required for serialisation; these ones save

[sgt/puzzles] / sixteen.c

/*
 * sixteen.c: `16-puzzle', a sliding-tiles jigsaw which differs
 * from the 15-puzzle in that you toroidally rotate a row or column
 * at a time.
 */

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

#include "puzzles.h"

#define PREFERRED_TILE_SIZE 48
#define TILE_SIZE (ds->tilesize)
#define BORDER TILE_SIZE
#define HIGHLIGHT_WIDTH (TILE_SIZE / 20)
#define COORD(x)  ( (x) * TILE_SIZE + BORDER )
#define FROMCOORD(x)  ( ((x) - BORDER + 2*TILE_SIZE) / TILE_SIZE - 2 )

#define ANIM_TIME 0.13F
#define FLASH_FRAME 0.13F

#define X(state, i) ( (i) % (state)->w )
#define Y(state, i) ( (i) / (state)->w )
#define C(state, x, y) ( (y) * (state)->w + (x) )

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

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

struct game_state {
    int w, h, n;
    int *tiles;
    int completed;
    int just_used_solve;               /* used to suppress undo animation */
    int used_solve;                    /* used to suppress completion flash */
    int movecount, movetarget;
    int last_movement_sense;
};

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

    ret->w = ret->h = 4;
    ret->movetarget = 0;

    return ret;
}

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

    switch (i) {
      case 0: w = 3, h = 3; break;
      case 1: w = 4, h = 3; break;
      case 2: w = 4, h = 4; break;
      case 3: w = 5, h = 4; break;
      case 4: w = 5, h = 5; break;
      default: return FALSE;
    }

    sprintf(buf, "%dx%d", w, h);
    *name = dupstr(buf);
    *params = ret = snew(game_params);
    ret->w = w;
    ret->h = h;
    ret->movetarget = 0;
    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 *ret, char const *string)
{
    ret->w = ret->h = atoi(string);
    ret->movetarget = 0;
    while (*string && isdigit(*string)) string++;
    if (*string == 'x') {
        string++;
        ret->h = atoi(string);
        while (*string && isdigit((unsigned char)*string))
            string++;
    }
    if (*string == 'm') {
        string++;
        ret->movetarget = atoi(string);
        while (*string && isdigit((unsigned char)*string))
            string++;
    }
}

static char *encode_params(game_params *params, int full)
{
    char data[256];

    sprintf(data, "%dx%d", params->w, params->h);
    /* Shuffle limit is part of the limited parameters, because we have to
     * supply the target move count. */
    if (params->movetarget)
        sprintf(data + strlen(data), "m%d", params->movetarget);

    return dupstr(data);
}

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

    ret = snewn(4, config_item);

    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 = "Number of shuffling moves";
    ret[2].type = C_STRING;
    sprintf(buf, "%d", params->movetarget);
    ret[2].sval = dupstr(buf);
    ret[2].ival = 0;

    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->movetarget = atoi(cfg[2].sval);

    return ret;
}

static char *validate_params(game_params *params)
{
    if (params->w < 2 || params->h < 2)
        return "Width and height must both be at least two";

    return NULL;
}

static int perm_parity(int *perm, int n)
{
    int i, j, ret;

    ret = 0;

    for (i = 0; i < n-1; i++)
        for (j = i+1; j < n; j++)
            if (perm[i] > perm[j])
                ret = !ret;

    return ret;
}

static char *new_game_desc(game_params *params, random_state *rs,
                           game_aux_info **aux, int interactive)
{
    int stop, n, i, x;
    int x1, x2, p1, p2;
    int *tiles, *used;
    char *ret;
    int retlen;

    n = params->w * params->h;

    tiles = snewn(n, int);

    if (params->movetarget) {
        int prevoffset = -1;
        int max = (params->w > params->h ? params->w : params->h);
        int *prevmoves = snewn(max, int);

        /*
         * Shuffle the old-fashioned way, by making a series of
         * single moves on the grid.
         */

        for (i = 0; i < n; i++)
            tiles[i] = i;

        for (i = 0; i < params->movetarget; i++) {
            int start, offset, len, direction, index;
            int j, tmp;

            /*
             * Choose a move to make. We can choose from any row
             * or any column.
             */
            while (1) {
                j = random_upto(rs, params->w + params->h);

                if (j < params->w) {
                    /* Column. */
                    index = j;
                    start = j;
                    offset = params->w;
                    len = params->h;
                } else {
                    /* Row. */
                    index = j - params->w;
                    start = index * params->w;
                    offset = 1;
                    len = params->w;
                }

                direction = -1 + 2 * random_upto(rs, 2);

                /*
                 * To at least _try_ to avoid boring cases, check
                 * that this move doesn't directly undo a previous
                 * one, or repeat it so many times as to turn it
                 * into fewer moves in the opposite direction. (For
                 * example, in a row of length 4, we're allowed to
                 * move it the same way twice, but not three
                 * times.)
                 * 
                 * We track this for each individual row/column,
                 * and clear all the counters as soon as a
                 * perpendicular move is made. This isn't perfect
                 * (it _can't_ guaranteeably be perfect - there
                 * will always come a move count beyond which a
                 * shorter solution will be possible than the one
                 * which constructed the position) but it should
                 * sort out all the obvious cases.
                 */
                if (offset == prevoffset) {
                    tmp = prevmoves[index] + direction;
                    if (abs(2*tmp) > len || abs(tmp) < abs(prevmoves[index]))
                        continue;
                }

                /* If we didn't `continue', we've found an OK move to make. */
                if (offset != prevoffset) {
                    int i;
                    for (i = 0; i < max; i++)
                        prevmoves[i] = 0;
                    prevoffset = offset;
                }
                prevmoves[index] += direction;
                break;
            }

            /*
             * Make the move.
             */
            if (direction < 0) {
                start += (len-1) * offset;
                offset = -offset;
            }
            tmp = tiles[start];
            for (j = 0; j+1 < len; j++)
                tiles[start + j*offset] = tiles[start + (j+1)*offset];
            tiles[start + (len-1) * offset] = tmp;
        }

        sfree(prevmoves);

    } else {

        used = snewn(n, int);

        for (i = 0; i < n; i++) {
            tiles[i] = -1;
            used[i] = FALSE;
        }

        /*
         * If both dimensions are odd, there is a parity
         * constraint.
         */
        if (params->w & params->h & 1)
            stop = 2;
        else
            stop = 0;

        /*
         * Place everything except (possibly) the last two tiles.
         */
        for (x = 0, i = n; i > stop; i--) {
            int k = i > 1 ? random_upto(rs, i) : 0;
            int j;

            for (j = 0; j < n; j++)
                if (!used[j] && (k-- == 0))
                    break;

            assert(j < n && !used[j]);
            used[j] = TRUE;

            while (tiles[x] >= 0)
                x++;
            assert(x < n);
            tiles[x] = j;
        }

        if (stop) {
            /*
             * Find the last two locations, and the last two
             * pieces.
             */
            while (tiles[x] >= 0)
                x++;
            assert(x < n);
            x1 = x;
            x++;
            while (tiles[x] >= 0)
                x++;
            assert(x < n);
            x2 = x;

            for (i = 0; i < n; i++)
                if (!used[i])
                    break;
            p1 = i;
            for (i = p1+1; i < n; i++)
                if (!used[i])
                    break;
            p2 = i;

            /*
             * Try the last two tiles one way round. If that fails,
             * swap them.
             */
            tiles[x1] = p1;
            tiles[x2] = p2;
            if (perm_parity(tiles, n) != 0) {
                tiles[x1] = p2;
                tiles[x2] = p1;
                assert(perm_parity(tiles, n) == 0);
            }
        }

        sfree(used);
    }

    /*
     * Now construct the game description, by describing the tile
     * array as a simple sequence of comma-separated integers.
     */
    ret = NULL;
    retlen = 0;
    for (i = 0; i < n; i++) {
        char buf[80];
        int k;

        k = sprintf(buf, "%d,", tiles[i]+1);

        ret = sresize(ret, retlen + k + 1, char);
        strcpy(ret + retlen, buf);
        retlen += k;
    }
    ret[retlen-1] = '\0';              /* delete last comma */

    sfree(tiles);

    return ret;
}

static void game_free_aux_info(game_aux_info *aux)
{
    assert(!"Shouldn't happen");
}


static char *validate_desc(game_params *params, char *desc)
{
    char *p, *err;
    int i, area;
    int *used;

    area = params->w * params->h;
    p = desc;
    err = NULL;

    used = snewn(area, int);
    for (i = 0; i < area; i++)
        used[i] = FALSE;

    for (i = 0; i < area; i++) {
        char *q = p;
        int n;

        if (*p < '0' || *p > '9') {
            err = "Not enough numbers in string";
            goto leave;
        }
        while (*p >= '0' && *p <= '9')
            p++;
        if (i < area-1 && *p != ',') {
            err = "Expected comma after number";
            goto leave;
        }
        else if (i == area-1 && *p) {
            err = "Excess junk at end of string";
            goto leave;
        }
        n = atoi(q);
        if (n < 1 || n > area) {
            err = "Number out of range";
            goto leave;
        }
        if (used[n-1]) {
            err = "Number used twice";
            goto leave;
        }
        used[n-1] = TRUE;

        if (*p) p++;                   /* eat comma */
    }

    leave:
    sfree(used);
    return err;
}

static game_state *new_game(midend_data *me, game_params *params, char *desc)
{
    game_state *state = snew(game_state);
    int i;
    char *p;

    state->w = params->w;
    state->h = params->h;
    state->n = params->w * params->h;
    state->tiles = snewn(state->n, int);

    p = desc;
    i = 0;
    for (i = 0; i < state->n; i++) {
        assert(*p);
        state->tiles[i] = atoi(p);
        while (*p && *p != ',')
            p++;
        if (*p) p++;                   /* eat comma */
    }
    assert(!*p);

    state->completed = state->movecount = 0;
    state->movetarget = params->movetarget;
    state->used_solve = state->just_used_solve = FALSE;
    state->last_movement_sense = 0;

    return state;
}

static game_state *dup_game(game_state *state)
{
    game_state *ret = snew(game_state);

    ret->w = state->w;
    ret->h = state->h;
    ret->n = state->n;
    ret->tiles = snewn(state->w * state->h, int);
    memcpy(ret->tiles, state->tiles, state->w * state->h * sizeof(int));
    ret->completed = state->completed;
    ret->movecount = state->movecount;
    ret->movetarget = state->movetarget;
    ret->used_solve = state->used_solve;
    ret->just_used_solve = state->just_used_solve;
    ret->last_movement_sense = state->last_movement_sense;

    return ret;
}

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

static char *solve_game(game_state *state, game_state *currstate,
                        game_aux_info *aux, char **error)
{
    return dupstr("S");
}

static char *game_text_format(game_state *state)
{
    char *ret, *p, buf[80];
    int x, y, col, maxlen;

    /*
     * First work out how many characters we need to display each
     * number.
     */
    col = sprintf(buf, "%d", state->n);

    /*
     * Now we know the exact total size of the grid we're going to
     * produce: it's got h rows, each containing w lots of col, w-1
     * spaces and a trailing newline.
     */
    maxlen = state->h * state->w * (col+1);

    ret = snewn(maxlen+1, char);
    p = ret;

    for (y = 0; y < state->h; y++) {
        for (x = 0; x < state->w; x++) {
            int v = state->tiles[state->w*y+x];
            sprintf(buf, "%*d", col, v);
            memcpy(p, buf, col);
            p += col;
            if (x+1 == state->w)
                *p++ = '\n';
            else
                *p++ = ' ';
        }
    }

    assert(p - ret == maxlen);
    *p = '\0';
    return ret;
}

static game_ui *new_ui(game_state *state)
{
    return NULL;
}

static void free_ui(game_ui *ui)
{
}

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

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

static void game_changed_state(game_ui *ui, game_state *oldstate,
                               game_state *newstate)
{
}

struct game_drawstate {
    int started;
    int w, h, bgcolour;
    int *tiles;
    int tilesize;
};

static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
                            int x, int y, int button)
{
    int cx, cy, dx, dy;
    char buf[80];

    button &= ~MOD_MASK;
    if (button != LEFT_BUTTON && button != RIGHT_BUTTON)
        return NULL;

    cx = FROMCOORD(x);
    cy = FROMCOORD(y);
    if (cx == -1 && cy >= 0 && cy < state->h)
        dx = -1, dy = 0;
    else if (cx == state->w && cy >= 0 && cy < state->h)
        dx = +1, dy = 0;
    else if (cy == -1 && cx >= 0 && cx < state->w)
        dy = -1, dx = 0;
    else if (cy == state->h && cx >= 0 && cx < state->w)
        dy = +1, dx = 0;
    else
        return NULL;                   /* invalid click location */

    /* reverse direction if right hand button is pressed */
    if (button == RIGHT_BUTTON) {
        dx = -dx;
        dy = -dy;
    }

    if (dx)
        sprintf(buf, "R%d,%d", cy, dx);
    else
        sprintf(buf, "C%d,%d", cx, dy);
    return dupstr(buf);
}

static game_state *execute_move(game_state *from, char *move)
{
    int cx, cy, dx, dy;
    int tx, ty, n;
    game_state *ret;

    if (!strcmp(move, "S")) {
        int i;

        ret = dup_game(from);

        /*
         * Simply replace the grid with a solved one. For this game,
         * this isn't a useful operation for actually telling the user
         * what they should have done, but it is useful for
         * conveniently being able to get hold of a clean state from
         * which to practise manoeuvres.
         */
        for (i = 0; i < ret->n; i++)
            ret->tiles[i] = i+1;
        ret->used_solve = ret->just_used_solve = TRUE;
        ret->completed = ret->movecount = 1;

        return ret;
    }

    if (move[0] == 'R' && sscanf(move+1, "%d,%d", &cy, &dx) == 2 &&
        cy >= 0 && cy < from->h) {
        cx = dy = 0;
        n = from->w;
    } else if (move[0] == 'C' && sscanf(move+1, "%d,%d", &cx, &dy) == 2 &&
               cx >= 0 && cx < from->w) {
        cy = dx = 0;
        n = from->h;
    } else
        return NULL;

    ret = dup_game(from);
    ret->just_used_solve = FALSE;      /* zero this in a hurry */

    do {
        tx = (cx - dx + from->w) % from->w;
        ty = (cy - dy + from->h) % from->h;
        ret->tiles[C(ret, cx, cy)] = from->tiles[C(from, tx, ty)];
        cx = tx;
        cy = ty;
    } while (--n > 0);

    ret->movecount++;

    ret->last_movement_sense = dx+dy;

    /*
     * See if the game has been completed.
     */
    if (!ret->completed) {
        ret->completed = ret->movecount;
        for (n = 0; n < ret->n; n++)
            if (ret->tiles[n] != n+1)
                ret->completed = FALSE;
    }

    return ret;
}

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

static void game_size(game_params *params, game_drawstate *ds,
                      int *x, int *y, int expand)
{
    int tsx, tsy, ts;
    /*
     * Each window dimension equals the tile size times two more
     * than the grid dimension (the border is the same size as the
     * tiles).
     */
    tsx = *x / (params->w + 2);
    tsy = *y / (params->h + 2);
    ts = min(tsx, tsy);

    if (expand)
        ds->tilesize = ts;
    else
        ds->tilesize = min(ts, PREFERRED_TILE_SIZE);

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

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

    frontend_default_colour(fe, &ret[COL_BACKGROUND * 3]);

    /*
     * Drop the background colour so that the highlight is
     * noticeably brighter than it while still being under 1.
     */
    max = ret[COL_BACKGROUND*3];
    for (i = 1; i < 3; i++)
        if (ret[COL_BACKGROUND*3+i] > max)
            max = ret[COL_BACKGROUND*3+i];
    if (max * 1.2F > 1.0F) {
        for (i = 0; i < 3; i++)
            ret[COL_BACKGROUND*3+i] /= (max * 1.2F);
    }

    for (i = 0; i < 3; i++) {
        ret[COL_HIGHLIGHT * 3 + i] = ret[COL_BACKGROUND * 3 + i] * 1.2F;
        ret[COL_LOWLIGHT * 3 + i] = ret[COL_BACKGROUND * 3 + i] * 0.8F;
        ret[COL_TEXT * 3 + i] = 0.0;
    }

    *ncolours = NCOLOURS;
    return ret;
}

static game_drawstate *game_new_drawstate(game_state *state)
{
    struct game_drawstate *ds = snew(struct game_drawstate);
    int i;

    ds->started = FALSE;
    ds->w = state->w;
    ds->h = state->h;
    ds->bgcolour = COL_BACKGROUND;
    ds->tiles = snewn(ds->w*ds->h, int);
    ds->tilesize = 0;                  /* haven't decided yet */
    for (i = 0; i < ds->w*ds->h; i++)
        ds->tiles[i] = -1;

    return ds;
}

static void game_free_drawstate(game_drawstate *ds)
{
    sfree(ds->tiles);
    sfree(ds);
}

static void draw_tile(frontend *fe, game_drawstate *ds,
                      game_state *state, int x, int y,
                      int tile, int flash_colour)
{
    if (tile == 0) {
        draw_rect(fe, x, y, TILE_SIZE, TILE_SIZE,
                  flash_colour);
    } else {
        int coords[6];
        char str[40];

        coords[0] = x + TILE_SIZE - 1;
        coords[1] = y + TILE_SIZE - 1;
        coords[2] = x + TILE_SIZE - 1;
        coords[3] = y;
        coords[4] = x;
        coords[5] = y + TILE_SIZE - 1;
        draw_polygon(fe, coords, 3, TRUE, COL_LOWLIGHT);
        draw_polygon(fe, coords, 3, FALSE, COL_LOWLIGHT);

        coords[0] = x;
        coords[1] = y;
        draw_polygon(fe, coords, 3, TRUE, COL_HIGHLIGHT);
        draw_polygon(fe, coords, 3, FALSE, COL_HIGHLIGHT);

        draw_rect(fe, x + HIGHLIGHT_WIDTH, y + HIGHLIGHT_WIDTH,
                  TILE_SIZE - 2*HIGHLIGHT_WIDTH, TILE_SIZE - 2*HIGHLIGHT_WIDTH,
                  flash_colour);

        sprintf(str, "%d", tile);
        draw_text(fe, x + TILE_SIZE/2, y + TILE_SIZE/2,
                  FONT_VARIABLE, TILE_SIZE/3, ALIGN_VCENTRE | ALIGN_HCENTRE,
                  COL_TEXT, str);
    }
    draw_update(fe, x, y, TILE_SIZE, TILE_SIZE);
}

static void draw_arrow(frontend *fe, game_drawstate *ds,
                       int x, int y, int xdx, int xdy)
{
    int coords[14];
    int ydy = -xdx, ydx = xdy;

#define POINT(n, xx, yy) ( \
    coords[2*(n)+0] = x + (xx)*xdx + (yy)*ydx, \
    coords[2*(n)+1] = y + (xx)*xdy + (yy)*ydy)

    POINT(0, TILE_SIZE / 2, 3 * TILE_SIZE / 4);   /* top of arrow */
    POINT(1, 3 * TILE_SIZE / 4, TILE_SIZE / 2);   /* right corner */
    POINT(2, 5 * TILE_SIZE / 8, TILE_SIZE / 2);   /* right concave */
    POINT(3, 5 * TILE_SIZE / 8, TILE_SIZE / 4);   /* bottom right */
    POINT(4, 3 * TILE_SIZE / 8, TILE_SIZE / 4);   /* bottom left */
    POINT(5, 3 * TILE_SIZE / 8, TILE_SIZE / 2);   /* left concave */
    POINT(6,     TILE_SIZE / 4, TILE_SIZE / 2);   /* left corner */

    draw_polygon(fe, coords, 7, TRUE, COL_LOWLIGHT);
    draw_polygon(fe, coords, 7, FALSE, COL_TEXT);
}

static void game_redraw(frontend *fe, game_drawstate *ds, game_state *oldstate,
                 game_state *state, int dir, game_ui *ui,
                 float animtime, float flashtime)
{
    int i, bgcolour;

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

    if (!ds->started) {
        int coords[10];

        draw_rect(fe, 0, 0,
                  TILE_SIZE * state->w + 2 * BORDER,
                  TILE_SIZE * state->h + 2 * BORDER, COL_BACKGROUND);
        draw_update(fe, 0, 0,
                    TILE_SIZE * state->w + 2 * BORDER,
                    TILE_SIZE * state->h + 2 * BORDER);

        /*
         * Recessed area containing the whole puzzle.
         */
        coords[0] = COORD(state->w) + HIGHLIGHT_WIDTH - 1;
        coords[1] = COORD(state->h) + HIGHLIGHT_WIDTH - 1;
        coords[2] = COORD(state->w) + HIGHLIGHT_WIDTH - 1;
        coords[3] = COORD(0) - HIGHLIGHT_WIDTH;
        coords[4] = coords[2] - TILE_SIZE;
        coords[5] = coords[3] + TILE_SIZE;
        coords[8] = COORD(0) - HIGHLIGHT_WIDTH;
        coords[9] = COORD(state->h) + HIGHLIGHT_WIDTH - 1;
        coords[6] = coords[8] + TILE_SIZE;
        coords[7] = coords[9] - TILE_SIZE;
        draw_polygon(fe, coords, 5, TRUE, COL_HIGHLIGHT);
        draw_polygon(fe, coords, 5, FALSE, COL_HIGHLIGHT);

        coords[1] = COORD(0) - HIGHLIGHT_WIDTH;
        coords[0] = COORD(0) - HIGHLIGHT_WIDTH;
        draw_polygon(fe, coords, 5, TRUE, COL_LOWLIGHT);
        draw_polygon(fe, coords, 5, FALSE, COL_LOWLIGHT);

        /*
         * Arrows for making moves.
         */
        for (i = 0; i < state->w; i++) {
            draw_arrow(fe, ds, COORD(i), COORD(0), +1, 0);
            draw_arrow(fe, ds, COORD(i+1), COORD(state->h), -1, 0);
        }
        for (i = 0; i < state->h; i++) {
            draw_arrow(fe, ds, COORD(state->w), COORD(i), 0, +1);
            draw_arrow(fe, ds, COORD(0), COORD(i+1), 0, -1);
        }

        ds->started = TRUE;
    }

    /*
     * Now draw each tile.
     */

    clip(fe, COORD(0), COORD(0), TILE_SIZE*state->w, TILE_SIZE*state->h);

    for (i = 0; i < state->n; i++) {
        int t, t0;
        /*
         * Figure out what should be displayed at this
         * location. It's either a simple tile, or it's a
         * transition between two tiles (in which case we say
         * -1 because it must always be drawn).
         */

        if (oldstate && oldstate->tiles[i] != state->tiles[i])
            t = -1;
        else
            t = state->tiles[i];

        t0 = t;

        if (ds->bgcolour != bgcolour ||   /* always redraw when flashing */
            ds->tiles[i] != t || ds->tiles[i] == -1 || t == -1) {
            int x, y, x2, y2;

            /*
             * Figure out what to _actually_ draw, and where to
             * draw it.
             */
            if (t == -1) {
                int x0, y0, x1, y1, dx, dy;
                int j;
                float c;
                int sense;

                if (dir < 0) {
                    assert(oldstate);
                    sense = -oldstate->last_movement_sense;
                } else {
                    sense = state->last_movement_sense;
                }

                t = state->tiles[i];

                /*
                 * FIXME: must be prepared to draw a double
                 * tile in some situations.
                 */

                /*
                 * Find the coordinates of this tile in the old and
                 * new states.
                 */
                x1 = COORD(X(state, i));
                y1 = COORD(Y(state, i));
                for (j = 0; j < oldstate->n; j++)
                    if (oldstate->tiles[j] == state->tiles[i])
                        break;
                assert(j < oldstate->n);
                x0 = COORD(X(state, j));
                y0 = COORD(Y(state, j));

                dx = (x1 - x0);
                if (dx != 0 &&
                    dx != TILE_SIZE * sense) {
                    dx = (dx < 0 ? dx + TILE_SIZE * state->w :
                          dx - TILE_SIZE * state->w);
                    assert(abs(dx) == TILE_SIZE);
                }
                dy = (y1 - y0);
                if (dy != 0 &&
                    dy != TILE_SIZE * sense) {
                    dy = (dy < 0 ? dy + TILE_SIZE * state->h :
                          dy - TILE_SIZE * state->h);
                    assert(abs(dy) == TILE_SIZE);
                }

                c = (animtime / ANIM_TIME);
                if (c < 0.0F) c = 0.0F;
                if (c > 1.0F) c = 1.0F;

                x = x0 + (int)(c * dx);
                y = y0 + (int)(c * dy);
                x2 = x1 - dx + (int)(c * dx);
                y2 = y1 - dy + (int)(c * dy);
            } else {
                x = COORD(X(state, i));
                y = COORD(Y(state, i));
                x2 = y2 = -1;
            }

            draw_tile(fe, ds, state, x, y, t, bgcolour);
            if (x2 != -1 || y2 != -1)
                draw_tile(fe, ds, state, x2, y2, t, bgcolour);
        }
        ds->tiles[i] = t0;
    }

    unclip(fe);

    ds->bgcolour = bgcolour;

    /*
     * Update the status bar.
     */
    {
        char statusbuf[256];

        /*
         * Don't show the new status until we're also showing the
         * new _state_ - after the game animation is complete.
         */
        if (oldstate)
            state = oldstate;

        if (state->used_solve)
            sprintf(statusbuf, "Moves since auto-solve: %d",
                    state->movecount - state->completed);
        else {
            sprintf(statusbuf, "%sMoves: %d",
                    (state->completed ? "COMPLETED! " : ""),
                    (state->completed ? state->completed : state->movecount));
            if (state->movetarget)
                sprintf(statusbuf+strlen(statusbuf), " (target %d)",
                        state->movetarget);
        }

        status_bar(fe, statusbuf);
    }
}

static float game_anim_length(game_state *oldstate,
                              game_state *newstate, int dir, game_ui *ui)
{
    if ((dir > 0 && newstate->just_used_solve) ||
        (dir < 0 && oldstate->just_used_solve))
        return 0.0F;
    else
        return ANIM_TIME;
}

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

static int game_wants_statusbar(void)
{
    return TRUE;
}

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

#ifdef COMBINED
#define thegame sixteen
#endif

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