[WIP] Port of Simon Tatham's Puzzle Collectionpuzzles

Original revision: 5123b1bf68777ffa86e651f178046b26a87cf2d9

MIT Licensed. Some games still crash and others are unplayable due to
issues with controls. Still need a "real" polygon filling algorithm.

The following games are at least partially broken for various reasons:

Cube:     crash with certain settings
Galaxies: crash
Inertia:  crash
Keen:     input issues
Loopy:    weird stuff happens
Map:      crash on input
Mines:    weird stuff happens on target
Palisade: input issues
Signpost: crash on input
Solo:     input issues
Towers:   input and drawing issues
Train Tracks: drawing issues
Twiddle:  weird animation on target
Undead:   input and drawing issues
Unequal:  input and drawing issues
Untangle: input issues

All in all, about 40% of the games are at least partially broken.

Change-Id: I7c69b6860ab115f973c8d76799502e9bb3d52368

1 files changed, 2278 insertions, 0 deletions

diff --git a/apps/plugins/puzzles/slant.c b/apps/plugins/puzzles/slant.c
new file mode 100644
index 0000000..3ab4d30
--- /dev/null
+++ b/apps/plugins/puzzles/slant.c
@@ -0,0 +1,2278 @@
+/*
+ * slant.c: Puzzle from nikoli.co.jp involving drawing a diagonal
+ * line through each square of a grid.
+ */
+
+/*
+ * In this puzzle you have a grid of squares, each of which must
+ * contain a diagonal line; you also have clue numbers placed at
+ * _points_ of that grid, which means there's a (w+1) x (h+1) array
+ * of possible clue positions.
+ * 
+ * I'm therefore going to adopt a rigid convention throughout this
+ * source file of using w and h for the dimensions of the grid of
+ * squares, and W and H for the dimensions of the grid of points.
+ * Thus, W == w+1 and H == h+1 always.
+ * 
+ * Clue arrays will be W*H `signed char's, and the clue at each
+ * point will be a number from 0 to 4, or -1 if there's no clue.
+ * 
+ * Solution arrays will be W*H `signed char's, and the number at
+ * each point will be +1 for a forward slash (/), -1 for a
+ * backslash (\), and 0 for unknown.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <string.h>
+#include "rbassert.h"
+#include <ctype.h>
+#include <math.h>
+
+#include "puzzles.h"
+
+enum {
+    COL_BACKGROUND,
+    COL_GRID,
+    COL_INK,
+    COL_SLANT1,
+    COL_SLANT2,
+    COL_ERROR,
+    COL_CURSOR,
+    COL_FILLEDSQUARE,
+    NCOLOURS
+};
+
+/*
+ * In standalone solver mode, `verbose' is a variable which can be
+ * set by command-line option; in debugging mode it's simply always
+ * true.
+ */
+#if defined STANDALONE_SOLVER
+#define SOLVER_DIAGNOSTICS
+int verbose = FALSE;
+#elif defined SOLVER_DIAGNOSTICS
+#define verbose TRUE
+#endif
+
+/*
+ * Difficulty levels. I do some macro ickery here to ensure that my
+ * enum and the various forms of my name list always match up.
+ */
+#define DIFFLIST(A) \
+    A(EASY,Easy,e) \
+    A(HARD,Hard,h)
+#define ENUM(upper,title,lower) DIFF_ ## upper,
+#define TITLE(upper,title,lower) #title,
+#define ENCODE(upper,title,lower) #lower
+#define CONFIG(upper,title,lower) ":" #title
+enum { DIFFLIST(ENUM) DIFFCOUNT };
+static char const *const slant_diffnames[] = { DIFFLIST(TITLE) };
+static char const slant_diffchars[] = DIFFLIST(ENCODE);
+#define DIFFCONFIG DIFFLIST(CONFIG)
+
+struct game_params {
+    int w, h, diff;
+};
+
+typedef struct game_clues {
+    int w, h;
+    signed char *clues;
+    int *tmpdsf;
+    int refcount;
+} game_clues;
+
+#define ERR_VERTEX 1
+#define ERR_SQUARE 2
+
+struct game_state {
+    struct game_params p;
+    game_clues *clues;
+    signed char *soln;
+    unsigned char *errors;
+    int completed;
+    int used_solve;		       /* used to suppress completion flash */
+};
+
+static game_params *default_params(void)
+{
+    game_params *ret = snew(game_params);
+
+    ret->w = ret->h = 8;
+    ret->diff = DIFF_EASY;
+
+    return ret;
+}
+
+static const struct game_params slant_presets[] = {
+    {5, 5, DIFF_EASY},
+    {5, 5, DIFF_HARD},
+    {8, 8, DIFF_EASY},
+    {8, 8, DIFF_HARD},
+    {12, 10, DIFF_EASY},
+    {12, 10, DIFF_HARD},
+};
+
+static int game_fetch_preset(int i, char **name, game_params **params)
+{
+    game_params *ret;
+    char str[80];
+
+    if (i < 0 || i >= lenof(slant_presets))
+        return FALSE;
+
+    ret = snew(game_params);
+    *ret = slant_presets[i];
+
+    sprintf(str, "%dx%d %s", ret->w, ret->h, slant_diffnames[ret->diff]);
+
+    *name = dupstr(str);
+    *params = ret;
+    return TRUE;
+}
+
+static void free_params(game_params *params)
+{
+    sfree(params);
+}
+
+static game_params *dup_params(const 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);
+    while (*string && isdigit((unsigned char)*string)) string++;
+    if (*string == 'x') {
+        string++;
+        ret->h = atoi(string);
+	while (*string && isdigit((unsigned char)*string)) string++;
+    }
+    if (*string == 'd') {
+	int i;
+	string++;
+	for (i = 0; i < DIFFCOUNT; i++)
+	    if (*string == slant_diffchars[i])
+		ret->diff = i;
+	if (*string) string++;
+    }
+}
+
+static char *encode_params(const game_params *params, int full)
+{
+    char data[256];
+
+    sprintf(data, "%dx%d", params->w, params->h);
+    if (full)
+	sprintf(data + strlen(data), "d%c", slant_diffchars[params->diff]);
+
+    return dupstr(data);
+}
+
+static config_item *game_configure(const 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 = "Difficulty";
+    ret[2].type = C_CHOICES;
+    ret[2].sval = DIFFCONFIG;
+    ret[2].ival = params->diff;
+
+    ret[3].name = NULL;
+    ret[3].type = C_END;
+    ret[3].sval = NULL;
+    ret[3].ival = 0;
+
+    return ret;
+}
+
+static game_params *custom_params(const config_item *cfg)
+{
+    game_params *ret = snew(game_params);
+
+    ret->w = atoi(cfg[0].sval);
+    ret->h = atoi(cfg[1].sval);
+    ret->diff = cfg[2].ival;
+
+    return ret;
+}
+
+static char *validate_params(const game_params *params, int full)
+{
+    /*
+     * (At least at the time of writing this comment) The grid
+     * generator is actually capable of handling even zero grid
+     * dimensions without crashing. Puzzles with a zero-area grid
+     * are a bit boring, though, because they're already solved :-)
+     * And puzzles with a dimension of 1 can't be made Hard, which
+     * means the simplest thing is to forbid them altogether.
+     */
+
+    if (params->w < 2 || params->h < 2)
+	return "Width and height must both be at least two";
+
+    return NULL;
+}
+
+/*
+ * Scratch space for solver.
+ */
+struct solver_scratch {
+    /*
+     * Disjoint set forest which tracks the connected sets of
+     * points.
+     */
+    int *connected;
+
+    /*
+     * Counts the number of possible exits from each connected set
+     * of points. (That is, the number of possible _simultaneous_
+     * exits: an unconnected point labelled 2 has an exit count of
+     * 2 even if all four possible edges are still under
+     * consideration.)
+     */
+    int *exits;
+
+    /*
+     * Tracks whether each connected set of points includes a
+     * border point.
+     */
+    unsigned char *border;
+
+    /*
+     * Another disjoint set forest. This one tracks _squares_ which
+     * are known to slant in the same direction.
+     */
+    int *equiv;
+
+    /*
+     * Stores slash values which we know for an equivalence class.
+     * When we fill in a square, we set slashval[canonify(x)] to
+     * the same value as soln[x], so that we can then spot other
+     * squares equivalent to it and fill them in immediately via
+     * their known equivalence.
+     */
+    signed char *slashval;
+
+    /*
+     * Stores possible v-shapes. This array is w by h in size, but
+     * not every bit of every entry is meaningful. The bits mean:
+     * 
+     *  - bit 0 for a square means that that square and the one to
+     *    its right might form a v-shape between them
+     *  - bit 1 for a square means that that square and the one to
+     *    its right might form a ^-shape between them
+     *  - bit 2 for a square means that that square and the one
+     *    below it might form a >-shape between them
+     *  - bit 3 for a square means that that square and the one
+     *    below it might form a <-shape between them
+     * 
+     * Any starting 1 or 3 clue rules out four bits in this array
+     * immediately; a 2 clue propagates any ruled-out bit past it
+     * (if the two squares on one side of a 2 cannot be a v-shape,
+     * then neither can the two on the other side be the same
+     * v-shape); we can rule out further bits during play using
+     * partially filled 2 clues; whenever a pair of squares is
+     * known not to be _either_ kind of v-shape, we can mark them
+     * as equivalent.
+     */
+    unsigned char *vbitmap;
+
+    /*
+     * Useful to have this information automatically passed to
+     * solver subroutines. (This pointer is not dynamically
+     * allocated by new_scratch and free_scratch.)
+     */
+    const signed char *clues;
+};
+
+static struct solver_scratch *new_scratch(int w, int h)
+{
+    int W = w+1, H = h+1;
+    struct solver_scratch *ret = snew(struct solver_scratch);
+    ret->connected = snewn(W*H, int);
+    ret->exits = snewn(W*H, int);
+    ret->border = snewn(W*H, unsigned char);
+    ret->equiv = snewn(w*h, int);
+    ret->slashval = snewn(w*h, signed char);
+    ret->vbitmap = snewn(w*h, unsigned char);
+    return ret;
+}
+
+static void free_scratch(struct solver_scratch *sc)
+{
+    sfree(sc->vbitmap);
+    sfree(sc->slashval);
+    sfree(sc->equiv);
+    sfree(sc->border);
+    sfree(sc->exits);
+    sfree(sc->connected);
+    sfree(sc);
+}
+
+/*
+ * Wrapper on dsf_merge() which updates the `exits' and `border'
+ * arrays.
+ */
+static void merge_vertices(int *connected,
+			   struct solver_scratch *sc, int i, int j)
+{
+    int exits = -1, border = FALSE;    /* initialise to placate optimiser */
+
+    if (sc) {
+	i = dsf_canonify(connected, i);
+	j = dsf_canonify(connected, j);
+
+	/*
+	 * We have used one possible exit from each of the two
+	 * classes. Thus, the viable exit count of the new class is
+	 * the sum of the old exit counts minus two.
+	 */
+	exits = sc->exits[i] + sc->exits[j] - 2;
+
+	border = sc->border[i] || sc->border[j];
+    }
+
+    dsf_merge(connected, i, j);
+
+    if (sc) {
+	i = dsf_canonify(connected, i);
+	sc->exits[i] = exits;
+	sc->border[i] = border;
+    }
+}
+
+/*
+ * Called when we have just blocked one way out of a particular
+ * point. If that point is a non-clue point (thus has a variable
+ * number of exits), we have therefore decreased its potential exit
+ * count, so we must decrement the exit count for the group as a
+ * whole.
+ */
+static void decr_exits(struct solver_scratch *sc, int i)
+{
+    if (sc->clues[i] < 0) {
+	i = dsf_canonify(sc->connected, i);
+	sc->exits[i]--;
+    }
+}
+
+static void fill_square(int w, int h, int x, int y, int v,
+			signed char *soln,
+			int *connected, struct solver_scratch *sc)
+{
+    int W = w+1 /*, H = h+1 */;
+
+    assert(x >= 0 && x < w && y >= 0 && y < h);
+
+    if (soln[y*w+x] != 0) {
+	return;			       /* do nothing */
+    }
+
+#ifdef SOLVER_DIAGNOSTICS
+    if (verbose)
+	printf("  placing %c in %d,%d\n", v == -1 ? '\\' : '/', x, y);
+#endif
+
+    soln[y*w+x] = v;
+
+    if (sc) {
+	int c = dsf_canonify(sc->equiv, y*w+x);
+	sc->slashval[c] = v;
+    }
+
+    if (v < 0) {
+	merge_vertices(connected, sc, y*W+x, (y+1)*W+(x+1));
+	if (sc) {
+	    decr_exits(sc, y*W+(x+1));
+	    decr_exits(sc, (y+1)*W+x);
+	}
+    } else {
+	merge_vertices(connected, sc, y*W+(x+1), (y+1)*W+x);
+	if (sc) {
+	    decr_exits(sc, y*W+x);
+	    decr_exits(sc, (y+1)*W+(x+1));
+	}
+    }
+}
+
+static int vbitmap_clear(int w, int h, struct solver_scratch *sc,
+                         int x, int y, int vbits, char *reason, ...)
+{
+    int done_something = FALSE;
+    int vbit;
+
+    for (vbit = 1; vbit <= 8; vbit <<= 1)
+        if (vbits & sc->vbitmap[y*w+x] & vbit) {
+            done_something = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+            if (verbose) {
+                va_list ap;
+
+                printf("ruling out %c shape at (%d,%d)-(%d,%d) (",
+                       "!v^!>!!!<"[vbit], x, y,
+                       x+((vbit&0x3)!=0), y+((vbit&0xC)!=0));
+
+                va_start(ap, reason);
+                vprintf(reason, ap);
+                va_end(ap);
+
+                printf(")\n");
+            }
+#endif
+            sc->vbitmap[y*w+x] &= ~vbit;
+        }
+
+    return done_something;
+}
+
+/*
+ * Solver. Returns 0 for impossibility, 1 for success, 2 for
+ * ambiguity or failure to converge.
+ */
+static int slant_solve(int w, int h, const signed char *clues,
+		       signed char *soln, struct solver_scratch *sc,
+		       int difficulty)
+{
+    int W = w+1, H = h+1;
+    int x, y, i, j;
+    int done_something;
+
+    /*
+     * Clear the output.
+     */
+    memset(soln, 0, w*h);
+
+    sc->clues = clues;
+
+    /*
+     * Establish a disjoint set forest for tracking connectedness
+     * between grid points.
+     */
+    dsf_init(sc->connected, W*H);
+
+    /*
+     * Establish a disjoint set forest for tracking which squares
+     * are known to slant in the same direction.
+     */
+    dsf_init(sc->equiv, w*h);
+
+    /*
+     * Clear the slashval array.
+     */
+    memset(sc->slashval, 0, w*h);
+
+    /*
+     * Set up the vbitmap array. Initially all types of v are possible.
+     */
+    memset(sc->vbitmap, 0xF, w*h);
+
+    /*
+     * Initialise the `exits' and `border' arrays. These are used
+     * to do second-order loop avoidance: the dual of the no loops
+     * constraint is that every point must be somehow connected to
+     * the border of the grid (otherwise there would be a solid
+     * loop around it which prevented this).
+     * 
+     * I define a `dead end' to be a connected group of points
+     * which contains no border point, and which can form at most
+     * one new connection outside itself. Then I forbid placing an
+     * edge so that it connects together two dead-end groups, since
+     * this would yield a non-border-connected isolated subgraph
+     * with no further scope to extend it.
+     */
+    for (y = 0; y < H; y++)
+	for (x = 0; x < W; x++) {
+	    if (y == 0 || y == H-1 || x == 0 || x == W-1)
+		sc->border[y*W+x] = TRUE;
+	    else
+		sc->border[y*W+x] = FALSE;
+
+	    if (clues[y*W+x] < 0)
+		sc->exits[y*W+x] = 4;
+	    else
+		sc->exits[y*W+x] = clues[y*W+x];
+	}
+
+    /*
+     * Repeatedly try to deduce something until we can't.
+     */
+    do {
+	done_something = FALSE;
+
+	/*
+	 * Any clue point with the number of remaining lines equal
+	 * to zero or to the number of remaining undecided
+	 * neighbouring squares can be filled in completely.
+	 */
+	for (y = 0; y < H; y++)
+	    for (x = 0; x < W; x++) {
+		struct {
+		    int pos, slash;
+		} neighbours[4];
+		int nneighbours;
+		int nu, nl, c, s, eq, eq2, last, meq, mj1, mj2;
+
+		if ((c = clues[y*W+x]) < 0)
+		    continue;
+
+		/*
+		 * We have a clue point. Start by listing its
+		 * neighbouring squares, in order around the point,
+		 * together with the type of slash that would be
+		 * required in that square to connect to the point.
+		 */
+		nneighbours = 0;
+		if (x > 0 && y > 0) {
+		    neighbours[nneighbours].pos = (y-1)*w+(x-1);
+		    neighbours[nneighbours].slash = -1;
+		    nneighbours++;
+		}
+		if (x > 0 && y < h) {
+		    neighbours[nneighbours].pos = y*w+(x-1);
+		    neighbours[nneighbours].slash = +1;
+		    nneighbours++;
+		}
+		if (x < w && y < h) {
+		    neighbours[nneighbours].pos = y*w+x;
+		    neighbours[nneighbours].slash = -1;
+		    nneighbours++;
+		}
+		if (x < w && y > 0) {
+		    neighbours[nneighbours].pos = (y-1)*w+x;
+		    neighbours[nneighbours].slash = +1;
+		    nneighbours++;
+		}
+
+		/*
+		 * Count up the number of undecided neighbours, and
+		 * also the number of lines already present.
+		 *
+		 * If we're not on DIFF_EASY, then in this loop we
+		 * also track whether we've seen two adjacent empty
+		 * squares belonging to the same equivalence class
+		 * (meaning they have the same type of slash). If
+		 * so, we count them jointly as one line.
+		 */
+		nu = 0;
+		nl = c;
+		last = neighbours[nneighbours-1].pos;
+		if (soln[last] == 0)
+		    eq = dsf_canonify(sc->equiv, last);
+		else
+		    eq = -1;
+		meq = mj1 = mj2 = -1;
+		for (i = 0; i < nneighbours; i++) {
+		    j = neighbours[i].pos;
+		    s = neighbours[i].slash;
+		    if (soln[j] == 0) {
+			nu++;	       /* undecided */
+			if (meq < 0 && difficulty > DIFF_EASY) {
+			    eq2 = dsf_canonify(sc->equiv, j);
+			    if (eq == eq2 && last != j) {
+				/*
+				 * We've found an equivalent pair.
+				 * Mark it. This also inhibits any
+				 * further equivalence tracking
+				 * around this square, since we can
+				 * only handle one pair (and in
+				 * particular we want to avoid
+				 * being misled by two overlapping
+				 * equivalence pairs).
+				 */
+				meq = eq;
+				mj1 = last;
+				mj2 = j;
+				nl--;   /* count one line */
+				nu -= 2;   /* and lose two undecideds */
+			    } else
+				eq = eq2;
+			}
+		    } else {
+			eq = -1;
+			if (soln[j] == s)
+			    nl--;      /* here's a line */
+		    }
+		    last = j;
+		}
+
+		/*
+		 * Check the counts.
+		 */
+		if (nl < 0 || nl > nu) {
+		    /*
+		     * No consistent value for this at all!
+		     */
+#ifdef SOLVER_DIAGNOSTICS
+		    if (verbose)
+			printf("need %d / %d lines around clue point at %d,%d!\n",
+			       nl, nu, x, y);
+#endif
+		    return 0;	       /* impossible */
+		}
+
+		if (nu > 0 && (nl == 0 || nl == nu)) {
+#ifdef SOLVER_DIAGNOSTICS
+		    if (verbose) {
+			if (meq >= 0)
+			    printf("partially (since %d,%d == %d,%d) ",
+				   mj1%w, mj1/w, mj2%w, mj2/w);
+			printf("%s around clue point at %d,%d\n",
+			       nl ? "filling" : "emptying", x, y);
+		    }
+#endif
+		    for (i = 0; i < nneighbours; i++) {
+			j = neighbours[i].pos;
+			s = neighbours[i].slash;
+			if (soln[j] == 0 && j != mj1 && j != mj2)
+			    fill_square(w, h, j%w, j/w, (nl ? s : -s), soln,
+					sc->connected, sc);
+		    }
+
+		    done_something = TRUE;
+		} else if (nu == 2 && nl == 1 && difficulty > DIFF_EASY) {
+		    /*
+		     * If we have precisely two undecided squares
+		     * and precisely one line to place between
+		     * them, _and_ those squares are adjacent, then
+		     * we can mark them as equivalent to one
+		     * another.
+		     * 
+		     * This even applies if meq >= 0: if we have a
+		     * 2 clue point and two of its neighbours are
+		     * already marked equivalent, we can indeed
+		     * mark the other two as equivalent.
+		     * 
+		     * We don't bother with this on DIFF_EASY,
+		     * since we wouldn't have used the results
+		     * anyway.
+		     */
+		    last = -1;
+		    for (i = 0; i < nneighbours; i++) {
+			j = neighbours[i].pos;
+			if (soln[j] == 0 && j != mj1 && j != mj2) {
+			    if (last < 0)
+				last = i;
+			    else if (last == i-1 || (last == 0 && i == 3))
+				break; /* found a pair */
+			}
+		    }
+		    if (i < nneighbours) {
+			int sv1, sv2;
+
+			assert(last >= 0);
+			/*
+			 * neighbours[last] and neighbours[i] are
+			 * the pair. Mark them equivalent.
+			 */
+#ifdef SOLVER_DIAGNOSTICS
+			if (verbose) {
+			    if (meq >= 0)
+				printf("since %d,%d == %d,%d, ",
+				       mj1%w, mj1/w, mj2%w, mj2/w);
+			}
+#endif
+			mj1 = neighbours[last].pos;
+			mj2 = neighbours[i].pos;
+#ifdef SOLVER_DIAGNOSTICS
+			if (verbose)
+			    printf("clue point at %d,%d implies %d,%d == %d,"
+				   "%d\n", x, y, mj1%w, mj1/w, mj2%w, mj2/w);
+#endif
+			mj1 = dsf_canonify(sc->equiv, mj1);
+			sv1 = sc->slashval[mj1];
+			mj2 = dsf_canonify(sc->equiv, mj2);
+			sv2 = sc->slashval[mj2];
+			if (sv1 != 0 && sv2 != 0 && sv1 != sv2) {
+#ifdef SOLVER_DIAGNOSTICS
+			    if (verbose)
+				printf("merged two equivalence classes with"
+				       " different slash values!\n");
+#endif
+			    return 0;
+			}
+			sv1 = sv1 ? sv1 : sv2;
+			dsf_merge(sc->equiv, mj1, mj2);
+			mj1 = dsf_canonify(sc->equiv, mj1);
+			sc->slashval[mj1] = sv1;
+		    }
+		}
+	    }
+
+	if (done_something)
+	    continue;
+
+	/*
+	 * Failing that, we now apply the second condition, which
+	 * is that no square may be filled in such a way as to form
+	 * a loop. Also in this loop (since it's over squares
+	 * rather than points), we check slashval to see if we've
+	 * already filled in another square in the same equivalence
+	 * class.
+	 * 
+	 * The slashval check is disabled on DIFF_EASY, as is dead
+	 * end avoidance. Only _immediate_ loop avoidance remains.
+	 */
+	for (y = 0; y < h; y++)
+	    for (x = 0; x < w; x++) {
+		int fs, bs, v;
+		int c1, c2;
+#ifdef SOLVER_DIAGNOSTICS
+		char *reason = "<internal error>";
+#endif
+
+		if (soln[y*w+x])
+		    continue;	       /* got this one already */
+
+		fs = FALSE;
+		bs = FALSE;
+
+		if (difficulty > DIFF_EASY)
+		    v = sc->slashval[dsf_canonify(sc->equiv, y*w+x)];
+		else
+		    v = 0;
+
+		/*
+		 * Try to rule out connectivity between (x,y) and
+		 * (x+1,y+1); if successful, we will deduce that we
+		 * must have a forward slash.
+		 */
+		c1 = dsf_canonify(sc->connected, y*W+x);
+		c2 = dsf_canonify(sc->connected, (y+1)*W+(x+1));
+		if (c1 == c2) {
+		    fs = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+		    reason = "simple loop avoidance";
+#endif
+		}
+		if (difficulty > DIFF_EASY &&
+		    !sc->border[c1] && !sc->border[c2] &&
+		    sc->exits[c1] <= 1 && sc->exits[c2] <= 1) {
+		    fs = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+		    reason = "dead end avoidance";
+#endif
+		}
+		if (v == +1) {
+		    fs = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+		    reason = "equivalence to an already filled square";
+#endif
+		}
+
+		/*
+		 * Now do the same between (x+1,y) and (x,y+1), to
+		 * see if we are required to have a backslash.
+		 */
+		c1 = dsf_canonify(sc->connected, y*W+(x+1));
+		c2 = dsf_canonify(sc->connected, (y+1)*W+x);
+		if (c1 == c2) {
+		    bs = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+		    reason = "simple loop avoidance";
+#endif
+		}
+		if (difficulty > DIFF_EASY &&
+		    !sc->border[c1] && !sc->border[c2] &&
+		    sc->exits[c1] <= 1 && sc->exits[c2] <= 1) {
+		    bs = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+		    reason = "dead end avoidance";
+#endif
+		}
+		if (v == -1) {
+		    bs = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+		    reason = "equivalence to an already filled square";
+#endif
+		}
+
+		if (fs && bs) {
+		    /*
+		     * No consistent value for this at all!
+		     */
+#ifdef SOLVER_DIAGNOSTICS
+		    if (verbose)
+			printf("%d,%d has no consistent slash!\n", x, y);
+#endif
+		    return 0;          /* impossible */
+		}
+
+		if (fs) {
+#ifdef SOLVER_DIAGNOSTICS
+		    if (verbose)
+			printf("employing %s\n", reason);
+#endif
+		    fill_square(w, h, x, y, +1, soln, sc->connected, sc);
+		    done_something = TRUE;
+		} else if (bs) {
+#ifdef SOLVER_DIAGNOSTICS
+		    if (verbose)
+			printf("employing %s\n", reason);
+#endif
+		    fill_square(w, h, x, y, -1, soln, sc->connected, sc);
+		    done_something = TRUE;
+		}
+	    }
+
+	if (done_something)
+	    continue;
+
+        /*
+         * Now see what we can do with the vbitmap array. All
+         * vbitmap deductions are disabled at Easy level.
+         */
+        if (difficulty <= DIFF_EASY)
+            continue;
+
+	for (y = 0; y < h; y++)
+	    for (x = 0; x < w; x++) {
+                int s, c;
+
+                /*
+                 * Any line already placed in a square must rule
+                 * out any type of v which contradicts it.
+                 */
+                if ((s = soln[y*w+x]) != 0) {
+                    if (x > 0)
+                        done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y, (s < 0 ? 0x1 : 0x2),
+                                      "contradicts known edge at (%d,%d)",x,y);
+                    if (x+1 < w)
+                        done_something |=
+                        vbitmap_clear(w, h, sc, x, y, (s < 0 ? 0x2 : 0x1),
+                                      "contradicts known edge at (%d,%d)",x,y);
+                    if (y > 0)
+                        done_something |=
+                        vbitmap_clear(w, h, sc, x, y-1, (s < 0 ? 0x4 : 0x8),
+                                      "contradicts known edge at (%d,%d)",x,y);
+                    if (y+1 < h)
+                        done_something |=
+                        vbitmap_clear(w, h, sc, x, y, (s < 0 ? 0x8 : 0x4),
+                                      "contradicts known edge at (%d,%d)",x,y);
+                }
+
+                /*
+                 * If both types of v are ruled out for a pair of
+                 * adjacent squares, mark them as equivalent.
+                 */
+                if (x+1 < w && !(sc->vbitmap[y*w+x] & 0x3)) {
+                    int n1 = y*w+x, n2 = y*w+(x+1);
+                    if (dsf_canonify(sc->equiv, n1) !=
+                        dsf_canonify(sc->equiv, n2)) {
+                        dsf_merge(sc->equiv, n1, n2);
+                        done_something = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+                        if (verbose)
+                            printf("(%d,%d) and (%d,%d) must be equivalent"
+                                   " because both v-shapes are ruled out\n",
+                                   x, y, x+1, y);
+#endif
+                    }
+                }
+                if (y+1 < h && !(sc->vbitmap[y*w+x] & 0xC)) {
+                    int n1 = y*w+x, n2 = (y+1)*w+x;
+                    if (dsf_canonify(sc->equiv, n1) !=
+                        dsf_canonify(sc->equiv, n2)) {
+                        dsf_merge(sc->equiv, n1, n2);
+                        done_something = TRUE;
+#ifdef SOLVER_DIAGNOSTICS
+                        if (verbose)
+                            printf("(%d,%d) and (%d,%d) must be equivalent"
+                                   " because both v-shapes are ruled out\n",
+                                   x, y, x, y+1);
+#endif
+                    }
+                }
+
+                /*
+                 * The remaining work in this loop only works
+                 * around non-edge clue points.
+                 */
+                if (y == 0 || x == 0)
+                    continue;
+		if ((c = clues[y*W+x]) < 0)
+		    continue;
+
+                /*
+                 * x,y marks a clue point not on the grid edge. See
+                 * if this clue point allows us to rule out any v
+                 * shapes.
+                 */
+
+                if (c == 1) {
+                    /*
+                     * A 1 clue can never have any v shape pointing
+                     * at it.
+                     */
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y-1, 0x5,
+                                      "points at 1 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y, 0x2,
+                                      "points at 1 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x, y-1, 0x8,
+                                      "points at 1 clue at (%d,%d)", x, y);
+                } else if (c == 3) {
+                    /*
+                     * A 3 clue can never have any v shape pointing
+                     * away from it.
+                     */
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y-1, 0xA,
+                                      "points away from 3 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y, 0x1,
+                                      "points away from 3 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x, y-1, 0x4,
+                                      "points away from 3 clue at (%d,%d)", x, y);
+                } else if (c == 2) {
+                    /*
+                     * If a 2 clue has any kind of v ruled out on
+                     * one side of it, the same v is ruled out on
+                     * the other side.
+                     */
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y-1,
+                                      (sc->vbitmap[(y  )*w+(x-1)] & 0x3) ^ 0x3,
+                                      "propagated by 2 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y-1,
+                                      (sc->vbitmap[(y-1)*w+(x  )] & 0xC) ^ 0xC,
+                                      "propagated by 2 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x-1, y,
+                                      (sc->vbitmap[(y-1)*w+(x-1)] & 0x3) ^ 0x3,
+                                      "propagated by 2 clue at (%d,%d)", x, y);
+                    done_something |=
+                        vbitmap_clear(w, h, sc, x, y-1,
+                                      (sc->vbitmap[(y-1)*w+(x-1)] & 0xC) ^ 0xC,
+                                      "propagated by 2 clue at (%d,%d)", x, y);
+                }
+
+#undef CLEARBITS
+
+            }
+
+    } while (done_something);
+
+    /*
+     * Solver can make no more progress. See if the grid is full.
+     */
+    for (i = 0; i < w*h; i++)
+	if (!soln[i])
+	    return 2;		       /* failed to converge */
+    return 1;			       /* success */
+}
+
+/*
+ * Filled-grid generator.
+ */
+static void slant_generate(int w, int h, signed char *soln, random_state *rs)
+{
+    int W = w+1, H = h+1;
+    int x, y, i;
+    int *connected, *indices;
+
+    /*
+     * Clear the output.
+     */
+    memset(soln, 0, w*h);
+
+    /*
+     * Establish a disjoint set forest for tracking connectedness
+     * between grid points.
+     */
+    connected = snew_dsf(W*H);
+
+    /*
+     * Prepare a list of the squares in the grid, and fill them in
+     * in a random order.
+     */
+    indices = snewn(w*h, int);
+    for (i = 0; i < w*h; i++)
+	indices[i] = i;
+    shuffle(indices, w*h, sizeof(*indices), rs);
+
+    /*
+     * Fill in each one in turn.
+     */
+    for (i = 0; i < w*h; i++) {
+	int fs, bs, v;
+
+	y = indices[i] / w;
+	x = indices[i] % w;
+
+	fs = (dsf_canonify(connected, y*W+x) ==
+	      dsf_canonify(connected, (y+1)*W+(x+1)));
+	bs = (dsf_canonify(connected, (y+1)*W+x) ==
+	      dsf_canonify(connected, y*W+(x+1)));
+
+	/*
+	 * It isn't possible to get into a situation where we
+	 * aren't allowed to place _either_ type of slash in a
+	 * square. Thus, filled-grid generation never has to
+	 * backtrack.
+	 * 
+	 * Proof (thanks to Gareth Taylor):
+	 * 
+	 * If it were possible, it would have to be because there
+	 * was an existing path (not using this square) between the
+	 * top-left and bottom-right corners of this square, and
+	 * another between the other two. These two paths would
+	 * have to cross at some point.
+	 * 
+	 * Obviously they can't cross in the middle of a square, so
+	 * they must cross by sharing a point in common. But this
+	 * isn't possible either: if you chessboard-colour all the
+	 * points on the grid, you find that any continuous
+	 * diagonal path is entirely composed of points of the same
+	 * colour. And one of our two hypothetical paths is between
+	 * two black points, and the other is between two white
+	 * points - therefore they can have no point in common. []
+	 */
+	assert(!(fs && bs));
+
+	v = fs ? +1 : bs ? -1 : 2 * random_upto(rs, 2) - 1;
+	fill_square(w, h, x, y, v, soln, connected, NULL);
+    }
+
+    sfree(indices);
+    sfree(connected);
+}
+
+static char *new_game_desc(const game_params *params, random_state *rs,
+			   char **aux, int interactive)
+{
+    int w = params->w, h = params->h, W = w+1, H = h+1;
+    signed char *soln, *tmpsoln, *clues;
+    int *clueindices;
+    struct solver_scratch *sc;
+    int x, y, v, i, j;
+    char *desc;
+
+    soln = snewn(w*h, signed char);
+    tmpsoln = snewn(w*h, signed char);
+    clues = snewn(W*H, signed char);
+    clueindices = snewn(W*H, int);
+    sc = new_scratch(w, h);
+
+    do {
+	/*
+	 * Create the filled grid.
+	 */
+	slant_generate(w, h, soln, rs);
+
+	/*
+	 * Fill in the complete set of clues.
+	 */
+	for (y = 0; y < H; y++)
+	    for (x = 0; x < W; x++) {
+		v = 0;
+
+		if (x > 0 && y > 0 && soln[(y-1)*w+(x-1)] == -1) v++;
+		if (x > 0 && y < h && soln[y*w+(x-1)] == +1) v++;
+		if (x < w && y > 0 && soln[(y-1)*w+x] == +1) v++;
+		if (x < w && y < h && soln[y*w+x] == -1) v++;
+
+		clues[y*W+x] = v;
+	    }
+
+	/*
+	 * With all clue points filled in, all puzzles are easy: we can
+	 * simply process the clue points in lexicographic order, and
+	 * at each clue point we will always have at most one square
+	 * undecided, which we can then fill in uniquely.
+	 */
+	assert(slant_solve(w, h, clues, tmpsoln, sc, DIFF_EASY) == 1);
+
+	/*
+	 * Remove as many clues as possible while retaining solubility.
+	 *
+	 * In DIFF_HARD mode, we prioritise the removal of obvious
+	 * starting points (4s, 0s, border 2s and corner 1s), on
+	 * the grounds that having as few of these as possible
+	 * seems like a good thing. In particular, we can often get
+	 * away without _any_ completely obvious starting points,
+	 * which is even better.
+	 */
+	for (i = 0; i < W*H; i++)
+	    clueindices[i] = i;
+	shuffle(clueindices, W*H, sizeof(*clueindices), rs);
+	for (j = 0; j < 2; j++) {
+	    for (i = 0; i < W*H; i++) {
+		int pass, yb, xb;
+
+		y = clueindices[i] / W;
+		x = clueindices[i] % W;
+		v = clues[y*W+x];
+
+		/*
+		 * Identify which pass we should process this point
+		 * in. If it's an obvious start point, _or_ we're
+		 * in DIFF_EASY, then it goes in pass 0; otherwise
+		 * pass 1.
+		 */
+		xb = (x == 0 || x == W-1);
+		yb = (y == 0 || y == H-1);
+		if (params->diff == DIFF_EASY || v == 4 || v == 0 ||
+		    (v == 2 && (xb||yb)) || (v == 1 && xb && yb))
+		    pass = 0;
+		else
+		    pass = 1;
+
+		if (pass == j) {
+		    clues[y*W+x] = -1;
+		    if (slant_solve(w, h, clues, tmpsoln, sc,
+				    params->diff) != 1)
+			clues[y*W+x] = v;	       /* put it back */
+		}
+	    }
+	}
+
+	/*
+	 * And finally, verify that the grid is of _at least_ the
+	 * requested difficulty, by running the solver one level
+	 * down and verifying that it can't manage it.
+	 */
+    } while (params->diff > 0 &&
+	     slant_solve(w, h, clues, tmpsoln, sc, params->diff - 1) <= 1);
+
+    /*
+     * Now we have the clue set as it will be presented to the
+     * user. Encode it in a game desc.
+     */
+    {
+	char *p;
+	int run, i;
+
+	desc = snewn(W*H+1, char);
+	p = desc;
+	run = 0;
+	for (i = 0; i <= W*H; i++) {
+	    int n = (i < W*H ? clues[i] : -2);
+
+	    if (n == -1)
+		run++;
+	    else {
+		if (run) {
+		    while (run > 0) {
+			int c = 'a' - 1 + run;
+			if (run > 26)
+			    c = 'z';
+			*p++ = c;
+			run -= c - ('a' - 1);
+		    }
+		}
+		if (n >= 0)
+		    *p++ = '0' + n;
+		run = 0;
+	    }
+	}
+	assert(p - desc <= W*H);
+	*p++ = '\0';
+	desc = sresize(desc, p - desc, char);
+    }
+
+    /*
+     * Encode the solution as an aux_info.
+     */
+    {
+	char *auxbuf;
+	*aux = auxbuf = snewn(w*h+1, char);
+	for (i = 0; i < w*h; i++)
+	    auxbuf[i] = soln[i] < 0 ? '\\' : '/';
+	auxbuf[w*h] = '\0';
+    }
+
+    free_scratch(sc);
+    sfree(clueindices);
+    sfree(clues);
+    sfree(tmpsoln);
+    sfree(soln);
+
+    return desc;
+}
+
+static char *validate_desc(const game_params *params, const char *desc)
+{
+    int w = params->w, h = params->h, W = w+1, H = h+1;
+    int area = W*H;
+    int squares = 0;
+
+    while (*desc) {
+        int n = *desc++;
+        if (n >= 'a' && n <= 'z') {
+            squares += n - 'a' + 1;
+        } else if (n >= '0' && n <= '4') {
+            squares++;
+        } else
+            return "Invalid character in game description";
+    }
+
+    if (squares < area)
+        return "Not enough data to fill grid";
+
+    if (squares > area)
+        return "Too much data to fit in grid";
+
+    return NULL;
+}
+
+static game_state *new_game(midend *me, const game_params *params,
+                            const char *desc)
+{
+    int w = params->w, h = params->h, W = w+1, H = h+1;
+    game_state *state = snew(game_state);
+    int area = W*H;
+    int squares = 0;
+
+    state->p = *params;
+    state->soln = snewn(w*h, signed char);
+    memset(state->soln, 0, w*h);
+    state->completed = state->used_solve = FALSE;
+    state->errors = snewn(W*H, unsigned char);
+    memset(state->errors, 0, W*H);
+
+    state->clues = snew(game_clues);
+    state->clues->w = w;
+    state->clues->h = h;
+    state->clues->clues = snewn(W*H, signed char);
+    state->clues->refcount = 1;
+    state->clues->tmpdsf = snewn(W*H*2+W+H, int);
+    memset(state->clues->clues, -1, W*H);
+    while (*desc) {
+        int n = *desc++;
+        if (n >= 'a' && n <= 'z') {
+            squares += n - 'a' + 1;
+        } else if (n >= '0' && n <= '4') {
+            state->clues->clues[squares++] = n - '0';
+        } else
+	    assert(!"can't get here");
+    }
+    assert(squares == area);
+
+    return state;
+}
+
+static game_state *dup_game(const game_state *state)
+{
+    int w = state->p.w, h = state->p.h, W = w+1, H = h+1;
+    game_state *ret = snew(game_state);
+
+    ret->p = state->p;
+    ret->clues = state->clues;
+    ret->clues->refcount++;
+    ret->completed = state->completed;
+    ret->used_solve = state->used_solve;
+
+    ret->soln = snewn(w*h, signed char);
+    memcpy(ret->soln, state->soln, w*h);
+
+    ret->errors = snewn(W*H, unsigned char);
+    memcpy(ret->errors, state->errors, W*H);
+
+    return ret;
+}
+
+static void free_game(game_state *state)
+{
+    sfree(state->errors);
+    sfree(state->soln);
+    assert(state->clues);
+    if (--state->clues->refcount <= 0) {
+        sfree(state->clues->clues);
+        sfree(state->clues->tmpdsf);
+        sfree(state->clues);
+    }
+    sfree(state);
+}
+
+/*
+ * Utility function to return the current degree of a vertex. If
+ * `anti' is set, it returns the number of filled-in edges
+ * surrounding the point which _don't_ connect to it; thus 4 minus
+ * its anti-degree is the maximum degree it could have if all the
+ * empty spaces around it were filled in.
+ * 
+ * (Yes, _4_ minus its anti-degree even if it's a border vertex.)
+ * 
+ * If ret > 0, *sx and *sy are set to the coordinates of one of the
+ * squares that contributed to it.
+ */
+static int vertex_degree(int w, int h, signed char *soln, int x, int y,
+                         int anti, int *sx, int *sy)
+{
+    int ret = 0;
+
+    assert(x >= 0 && x <= w && y >= 0 && y <= h);
+    if (x > 0 && y > 0 && soln[(y-1)*w+(x-1)] - anti < 0) {
+        if (sx) *sx = x-1;
+        if (sy) *sy = y-1;
+        ret++;
+    }
+    if (x > 0 && y < h && soln[y*w+(x-1)] + anti > 0) {
+        if (sx) *sx = x-1;
+        if (sy) *sy = y;
+        ret++;
+    }
+    if (x < w && y > 0 && soln[(y-1)*w+x] + anti > 0) {
+        if (sx) *sx = x;
+        if (sy) *sy = y-1;
+        ret++;
+    }
+    if (x < w && y < h && soln[y*w+x] - anti < 0) {
+        if (sx) *sx = x;
+        if (sy) *sy = y;
+        ret++;
+    }
+
+    return anti ? 4 - ret : ret;
+}
+
+struct slant_neighbour_ctx {
+    const game_state *state;
+    int i, n, neighbours[4];
+};
+static int slant_neighbour(int vertex, void *vctx)
+{
+    struct slant_neighbour_ctx *ctx = (struct slant_neighbour_ctx *)vctx;
+
+    if (vertex >= 0) {
+        int w = ctx->state->p.w, h = ctx->state->p.h, W = w+1;
+        int x = vertex % W, y = vertex / W;
+        ctx->n = ctx->i = 0;
+        if (x < w && y < h && ctx->state->soln[y*w+x] < 0)
+            ctx->neighbours[ctx->n++] = (y+1)*W+(x+1);
+        if (x > 0 && y > 0 && ctx->state->soln[(y-1)*w+(x-1)] < 0)
+            ctx->neighbours[ctx->n++] = (y-1)*W+(x-1);
+        if (x > 0 && y < h && ctx->state->soln[y*w+(x-1)] > 0)
+            ctx->neighbours[ctx->n++] = (y+1)*W+(x-1);
+        if (x < w && y > 0 && ctx->state->soln[(y-1)*w+x] > 0)
+            ctx->neighbours[ctx->n++] = (y-1)*W+(x+1);
+    }
+
+    if (ctx->i < ctx->n)
+        return ctx->neighbours[ctx->i++];
+    else
+        return -1;
+}
+
+static int check_completion(game_state *state)
+{
+    int w = state->p.w, h = state->p.h, W = w+1, H = h+1;
+    int x, y, err = FALSE;
+
+    memset(state->errors, 0, W*H);
+
+    /*
+     * Detect and error-highlight loops in the grid.
+     */
+    {
+        struct findloopstate *fls = findloop_new_state(W*H);
+        struct slant_neighbour_ctx ctx;
+        ctx.state = state;
+
+        if (findloop_run(fls, W*H, slant_neighbour, &ctx))
+            err = TRUE;
+        for (y = 0; y < h; y++) {
+            for (x = 0; x < w; x++) {
+                int u, v;
+                if (state->soln[y*w+x] == 0) {
+                    continue;
+                } else if (state->soln[y*w+x] > 0) {
+                    u = y*W+(x+1);
+                    v = (y+1)*W+x;
+                } else {
+                    u = (y+1)*W+(x+1);
+                    v = y*W+x;
+                }
+                if (findloop_is_loop_edge(fls, u, v))
+                    state->errors[y*W+x] |= ERR_SQUARE;
+	    }
+        }
+
+        findloop_free_state(fls);
+    }
+
+    /*
+     * Now go through and check the degree of each clue vertex, and
+     * mark it with ERR_VERTEX if it cannot be fulfilled.
+     */
+    for (y = 0; y < H; y++)
+        for (x = 0; x < W; x++) {
+            int c;
+
+	    if ((c = state->clues->clues[y*W+x]) < 0)
+		continue;
+
+            /*
+             * Check to see if there are too many connections to
+             * this vertex _or_ too many non-connections. Either is
+             * grounds for marking the vertex as erroneous.
+             */
+            if (vertex_degree(w, h, state->soln, x, y,
+                              FALSE, NULL, NULL) > c ||
+                vertex_degree(w, h, state->soln, x, y,
+                              TRUE, NULL, NULL) > 4-c) {
+                state->errors[y*W+x] |= ERR_VERTEX;
+                err = TRUE;
+            }
+        }
+
+    /*
+     * Now our actual victory condition is that (a) none of the
+     * above code marked anything as erroneous, and (b) every
+     * square has an edge in it.
+     */
+
+    if (err)
+        return FALSE;
+
+    for (y = 0; y < h; y++)
+	for (x = 0; x < w; x++)
+	    if (state->soln[y*w+x] == 0)
+		return FALSE;
+
+    return TRUE;
+}
+
+static char *solve_game(const game_state *state, const game_state *currstate,
+                        const char *aux, char **error)
+{
+    int w = state->p.w, h = state->p.h;
+    signed char *soln;
+    int bs, ret;
+    int free_soln = FALSE;
+    char *move, buf[80];
+    int movelen, movesize;
+    int x, y;
+
+    if (aux) {
+	/*
+	 * If we already have the solution, save ourselves some
+	 * time.
+	 */
+	soln = (signed char *)aux;
+	bs = (signed char)'\\';
+	free_soln = FALSE;
+    } else {
+	struct solver_scratch *sc = new_scratch(w, h);
+	soln = snewn(w*h, signed char);
+	bs = -1;
+	ret = slant_solve(w, h, state->clues->clues, soln, sc, DIFF_HARD);
+	free_scratch(sc);
+	if (ret != 1) {
+	    sfree(soln);
+	    if (ret == 0)
+		*error = "This puzzle is not self-consistent";
+	    else
+		*error = "Unable to find a unique solution for this puzzle";
+            return NULL;
+	}
+	free_soln = TRUE;
+    }
+
+    /*
+     * Construct a move string which turns the current state into
+     * the solved state.
+     */
+    movesize = 256;
+    move = snewn(movesize, char);
+    movelen = 0;
+    move[movelen++] = 'S';
+    move[movelen] = '\0';
+    for (y = 0; y < h; y++)
+	for (x = 0; x < w; x++) {
+	    int v = (soln[y*w+x] == bs ? -1 : +1);
+	    if (state->soln[y*w+x] != v) {
+		int len = sprintf(buf, ";%c%d,%d", (int)(v < 0 ? '\\' : '/'), x, y);
+		if (movelen + len >= movesize) {
+		    movesize = movelen + len + 256;
+		    move = sresize(move, movesize, char);
+		}
+		strcpy(move + movelen, buf);
+		movelen += len;
+	    }
+	}
+
+    if (free_soln)
+	sfree(soln);
+
+    return move;
+}
+
+static int game_can_format_as_text_now(const game_params *params)
+{
+    return TRUE;
+}
+
+static char *game_text_format(const game_state *state)
+{
+    int w = state->p.w, h = state->p.h, W = w+1, H = h+1;
+    int x, y, len;
+    char *ret, *p;
+
+    /*
+     * There are h+H rows of w+W columns.
+     */
+    len = (h+H) * (w+W+1) + 1;
+    ret = snewn(len, char);
+    p = ret;
+
+    for (y = 0; y < H; y++) {
+	for (x = 0; x < W; x++) {
+	    if (state->clues->clues[y*W+x] >= 0)
+		*p++ = state->clues->clues[y*W+x] + '0';
+	    else
+		*p++ = '+';
+	    if (x < w)
+		*p++ = '-';
+	}
+	*p++ = '\n';
+	if (y < h) {
+	    for (x = 0; x < W; x++) {
+		*p++ = '|';
+		if (x < w) {
+		    if (state->soln[y*w+x] != 0)
+			*p++ = (state->soln[y*w+x] < 0 ? '\\' : '/');
+		    else
+			*p++ = ' ';
+		}
+	    }
+	    *p++ = '\n';
+	}
+    }
+    *p++ = '\0';
+
+    assert(p - ret == len);
+    return ret;
+}
+
+struct game_ui {
+    int cur_x, cur_y, cur_visible;
+};
+
+static game_ui *new_ui(const game_state *state)
+{
+    game_ui *ui = snew(game_ui);
+    ui->cur_x = ui->cur_y = ui->cur_visible = 0;
+    return ui;
+}
+
+static void free_ui(game_ui *ui)
+{
+    sfree(ui);
+}
+
+static char *encode_ui(const game_ui *ui)
+{
+    return NULL;
+}
+
+static void decode_ui(game_ui *ui, const char *encoding)
+{
+}
+
+static void game_changed_state(game_ui *ui, const game_state *oldstate,
+                               const game_state *newstate)
+{
+}
+
+#define PREFERRED_TILESIZE 32
+#define TILESIZE (ds->tilesize)
+#define BORDER TILESIZE
+#define CLUE_RADIUS (TILESIZE / 3)
+#define CLUE_TEXTSIZE (TILESIZE / 2)
+#define COORD(x)  ( (x) * TILESIZE + BORDER )
+#define FROMCOORD(x)  ( ((x) - BORDER + TILESIZE) / TILESIZE - 1 )
+
+#define FLASH_TIME 0.30F
+
+/*
+ * Bit fields in the `grid' and `todraw' elements of the drawstate.
+ */
+#define BACKSLASH 0x00000001L
+#define FORWSLASH 0x00000002L
+#define L_T       0x00000004L
+#define ERR_L_T   0x00000008L
+#define L_B       0x00000010L
+#define ERR_L_B   0x00000020L
+#define T_L       0x00000040L
+#define ERR_T_L   0x00000080L
+#define T_R       0x00000100L
+#define ERR_T_R   0x00000200L
+#define C_TL      0x00000400L
+#define ERR_C_TL  0x00000800L
+#define FLASH     0x00001000L
+#define ERRSLASH  0x00002000L
+#define ERR_TL    0x00004000L
+#define ERR_TR    0x00008000L
+#define ERR_BL    0x00010000L
+#define ERR_BR    0x00020000L
+#define CURSOR    0x00040000L
+
+struct game_drawstate {
+    int tilesize;
+    int started;
+    long *grid;
+    long *todraw;
+};
+
+static char *interpret_move(const game_state *state, game_ui *ui,
+                            const game_drawstate *ds,
+                            int x, int y, int button)
+{
+    int w = state->p.w, h = state->p.h;
+    int v;
+    char buf[80];
+    enum { CLOCKWISE, ANTICLOCKWISE, NONE } action = NONE;
+
+    if (button == LEFT_BUTTON || button == RIGHT_BUTTON) {
+	/*
+	 * This is an utterly awful hack which I should really sort out
+	 * by means of a proper configuration mechanism. One Slant
+	 * player has observed that they prefer the mouse buttons to
+	 * function exactly the opposite way round, so here's a
+	 * mechanism for environment-based configuration. I cache the
+	 * result in a global variable - yuck! - to avoid repeated
+	 * lookups.
+	 */
+	{
+	    static int swap_buttons = -1;
+	    if (swap_buttons < 0) {
+		char *env = getenv("SLANT_SWAP_BUTTONS");
+		swap_buttons = (env && (env[0] == 'y' || env[0] == 'Y'));
+	    }
+	    if (swap_buttons) {
+		if (button == LEFT_BUTTON)
+		    button = RIGHT_BUTTON;
+		else
+		    button = LEFT_BUTTON;
+	    }
+	}
+        action = (button == LEFT_BUTTON) ? CLOCKWISE : ANTICLOCKWISE;
+
+        x = FROMCOORD(x);
+        y = FROMCOORD(y);
+        if (x < 0 || y < 0 || x >= w || y >= h)
+            return NULL;
+        ui->cur_visible = 0;
+    } else if (IS_CURSOR_SELECT(button)) {
+        if (!ui->cur_visible) {
+            ui->cur_visible = 1;
+            return "";
+        }
+        x = ui->cur_x;
+        y = ui->cur_y;
+
+        action = (button == CURSOR_SELECT2) ? ANTICLOCKWISE : CLOCKWISE;
+    } else if (IS_CURSOR_MOVE(button)) {
+        move_cursor(button, &ui->cur_x, &ui->cur_y, w, h, 0);
+        ui->cur_visible = 1;
+        return "";
+    } else if (button == '\\' || button == '\b' || button == '/') {
+	int x = ui->cur_x, y = ui->cur_y;
+	if (button == ("\\" "\b" "/")[state->soln[y*w + x] + 1]) return NULL;
+	sprintf(buf, "%c%d,%d", button == '\b' ? 'C' : button, x, y);
+	return dupstr(buf);
+    }
+
+    if (action != NONE) {
+        if (action == CLOCKWISE) {
+            /*
+             * Left-clicking cycles blank -> \ -> / -> blank.
+             */
+            v = state->soln[y*w+x] - 1;
+            if (v == -2)
+                v = +1;
+        } else {
+            /*
+             * Right-clicking cycles blank -> / -> \ -> blank.
+             */
+            v = state->soln[y*w+x] + 1;
+            if (v == +2)
+                v = -1;
+        }
+
+        sprintf(buf, "%c%d,%d", (int)(v==-1 ? '\\' : v==+1 ? '/' : 'C'), x, y);
+        return dupstr(buf);
+    }
+
+    return NULL;
+}
+
+static game_state *execute_move(const game_state *state, const char *move)
+{
+    int w = state->p.w, h = state->p.h;
+    char c;
+    int x, y, n;
+    game_state *ret = dup_game(state);
+
+    while (*move) {
+        c = *move;
+	if (c == 'S') {
+	    ret->used_solve = TRUE;
+	    move++;
+	} else if (c == '\\' || c == '/' || c == 'C') {
+            move++;
+            if (sscanf(move, "%d,%d%n", &x, &y, &n) != 2 ||
+                x < 0 || y < 0 || x >= w || y >= h) {
+                free_game(ret);
+                return NULL;
+            }
+            ret->soln[y*w+x] = (c == '\\' ? -1 : c == '/' ? +1 : 0);
+            move += n;
+        } else {
+            free_game(ret);
+            return NULL;
+        }
+        if (*move == ';')
+            move++;
+        else if (*move) {
+            free_game(ret);
+            return NULL;
+        }
+    }
+
+    /*
+     * We never clear the `completed' flag, but we must always
+     * re-run the completion check because it also highlights
+     * errors in the grid.
+     */
+    ret->completed = check_completion(ret) || ret->completed;
+
+    return ret;
+}
+
+/* ----------------------------------------------------------------------
+ * Drawing routines.
+ */
+
+static void game_compute_size(const game_params *params, int tilesize,
+                              int *x, int *y)
+{
+    /* fool the macros */
+    struct dummy { int tilesize; } dummy, *ds = &dummy;
+    dummy.tilesize = tilesize;
+
+    *x = 2 * BORDER + params->w * TILESIZE + 1;
+    *y = 2 * BORDER + params->h * TILESIZE + 1;
+}
+
+static void game_set_size(drawing *dr, game_drawstate *ds,
+                          const game_params *params, int tilesize)
+{
+    ds->tilesize = tilesize;
+}
+
+static float *game_colours(frontend *fe, int *ncolours)
+{
+    float *ret = snewn(3 * NCOLOURS, float);
+
+    /* CURSOR colour is a background highlight. */
+    game_mkhighlight(fe, ret, COL_BACKGROUND, COL_CURSOR, -1);
+
+    ret[COL_FILLEDSQUARE * 3 + 0] = ret[COL_BACKGROUND * 3 + 0];
+    ret[COL_FILLEDSQUARE * 3 + 1] = ret[COL_BACKGROUND * 3 + 1];
+    ret[COL_FILLEDSQUARE * 3 + 2] = ret[COL_BACKGROUND * 3 + 2];
+
+    ret[COL_GRID * 3 + 0] = ret[COL_BACKGROUND * 3 + 0] * 0.7F;
+    ret[COL_GRID * 3 + 1] = ret[COL_BACKGROUND * 3 + 1] * 0.7F;
+    ret[COL_GRID * 3 + 2] = ret[COL_BACKGROUND * 3 + 2] * 0.7F;
+
+    ret[COL_INK * 3 + 0] = 0.0F;
+    ret[COL_INK * 3 + 1] = 0.0F;
+    ret[COL_INK * 3 + 2] = 0.0F;
+
+    ret[COL_SLANT1 * 3 + 0] = 0.0F;
+    ret[COL_SLANT1 * 3 + 1] = 0.0F;
+    ret[COL_SLANT1 * 3 + 2] = 0.0F;
+
+    ret[COL_SLANT2 * 3 + 0] = 0.0F;
+    ret[COL_SLANT2 * 3 + 1] = 0.0F;
+    ret[COL_SLANT2 * 3 + 2] = 0.0F;
+
+    ret[COL_ERROR * 3 + 0] = 1.0F;
+    ret[COL_ERROR * 3 + 1] = 0.0F;
+    ret[COL_ERROR * 3 + 2] = 0.0F;
+
+    *ncolours = NCOLOURS;
+    return ret;
+}
+
+static game_drawstate *game_new_drawstate(drawing *dr, const game_state *state)
+{
+    int w = state->p.w, h = state->p.h;
+    int i;
+    struct game_drawstate *ds = snew(struct game_drawstate);
+
+    ds->tilesize = 0;
+    ds->started = FALSE;
+    ds->grid = snewn((w+2)*(h+2), long);
+    ds->todraw = snewn((w+2)*(h+2), long);
+    for (i = 0; i < (w+2)*(h+2); i++)
+	ds->grid[i] = ds->todraw[i] = -1;
+
+    return ds;
+}
+
+static void game_free_drawstate(drawing *dr, game_drawstate *ds)
+{
+    sfree(ds->todraw);
+    sfree(ds->grid);
+    sfree(ds);
+}
+
+static void draw_clue(drawing *dr, game_drawstate *ds,
+		      int x, int y, long v, long err, int bg, int colour)
+{
+    char p[2];
+    int ccol = colour >= 0 ? colour : ((x ^ y) & 1) ? COL_SLANT1 : COL_SLANT2;
+    int tcol = colour >= 0 ? colour : err ? COL_ERROR : COL_INK;
+
+    if (v < 0)
+	return;
+
+    p[0] = (char)v + '0';
+    p[1] = '\0';
+    draw_circle(dr, COORD(x), COORD(y), CLUE_RADIUS,
+		bg >= 0 ? bg : COL_BACKGROUND, ccol);
+    draw_text(dr, COORD(x), COORD(y), FONT_VARIABLE,
+	      CLUE_TEXTSIZE, ALIGN_VCENTRE|ALIGN_HCENTRE, tcol, p);
+}
+
+static void draw_tile(drawing *dr, game_drawstate *ds, game_clues *clues,
+		      int x, int y, long v)
+{
+    int w = clues->w, h = clues->h, W = w+1 /*, H = h+1 */;
+    int chesscolour = (x ^ y) & 1;
+    int fscol = chesscolour ? COL_SLANT2 : COL_SLANT1;
+    int bscol = chesscolour ? COL_SLANT1 : COL_SLANT2;
+
+    clip(dr, COORD(x), COORD(y), TILESIZE, TILESIZE);
+
+    draw_rect(dr, COORD(x), COORD(y), TILESIZE, TILESIZE,
+	      (v & FLASH) ? COL_GRID :
+              (v & CURSOR) ? COL_CURSOR :
+	      (v & (BACKSLASH | FORWSLASH)) ? COL_FILLEDSQUARE :
+	      COL_BACKGROUND);
+
+    /*
+     * Draw the grid lines.
+     */
+    if (x >= 0 && x < w && y >= 0)
+        draw_rect(dr, COORD(x), COORD(y), TILESIZE+1, 1, COL_GRID);
+    if (x >= 0 && x < w && y < h)
+        draw_rect(dr, COORD(x), COORD(y+1), TILESIZE+1, 1, COL_GRID);
+    if (y >= 0 && y < h && x >= 0)
+        draw_rect(dr, COORD(x), COORD(y), 1, TILESIZE+1, COL_GRID);
+    if (y >= 0 && y < h && x < w)
+        draw_rect(dr, COORD(x+1), COORD(y), 1, TILESIZE+1, COL_GRID);
+    if (x == -1 && y == -1)
+        draw_rect(dr, COORD(x+1), COORD(y+1), 1, 1, COL_GRID);
+    if (x == -1 && y == h)
+        draw_rect(dr, COORD(x+1), COORD(y), 1, 1, COL_GRID);
+    if (x == w && y == -1)
+        draw_rect(dr, COORD(x), COORD(y+1), 1, 1, COL_GRID);
+    if (x == w && y == h)
+        draw_rect(dr, COORD(x), COORD(y), 1, 1, COL_GRID);
+
+    /*
+     * Draw the slash.
+     */
+    if (v & BACKSLASH) {
+        int scol = (v & ERRSLASH) ? COL_ERROR : bscol;
+	draw_line(dr, COORD(x), COORD(y), COORD(x+1), COORD(y+1), scol);
+	draw_line(dr, COORD(x)+1, COORD(y), COORD(x+1), COORD(y+1)-1,
+		  scol);
+	draw_line(dr, COORD(x), COORD(y)+1, COORD(x+1)-1, COORD(y+1),
+		  scol);
+    } else if (v & FORWSLASH) {
+        int scol = (v & ERRSLASH) ? COL_ERROR : fscol;
+	draw_line(dr, COORD(x+1), COORD(y), COORD(x), COORD(y+1), scol);
+	draw_line(dr, COORD(x+1)-1, COORD(y), COORD(x), COORD(y+1)-1,
+		  scol);
+	draw_line(dr, COORD(x+1), COORD(y)+1, COORD(x)+1, COORD(y+1),
+		  scol);
+    }
+
+    /*
+     * Draw dots on the grid corners that appear if a slash is in a
+     * neighbouring cell.
+     */
+    if (v & (L_T | BACKSLASH))
+	draw_rect(dr, COORD(x), COORD(y)+1, 1, 1,
+                  (v & ERR_L_T ? COL_ERROR : bscol));
+    if (v & (L_B | FORWSLASH))
+	draw_rect(dr, COORD(x), COORD(y+1)-1, 1, 1,
+                  (v & ERR_L_B ? COL_ERROR : fscol));
+    if (v & (T_L | BACKSLASH))
+	draw_rect(dr, COORD(x)+1, COORD(y), 1, 1,
+                  (v & ERR_T_L ? COL_ERROR : bscol));
+    if (v & (T_R | FORWSLASH))
+	draw_rect(dr, COORD(x+1)-1, COORD(y), 1, 1,
+                  (v & ERR_T_R ? COL_ERROR : fscol));
+    if (v & (C_TL | BACKSLASH))
+	draw_rect(dr, COORD(x), COORD(y), 1, 1,
+                  (v & ERR_C_TL ? COL_ERROR : bscol));
+
+    /*
+     * And finally the clues at the corners.
+     */
+    if (x >= 0 && y >= 0)
+        draw_clue(dr, ds, x, y, clues->clues[y*W+x], v & ERR_TL, -1, -1);
+    if (x < w && y >= 0)
+        draw_clue(dr, ds, x+1, y, clues->clues[y*W+(x+1)], v & ERR_TR, -1, -1);
+    if (x >= 0 && y < h)
+        draw_clue(dr, ds, x, y+1, clues->clues[(y+1)*W+x], v & ERR_BL, -1, -1);
+    if (x < w && y < h)
+        draw_clue(dr, ds, x+1, y+1, clues->clues[(y+1)*W+(x+1)], v & ERR_BR,
+		  -1, -1);
+
+    unclip(dr);
+    draw_update(dr, COORD(x), COORD(y), TILESIZE, TILESIZE);
+}
+
+static void game_redraw(drawing *dr, game_drawstate *ds,
+                        const game_state *oldstate, const game_state *state,
+                        int dir, const game_ui *ui,
+                        float animtime, float flashtime)
+{
+    int w = state->p.w, h = state->p.h, W = w+1, H = h+1;
+    int x, y;
+    int flashing;
+
+    if (flashtime > 0)
+	flashing = (int)(flashtime * 3 / FLASH_TIME) != 1;
+    else
+	flashing = FALSE;
+
+    if (!ds->started) {
+	int ww, wh;
+	game_compute_size(&state->p, TILESIZE, &ww, &wh);
+	draw_rect(dr, 0, 0, ww, wh, COL_BACKGROUND);
+	draw_update(dr, 0, 0, ww, wh);
+	ds->started = TRUE;
+    }
+
+    /*
+     * Loop over the grid and work out where all the slashes are.
+     * We need to do this because a slash in one square affects the
+     * drawing of the next one along.
+     */
+    for (y = -1; y <= h; y++)
+	for (x = -1; x <= w; x++) {
+            if (x >= 0 && x < w && y >= 0 && y < h)
+                ds->todraw[(y+1)*(w+2)+(x+1)] = flashing ? FLASH : 0;
+            else
+                ds->todraw[(y+1)*(w+2)+(x+1)] = 0;
+        }
+
+    for (y = 0; y < h; y++) {
+	for (x = 0; x < w; x++) {
+            int err = state->errors[y*W+x] & ERR_SQUARE;
+
+	    if (state->soln[y*w+x] < 0) {
+		ds->todraw[(y+1)*(w+2)+(x+1)] |= BACKSLASH;
+                ds->todraw[(y+2)*(w+2)+(x+1)] |= T_R;
+                ds->todraw[(y+1)*(w+2)+(x+2)] |= L_B;
+                ds->todraw[(y+2)*(w+2)+(x+2)] |= C_TL;
+                if (err) {
+                    ds->todraw[(y+1)*(w+2)+(x+1)] |= ERRSLASH | 
+			ERR_T_L | ERR_L_T | ERR_C_TL;
+                    ds->todraw[(y+2)*(w+2)+(x+1)] |= ERR_T_R;
+                    ds->todraw[(y+1)*(w+2)+(x+2)] |= ERR_L_B;
+                    ds->todraw[(y+2)*(w+2)+(x+2)] |= ERR_C_TL;
+                }
+	    } else if (state->soln[y*w+x] > 0) {
+		ds->todraw[(y+1)*(w+2)+(x+1)] |= FORWSLASH;
+                ds->todraw[(y+1)*(w+2)+(x+2)] |= L_T | C_TL;
+                ds->todraw[(y+2)*(w+2)+(x+1)] |= T_L | C_TL;
+                if (err) {
+                    ds->todraw[(y+1)*(w+2)+(x+1)] |= ERRSLASH |
+			ERR_L_B | ERR_T_R;
+                    ds->todraw[(y+1)*(w+2)+(x+2)] |= ERR_L_T | ERR_C_TL;
+                    ds->todraw[(y+2)*(w+2)+(x+1)] |= ERR_T_L | ERR_C_TL;
+                }
+	    }
+            if (ui->cur_visible && ui->cur_x == x && ui->cur_y == y)
+                ds->todraw[(y+1)*(w+2)+(x+1)] |= CURSOR;
+	}
+    }
+
+    for (y = 0; y < H; y++)
+        for (x = 0; x < W; x++)
+            if (state->errors[y*W+x] & ERR_VERTEX) {
+                ds->todraw[y*(w+2)+x] |= ERR_BR;
+                ds->todraw[y*(w+2)+(x+1)] |= ERR_BL;
+                ds->todraw[(y+1)*(w+2)+x] |= ERR_TR;
+                ds->todraw[(y+1)*(w+2)+(x+1)] |= ERR_TL;
+            }
+
+    /*
+     * Now go through and draw the grid squares.
+     */
+    for (y = -1; y <= h; y++) {
+	for (x = -1; x <= w; x++) {
+	    if (ds->todraw[(y+1)*(w+2)+(x+1)] != ds->grid[(y+1)*(w+2)+(x+1)]) {
+		draw_tile(dr, ds, state->clues, x, y,
+                          ds->todraw[(y+1)*(w+2)+(x+1)]);
+		ds->grid[(y+1)*(w+2)+(x+1)] = ds->todraw[(y+1)*(w+2)+(x+1)];
+	    }
+	}
+    }
+}
+
+static float game_anim_length(const game_state *oldstate,
+                              const game_state *newstate, int dir, game_ui *ui)
+{
+    return 0.0F;
+}
+
+static float game_flash_length(const game_state *oldstate,
+                               const game_state *newstate, int dir, game_ui *ui)
+{
+    if (!oldstate->completed && newstate->completed &&
+	!oldstate->used_solve && !newstate->used_solve)
+        return FLASH_TIME;
+
+    return 0.0F;
+}
+
+static int game_status(const game_state *state)
+{
+    return state->completed ? +1 : 0;
+}
+
+static int game_timing_state(const game_state *state, game_ui *ui)
+{
+    return TRUE;
+}
+
+static void game_print_size(const game_params *params, float *x, float *y)
+{
+    int pw, ph;
+
+    /*
+     * I'll use 6mm squares by default.
+     */
+    game_compute_size(params, 600, &pw, &ph);
+    *x = pw / 100.0F;
+    *y = ph / 100.0F;
+}
+
+static void game_print(drawing *dr, const game_state *state, int tilesize)
+{
+    int w = state->p.w, h = state->p.h, W = w+1;
+    int ink = print_mono_colour(dr, 0);
+    int paper = print_mono_colour(dr, 1);
+    int x, y;
+
+    /* Ick: fake up `ds->tilesize' for macro expansion purposes */
+    game_drawstate ads, *ds = &ads;
+    game_set_size(dr, ds, NULL, tilesize);
+
+    /*
+     * Border.
+     */
+    print_line_width(dr, TILESIZE / 16);
+    draw_rect_outline(dr, COORD(0), COORD(0), w*TILESIZE, h*TILESIZE, ink);
+
+    /*
+     * Grid.
+     */
+    print_line_width(dr, TILESIZE / 24);
+    for (x = 1; x < w; x++)
+	draw_line(dr, COORD(x), COORD(0), COORD(x), COORD(h), ink);
+    for (y = 1; y < h; y++)
+	draw_line(dr, COORD(0), COORD(y), COORD(w), COORD(y), ink);
+
+    /*
+     * Solution.
+     */
+    print_line_width(dr, TILESIZE / 12);
+    for (y = 0; y < h; y++)
+	for (x = 0; x < w; x++)
+	    if (state->soln[y*w+x]) {
+		int ly, ry;
+		/*
+		 * To prevent nasty line-ending artefacts at
+		 * corners, I'll do something slightly cunning
+		 * here.
+		 */
+		clip(dr, COORD(x), COORD(y), TILESIZE, TILESIZE);
+		if (state->soln[y*w+x] < 0)
+		    ly = y-1, ry = y+2;
+		else
+		    ry = y-1, ly = y+2;
+		draw_line(dr, COORD(x-1), COORD(ly), COORD(x+2), COORD(ry),
+			  ink);
+		unclip(dr);
+	    }
+
+    /*
+     * Clues.
+     */
+    print_line_width(dr, TILESIZE / 24);
+    for (y = 0; y <= h; y++)
+	for (x = 0; x <= w; x++)
+	    draw_clue(dr, ds, x, y, state->clues->clues[y*W+x],
+		      FALSE, paper, ink);
+}
+
+#ifdef COMBINED
+#define thegame slant
+#endif
+
+const struct game thegame = {
+    "Slant", "games.slant", "slant",
+    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,
+    TRUE, solve_game,
+    TRUE, game_can_format_as_text_now, game_text_format,
+    new_ui,
+    free_ui,
+    encode_ui,
+    decode_ui,
+    game_changed_state,
+    interpret_move,
+    execute_move,
+    PREFERRED_TILESIZE, game_compute_size, game_set_size,
+    game_colours,
+    game_new_drawstate,
+    game_free_drawstate,
+    game_redraw,
+    game_anim_length,
+    game_flash_length,
+    game_status,
+    TRUE, FALSE, game_print_size, game_print,
+    FALSE,			       /* wants_statusbar */
+    FALSE, game_timing_state,
+    0,				       /* flags */
+};
+
+#ifdef STANDALONE_SOLVER
+
+#include <stdarg.h>
+
+int main(int argc, char **argv)
+{
+    game_params *p;
+    game_state *s;
+    char *id = NULL, *desc, *err;
+    int grade = FALSE;
+    int ret, diff, really_verbose = FALSE;
+    struct solver_scratch *sc;
+
+    while (--argc > 0) {
+        char *p = *++argv;
+        if (!strcmp(p, "-v")) {
+            really_verbose = TRUE;
+        } else if (!strcmp(p, "-g")) {
+            grade = TRUE;
+        } else if (*p == '-') {
+            fprintf(stderr, "%s: unrecognised option `%s'\n", argv[0], p);
+            return 1;
+        } else {
+            id = p;
+        }
+    }
+
+    if (!id) {
+        fprintf(stderr, "usage: %s [-g | -v] <game_id>\n", argv[0]);
+        return 1;
+    }
+
+    desc = strchr(id, ':');
+    if (!desc) {
+        fprintf(stderr, "%s: game id expects a colon in it\n", argv[0]);
+        return 1;
+    }
+    *desc++ = '\0';
+
+    p = default_params();
+    decode_params(p, id);
+    err = validate_desc(p, desc);
+    if (err) {
+        fprintf(stderr, "%s: %s\n", argv[0], err);
+        return 1;
+    }
+    s = new_game(NULL, p, desc);
+
+    sc = new_scratch(p->w, p->h);
+
+    /*
+     * When solving an Easy puzzle, we don't want to bother the
+     * user with Hard-level deductions. For this reason, we grade
+     * the puzzle internally before doing anything else.
+     */
+    ret = -1;			       /* placate optimiser */
+    for (diff = 0; diff < DIFFCOUNT; diff++) {
+	ret = slant_solve(p->w, p->h, s->clues->clues,
+			  s->soln, sc, diff);
+	if (ret < 2)
+	    break;
+    }
+
+    if (diff == DIFFCOUNT) {
+	if (grade)
+	    printf("Difficulty rating: harder than Hard, or ambiguous\n");
+	else
+	    printf("Unable to find a unique solution\n");
+    } else {
+	if (grade) {
+	    if (ret == 0)
+		printf("Difficulty rating: impossible (no solution exists)\n");
+	    else if (ret == 1)
+		printf("Difficulty rating: %s\n", slant_diffnames[diff]);
+	} else {
+	    verbose = really_verbose;
+	    ret = slant_solve(p->w, p->h, s->clues->clues,
+			      s->soln, sc, diff);
+	    if (ret == 0)
+		printf("Puzzle is inconsistent\n");
+	    else
+		fputs(game_text_format(s), stdout);
+	}
+    }
+
+    return 0;
+}
+
+#endif
+
+/* vim: set shiftwidth=4 tabstop=8: */