shithub: puzzles

Download patch

ref: 7e4f870f6d825fb750b45cacfca78c8de4f9d5da
parent: b0ad387d0567089bb7b24a15e3ea6a6910c13a19
author: Simon Tatham <anakin@pobox.com>
date: Mon Feb 15 16:09:09 EST 2010

New puzzle! Setting what might be a record for how long we've sat on
a puzzle before it was ready to commit, here is 'Signpost': a clone
of janko.at's "Arrow Path", by James Harvey.

[originally from svn r8861]

--- a/icons/Makefile
+++ b/icons/Makefile
@@ -2,8 +2,8 @@
 
 PUZZLES = blackbox bridges cube dominosa fifteen filling flip galaxies guess \
 	  inertia keen lightup loopy magnets map mines net netslide pattern \
-	  pegs rect samegame singles sixteen slant solo tents towers twiddle \
-	  unequal untangle
+	  pegs rect samegame signpost singles sixteen slant solo tents towers \
+	  twiddle unequal untangle
 
 BASE = $(patsubst %,%-base.png,$(PUZZLES))
 WEB = $(patsubst %,%-web.png,$(PUZZLES))
@@ -71,6 +71,7 @@
 pattern-ibase.png : override CROP=384x384 223x223+0+0
 pegs-ibase.png : override CROP=263x263 147x147+116+0
 rect-ibase.png : override CROP=205x205 115x115+90+0
+signpost-ibase.png : override CROP=240x240 98x98+23+23
 singles-ibase.png : override CROP=224x224 98x98+15+15
 sixteen-ibase.png : override CROP=288x288 144x144+144+144
 slant-ibase.png : override CROP=321x321 160x160+160+160
--- /dev/null
+++ b/icons/signpost.sav
@@ -1,0 +1,23 @@
+SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
+VERSION :1:1
+GAME    :8:Signpost
+PARAMS  :4:4x4c
+CPARAMS :4:4x4c
+SEED    :15:230468784719861
+DESC    :19:1eceebecfbfhgcaa16a
+NSTATES :2:15
+STATEPOS:2:11
+MOVE    :8:L2,1-3,1
+MOVE    :8:L0,1-1,0
+MOVE    :8:L2,2-1,1
+MOVE    :8:L1,2-0,3
+MOVE    :8:L0,2-2,0
+MOVE    :8:L1,3-1,2
+MOVE    :8:L1,1-1,3
+MOVE    :8:L1,0-3,0
+MOVE    :8:L0,0-0,1
+MOVE    :8:L3,0-3,2
+MOVE    :8:L3,2-0,2
+MOVE    :8:L3,1-2,2
+MOVE    :8:L2,3-2,1
+MOVE    :8:L2,0-2,3
--- a/puzzles.but
+++ b/puzzles.but
@@ -2774,6 +2774,86 @@
 time, making the puzzle more difficult.
 
 
+\C{signpost} \i{Signpost}
+
+\cfg{winhelp-topic}{games.signpost}
+
+You have a grid of squares; each square (except the last one)
+contains an arrow, and some squares also contain numbers. Your job
+is to connect the squares to form a continuous list of numbers
+starting at 1 and linked in the direction of the arrows \dash so the
+arrow inside the square with the number 1 will point to the square
+containing the number 2, which will point to the square containing
+the number 3, etc. Each square can be any distance away from the
+previous one, as long as it is somewhere in the direction of the
+arrow.
+
+By convention the first and last numbers are shown; one or more
+interim numbers may also appear at the beginning. 
+
+Credit for this puzzle goes to \i{Janko} \k{janko-arrowpath}, who call it
+\q{Pfeilpfad} (\q{arrow path}).
+
+Signpost was contributed to this collection by James Harvey.
+
+\B{janko-arrowpath}
+\W{http://janko.at/Raetsel/Pfeilpfad/index.htm}\cw{http://janko.at/Raetsel/Pfeilpfad/index.htm}
+
+\H{signpost-controls} \I{controls, for Signpost}Signpost controls
+
+To play Signpost, you connect squares together by dragging from one
+square to another, indicating that they are adjacent in the
+sequence. Drag with the left button from a square to its successor,
+or with the right button from a square to its predecessor.
+
+If you connect together two squares in this way and one of them has
+a number in it, the appropriate number will appear in the other
+square. If you connect two non-numbered squares, they will be
+assigned temporary algebraic labels: on the first occasion, they
+will be labelled \cq{a} and \cq{a+1}, and then \cq{b} and \cq{b+1},
+and so on. Connecting more squares on to the ends of such a chain
+will cause them all to be labelled with the same letter.
+
+When you left-click or right-click in a square, the legal squares to
+connect it to will be shown.
+
+The arrow in each square starts off black, and goes grey once you
+connect the square to its successor. Also, each square which needs a
+predecessor has a small dot in the bottom left corner, which
+vanishes once you link a square to it. So your aim is always to
+connect a square with a black arrow to a square with a dot.
+
+To remove any links for a particular square (both incoming and
+outgoing), left-drag it off the grid. To remove a whole chain,
+right-drag any square in the chain off the grid.
+
+You can also use the cursor keys to move around the grid squares and
+lines. Pressing the return key when over a square starts a link
+operation, and pressing the return key again over a square will
+finish the link, if allowable. Pressing the space bar over a square
+will show the other squares pointing to it, and allow you to form a
+backward link, and pressing the space bar again cancels this.
+
+(All the actions described in \k{common-actions} are also available.)
+
+\H{signpost-parameters} \I{parameters, for Signpost}Signpost parameters
+
+These parameters are available from the \q{Custom...} option on the
+\q{Type} menu.
+
+\dt \e{Width}, \e{Height}
+
+\dd Size of grid in squares. There will be half \e{Width} \by \e{Height}
+dominoes in the grid: if this number is odd then one square will be blank.
+
+(Grids with at least one odd dimension tend to be easier to solve.)
+
+\dt \e{Force start/end to corners}
+
+\dd If true, the start and end squares are always placed in opposite corners
+(the start at the top left, and the end at the bottom right). If false the start
+and end squares are placed randomly (although always both shown). 
+
 
 \A{licence} \I{MIT licence}\ii{Licence}
 
--- /dev/null
+++ b/signpost.R
@@ -1,0 +1,23 @@
+# -*- makefile -*-
+
+SIGNPOST_EXTRA = dsf
+
+signpost : [X] GTK COMMON signpost SIGNPOST_EXTRA signpost-icon|no-icon
+signpost : [G] WINDOWS COMMON signpost SIGNPOST_EXTRA signpost.res|noicon.res
+
+signpostsolver : [U] signpost[STANDALONE_SOLVER] SIGNPOST_EXTRA STANDALONE m.lib
+signpostsolver : [C] signpost[STANDALONE_SOLVER] SIGNPOST_EXTRA STANDALONE
+
+ALL += signpost[COMBINED] SIGNPOST_EXTRA
+
+!begin gtk
+GAMES += signpost
+!end
+
+!begin >list.c
+    A(signpost) \
+!end
+
+!begin >wingames.lst
+signpost.exe:Signpost
+!end
--- /dev/null
+++ b/signpost.c
@@ -1,0 +1,2333 @@
+/*
+ * signpost.c: implementation of the janko game 'arrow path'
+ *
+ * Remaining troublesome games:
+ *
+ * 4x4#438520052525454
+ */
+
+#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 BLITTER_SIZE TILE_SIZE
+#define BORDER    (TILE_SIZE / 2)
+
+#define COORD(x)  ( (x) * TILE_SIZE + BORDER )
+#define FROMCOORD(x)  ( ((x) - BORDER + TILE_SIZE) / TILE_SIZE - 1 )
+
+#define INGRID(s,x,y) ((x) >= 0 && (x) < (s)->w && (y) >= 0 && (y) < (s)->h)
+
+#define FLASH_SPIN 0.7F
+
+#define NBACKGROUNDS 16
+
+enum {
+    COL_BACKGROUND, COL_HIGHLIGHT, COL_LOWLIGHT,
+    COL_GRID, COL_CURSOR, COL_ERROR, COL_DRAG_ORIGIN,
+    COL_ARROW, COL_ARROW_BG_DIM,
+    COL_NUMBER, COL_NUMBER_SET, COL_NUMBER_SET_MID,
+    COL_B0,                             /* background colours */
+    COL_M0 =   COL_B0 + 1*NBACKGROUNDS, /* mid arrow colours */
+    COL_D0 =   COL_B0 + 2*NBACKGROUNDS, /* dim arrow colours */
+    COL_X0 =   COL_B0 + 3*NBACKGROUNDS, /* dim arrow colours */
+    NCOLOURS = COL_B0 + 4*NBACKGROUNDS
+};
+
+struct game_params {
+    int w, h;
+    int force_corner_start;
+};
+
+enum { DIR_N = 0, DIR_NE, DIR_E, DIR_SE, DIR_S, DIR_SW, DIR_W, DIR_NW, DIR_MAX };
+static const char *dirstrings[8] = { "N ", "NE", "E ", "SE", "S ", "SW", "W ", "NW" };
+
+static const int dxs[DIR_MAX] = {  0,  1, 1, 1, 0, -1, -1, -1 };
+static const int dys[DIR_MAX] = { -1, -1, 0, 1, 1,  1,  0, -1 };
+
+#define DIR_OPPOSITE(d) ((d+4)%8)
+
+struct game_state {
+    int w, h, n;
+    int completed, used_solve, impossible;
+    int *dirs;                  /* direction enums, size n */
+    int *nums;                  /* numbers, size n */
+    unsigned int *flags;        /* flags, size n */
+    int *next, *prev;           /* links to other cell indexes, size n (-1 absent) */
+    int *dsf;                   /* connects regions with a dsf. */
+    int *numsi;                 /* for each number, which index is it in? (-1 absent) */
+};
+
+#define FLAG_IMMUTABLE  1
+#define FLAG_ERROR      2
+
+/* --- Generally useful functions --- */
+
+#define ISREALNUM(state, num) ((num) > 0 && (num) <= (state)->n)
+
+static int whichdir(int fromx, int fromy, int tox, int toy)
+{
+    int i, dx, dy;
+
+    dx = tox - fromx;
+    dy = toy - fromy;
+
+    if (dx && dy && abs(dx) != abs(dy)) return -1;
+
+    if (dx) dx = dx / abs(dx); /* limit to (-1, 0, 1) */
+    if (dy) dy = dy / abs(dy); /* ditto */
+
+    for (i = 0; i < DIR_MAX; i++) {
+        if (dx == dxs[i] && dy == dys[i]) return i;
+    }
+    return -1;
+}
+
+static int whichdiri(game_state *state, int fromi, int toi)
+{
+    int w = state->w;
+    return whichdir(fromi%w, fromi/w, toi%w, toi/w);
+}
+
+static int ispointing(game_state *state, int fromx, int fromy, int tox, int toy)
+{
+    int w = state->w, dir = state->dirs[fromy*w+fromx];
+
+    /* (by convention) squares do not point to themselves. */
+    if (fromx == tox && fromy == toy) return 0;
+
+    /* the final number points to nothing. */
+    if (state->nums[fromy*w + fromx] == state->n) return 0;
+
+    while (1) {
+        if (!INGRID(state, fromx, fromy)) return 0;
+        if (fromx == tox && fromy == toy) return 1;
+        fromx += dxs[dir]; fromy += dys[dir];
+    }
+    return 0; /* not reached */
+}
+
+static int ispointingi(game_state *state, int fromi, int toi)
+{
+    int w = state->w;
+    return ispointing(state, fromi%w, fromi/w, toi%w, toi/w);
+}
+
+/* Taking the number 'num', work out the gap between it and the next
+ * available number up or down (depending on d). Return 1 if the region
+ * at (x,y) will fit in that gap, or 0 otherwise. */
+static int move_couldfit(game_state *state, int num, int d, int x, int y)
+{
+    int n, gap, i = y*state->w+x, sz;
+
+    assert(d != 0);
+    /* The 'gap' is the number of missing numbers in the grid between
+     * our number and the next one in the sequence (up or down), or
+     * the end of the sequence (if we happen not to have 1/n present) */
+    for (n = num + d, gap = 0;
+         ISREALNUM(state, n) && state->numsi[n] == -1;
+         n += d, gap++) ; /* empty loop */
+
+    if (gap == 0) {
+        /* no gap, so the only allowable move is that that directly
+         * links the two numbers. */
+        n = state->nums[i];
+        return (n == num+d) ? 0 : 1;
+    }
+    if (state->prev[i] == -1 && state->next[i] == -1)
+        return 1; /* single unconnected square, always OK */
+
+    sz = dsf_size(state->dsf, i);
+    return (sz > gap) ? 0 : 1;
+}
+
+static int isvalidmove(game_state *state, int clever,
+                       int fromx, int fromy, int tox, int toy)
+{
+    int w = state->w, from = fromy*w+fromx, to = toy*w+tox;
+    int nfrom, nto;
+
+    if (!INGRID(state, fromx, fromy) || !INGRID(state, tox, toy))
+        return 0;
+
+    /* can only move where we point */
+    if (!ispointing(state, fromx, fromy, tox, toy))
+        return 0;
+
+    nfrom = state->nums[from]; nto = state->nums[to];
+
+    /* can't move _from_ the final number, or _to_ the 1. */
+    if (nfrom == state->n || nto == 1)
+        return 0;
+
+    /* can't create a new connection between cells in the same region
+     * as that would create a loop. */
+    if (dsf_canonify(state->dsf, from) == dsf_canonify(state->dsf, to))
+        return 0;
+
+    /* if both cells are actual numbers, can't drag if we're not
+     * one digit apart. */
+    if (ISREALNUM(state, nfrom) && ISREALNUM(state, nto)) {
+        if (nfrom != nto-1)
+            return 0;
+    } else if (clever && ISREALNUM(state, nfrom)) {
+        if (!move_couldfit(state, nfrom, +1, tox, toy))
+            return 0;
+    } else if (clever && ISREALNUM(state, nto)) {
+        if (!move_couldfit(state, nto, -1, fromx, fromy))
+            return 0;
+    }
+
+    return 1;
+}
+
+static void makelink(game_state *state, int from, int to)
+{
+    if (state->next[from] != -1)
+        state->prev[state->next[from]] = -1;
+    state->next[from] = to;
+
+    if (state->prev[to] != -1)
+        state->next[state->prev[to]] = -1;
+    state->prev[to] = from;
+}
+
+static int game_can_format_as_text_now(game_params *params)
+{
+    if (params->w * params->h >= 100) return 0;
+    return 1;
+}
+
+static char *game_text_format(game_state *state)
+{
+    int len = state->h * 2 * (4*state->w + 1) + state->h + 2;
+    int x, y, i, num, n, set;
+    char *ret, *p;
+
+    p = ret = snewn(len, char);
+
+    for (y = 0; y < state->h; y++) {
+        for (x = 0; x < state->h; x++) {
+            i = y*state->w+x;
+            *p++ = dirstrings[state->dirs[i]][0];
+            *p++ = dirstrings[state->dirs[i]][1];
+            *p++ = (state->flags[i] & FLAG_IMMUTABLE) ? 'I' : ' ';
+            *p++ = ' ';
+        }
+        *p++ = '\n';
+        for (x = 0; x < state->h; x++) {
+            i = y*state->w+x;
+            num = state->nums[i];
+            if (num == 0) {
+                *p++ = ' ';
+                *p++ = ' ';
+                *p++ = ' ';
+            } else {
+                n = num % (state->n+1);
+                set = num / (state->n+1);
+
+                assert(n <= 99); /* two digits only! */
+
+                if (set != 0)
+                    *p++ = set+'a'-1;
+
+                *p++ = (n >= 10) ? ('0' + (n/10)) : ' ';
+                *p++ = '0' + (n%10);
+
+                if (set == 0)
+                    *p++ = ' ';
+            }
+            *p++ = ' ';
+        }
+        *p++ = '\n';
+        *p++ = '\n';
+    }
+    *p++ = '\0';
+
+    return ret;
+}
+
+static void debug_state(const char *desc, game_state *state)
+{
+#ifdef DEBUGGING
+    char *dbg;
+    if (state->n >= 100) {
+        debug(("[ no game_text_format for this size ]"));
+        return;
+    }
+    dbg = game_text_format(state);
+    debug(("%s\n%s", desc, dbg));
+    sfree(dbg);
+#endif
+}
+
+
+static void strip_nums(game_state *state) {
+    int i;
+    for (i = 0; i < state->n; i++) {
+        if (!(state->flags[i] & FLAG_IMMUTABLE))
+            state->nums[i] = 0;
+    }
+    memset(state->next, -1, state->n*sizeof(int));
+    memset(state->prev, -1, state->n*sizeof(int));
+    memset(state->numsi, -1, (state->n+1)*sizeof(int));
+    dsf_init(state->dsf, state->n);
+}
+
+static int check_nums(game_state *orig, game_state *copy, int only_immutable)
+{
+    int i, ret = 1;
+    assert(copy->n == orig->n);
+    for (i = 0; i < copy->n; i++) {
+        if (only_immutable && !copy->flags[i] & FLAG_IMMUTABLE) continue;
+        assert(copy->nums[i] >= 0);
+        assert(copy->nums[i] <= copy->n);
+        if (copy->nums[i] != orig->nums[i]) {
+            debug(("check_nums: (%d,%d) copy=%d, orig=%d.",
+                   i%orig->w, i/orig->w, copy->nums[i], orig->nums[i]));
+            ret = 0;
+        }
+    }
+    return ret;
+}
+
+/* --- Game parameter/presets functions --- */
+
+static game_params *default_params(void)
+{
+    game_params *ret = snew(game_params);
+    ret->w = ret->h = 4;
+    ret->force_corner_start = 1;
+
+    return ret;
+}
+
+static const struct game_params signpost_presets[] = {
+  { 4, 4, 1 },
+  { 4, 4, 0 },
+  { 5, 5, 1 },
+  { 5, 5, 0 },
+  { 6, 6, 1 },
+  { 7, 7, 1 }
+};
+
+static int game_fetch_preset(int i, char **name, game_params **params)
+{
+    game_params *ret;
+    char buf[80];
+
+    if (i < 0 || i >= lenof(signpost_presets))
+        return FALSE;
+
+    ret = default_params();
+    *ret = signpost_presets[i];
+    *params = ret;
+
+    sprintf(buf, "%dx%d%s", ret->w, ret->h,
+            ret->force_corner_start ? "" : ", free ends");
+    *name = dupstr(buf);
+
+    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);
+    while (*string && isdigit((unsigned char)*string)) string++;
+    if (*string == 'x') {
+        string++;
+        ret->h = atoi(string);
+        while (*string && isdigit((unsigned char)*string)) string++;
+    }
+    ret->force_corner_start = 0;
+    if (*string == 'c') {
+        string++;
+        ret->force_corner_start = 1;
+    }
+
+}
+
+static char *encode_params(game_params *params, int full)
+{
+    char data[256];
+
+    if (full)
+        sprintf(data, "%dx%d%s", params->w, params->h,
+                params->force_corner_start ? "c" : "");
+    else
+        sprintf(data, "%dx%d", params->w, params->h);
+
+    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 = "Start and end in corners";
+    ret[2].type = C_BOOLEAN;
+    ret[2].sval = NULL;
+    ret[2].ival = params->force_corner_start;
+
+    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->force_corner_start = cfg[2].ival;
+
+    return ret;
+}
+
+static char *validate_params(game_params *params, int full)
+{
+    if (params->w < 2 || params->h < 2)
+	return "Width and height must both be at least two";
+
+    return NULL;
+}
+
+/* --- Game description string generation and unpicking --- */
+
+static void blank_game_into(game_state *state)
+{
+    memset(state->dirs, 0, state->n*sizeof(int));
+    memset(state->nums, 0, state->n*sizeof(int));
+    memset(state->flags, 0, state->n*sizeof(unsigned int));
+    memset(state->next, -1, state->n*sizeof(int));
+    memset(state->prev, -1, state->n*sizeof(int));
+    memset(state->numsi, -1, (state->n+1)*sizeof(int));
+}
+
+static game_state *blank_game(int w, int h)
+{
+    game_state *state = snew(game_state);
+
+    memset(state, 0, sizeof(game_state));
+    state->w = w;
+    state->h = h;
+    state->n = w*h;
+
+    state->dirs  = snewn(state->n, int);
+    state->nums  = snewn(state->n, int);
+    state->flags = snewn(state->n, unsigned int);
+    state->next  = snewn(state->n, int);
+    state->prev  = snewn(state->n, int);
+    state->dsf = snew_dsf(state->n);
+    state->numsi  = snewn(state->n+1, int);
+
+    blank_game_into(state);
+
+    return state;
+}
+
+static void dup_game_to(game_state *to, game_state *from)
+{
+    to->completed = from->completed;
+    to->used_solve = from->used_solve;
+    to->impossible = from->impossible;
+
+    memcpy(to->dirs, from->dirs, to->n*sizeof(int));
+    memcpy(to->flags, from->flags, to->n*sizeof(unsigned int));
+    memcpy(to->nums, from->nums, to->n*sizeof(int));
+
+    memcpy(to->next, from->next, to->n*sizeof(int));
+    memcpy(to->prev, from->prev, to->n*sizeof(int));
+
+    memcpy(to->dsf, from->dsf, to->n*sizeof(int));
+    memcpy(to->numsi, from->numsi, (to->n+1)*sizeof(int));
+}
+
+static game_state *dup_game(game_state *state)
+{
+    game_state *ret = blank_game(state->w, state->h);
+    dup_game_to(ret, state);
+    return ret;
+}
+
+static void free_game(game_state *state)
+{
+    sfree(state->dirs);
+    sfree(state->nums);
+    sfree(state->flags);
+    sfree(state->next);
+    sfree(state->prev);
+    sfree(state->dsf);
+    sfree(state->numsi);
+    sfree(state);
+}
+
+static void unpick_desc(game_params *params, char *desc,
+                        game_state **sout, char **mout)
+{
+    game_state *state = blank_game(params->w, params->h);
+    char *msg = NULL, c;
+    int num = 0, i = 0;
+
+    while (*desc) {
+        if (i >= state->n) {
+            msg = "Game description longer than expected";
+            goto done;
+        }
+
+        c = *desc;
+        if (isdigit(c)) {
+            num = (num*10) + (int)(c-'0');
+            if (num > state->n) {
+                msg = "Number too large";
+                goto done;
+            }
+        } else if ((c-'a') >= 0 && (c-'a') < DIR_MAX) {
+            state->nums[i] = num;
+            state->flags[i] = num ? FLAG_IMMUTABLE : 0;
+            num = 0;
+
+            state->dirs[i] = c - 'a';
+            i++;
+        } else if (!*desc) {
+            msg = "Game description shorter than expected";
+            goto done;
+        } else {
+            msg = "Game description contains unexpected characters";
+            goto done;
+        }
+        desc++;
+    }
+    if (i < state->n) {
+        msg = "Game description shorter than expected";
+        goto done;
+    }
+
+done:
+    if (msg) { /* sth went wrong. */
+        if (mout) *mout = msg;
+        free_game(state);
+    } else {
+        if (mout) *mout = NULL;
+        if (sout) *sout = state;
+        else free_game(state);
+    }
+}
+
+static char *generate_desc(game_state *state, int issolve)
+{
+    char *ret, buf[80];
+    int retlen, i, k;
+
+    ret = NULL; retlen = 0;
+    if (issolve) {
+        ret = sresize(ret, 2, char);
+        ret[0] = 'S'; ret[1] = '\0';
+        retlen += 1;
+    }
+    for (i = 0; i < state->n; i++) {
+        if (state->nums[i])
+            k = sprintf(buf, "%d%c", state->nums[i], (int)(state->dirs[i]+'a'));
+        else
+            k = sprintf(buf, "%c", (int)(state->dirs[i]+'a'));
+        ret = sresize(ret, retlen + k + 1, char);
+        strcpy(ret + retlen, buf);
+        retlen += k;
+    }
+    return ret;
+}
+
+/* --- Game generation --- */
+
+/* Fills in preallocated arrays ai (indices) and ad (directions)
+ * showing all non-numbered cells adjacent to index i, returns length */
+/* This function has been somewhat optimised... */
+static int cell_adj(game_state *state, int i, int *ai, int *ad)
+{
+    int n = 0, a, x, y, sx, sy, dx, dy, newi;
+    int w = state->w, h = state->h;
+
+    sx = i % w; sy = i / w;
+
+    for (a = 0; a < DIR_MAX; a++) {
+        x = sx; y = sy;
+        dx = dxs[a]; dy = dys[a];
+        while (1) {
+            x += dx; y += dy;
+            if (x < 0 || y < 0 || x >= w || y >= h) break;
+
+            newi = y*w + x;
+            if (state->nums[newi] == 0) {
+                ai[n] = newi;
+                ad[n] = a;
+                n++;
+            }
+        }
+    }
+    return n;
+}
+
+static int new_game_fill(game_state *state, random_state *rs,
+                         int headi, int taili)
+{
+    int nfilled, an, ret = 0, j;
+    int *aidx, *adir;
+
+    aidx = snewn(state->n, int);
+    adir = snewn(state->n, int);
+
+    debug(("new_game_fill: headi=%d, taili=%d.", headi, taili));
+
+    memset(state->nums, 0, state->n*sizeof(int));
+
+    state->nums[headi] = 1;
+    state->nums[taili] = state->n;
+
+    state->dirs[taili] = 0;
+    nfilled = 2;
+
+    while (nfilled < state->n) {
+        /* Try and expand _from_ headi; keep going if there's only one
+         * place to go to. */
+        an = cell_adj(state, headi, aidx, adir);
+        do {
+            if (an == 0) goto done;
+            j = random_upto(rs, an);
+            state->dirs[headi] = adir[j];
+            state->nums[aidx[j]] = state->nums[headi] + 1;
+            nfilled++;
+            headi = aidx[j];
+            an = cell_adj(state, headi, aidx, adir);
+        } while (an == 1);
+
+        /* Try and expand _to_ taili; keep going if there's only one
+         * place to go to. */
+        an = cell_adj(state, taili, aidx, adir);
+        do {
+            if (an == 0) goto done;
+            j = random_upto(rs, an);
+            state->dirs[aidx[j]] = DIR_OPPOSITE(adir[j]);
+            state->nums[aidx[j]] = state->nums[taili] - 1;
+            nfilled++;
+            taili = aidx[j];
+            an = cell_adj(state, taili, aidx, adir);
+        } while (an == 1);
+    }
+    /* If we get here we have headi and taili set but unconnected
+     * by direction: we need to set headi's direction so as to point
+     * at taili. */
+    state->dirs[headi] = whichdiri(state, headi, taili);
+
+    /* it could happen that our last two weren't in line; if that's the
+     * case, we have to start again. */
+    if (state->dirs[headi] != -1) ret = 1;
+
+done:
+    sfree(aidx);
+    sfree(adir);
+    return ret;
+}
+
+/* Better generator: with the 'generate, sprinkle numbers, solve,
+ * repeat' algorithm we're _never_ generating anything greater than
+ * 6x6, and spending all of our time in new_game_fill (and very little
+ * in solve_state).
+ *
+ * So, new generator steps:
+   * generate the grid, at random (same as now). Numbers 1 and N get
+      immutable flag immediately.
+   * squirrel that away for the solved state.
+   *
+   * (solve:) Try and solve it.
+   * If we solved it, we're done:
+     * generate the description from current immutable numbers,
+     * free stuff that needs freeing,
+     * return description + solved state.
+   * If we didn't solve it:
+     * count #tiles in state we've made deductions about.
+     * while (1):
+       * randomise a scratch array.
+       * for each index in scratch (in turn):
+         * if the cell isn't empty, continue (through scratch array)
+         * set number + immutable in state.
+         * try and solve state.
+         * if we've solved it, we're done.
+         * otherwise, count #tiles. If it's more than we had before:
+           * good, break from this loop and re-randomise.
+         * otherwise (number didn't help):
+           * remove number and try next in scratch array.
+       * if we've got to the end of the scratch array, no luck:
+          free everything we need to, and go back to regenerate the grid.
+   */
+
+static int solve_state(game_state *state);
+
+static void debug_desc(const char *what, game_state *state)
+{
+#if DEBUGGING
+    {
+        char *desc = generate_desc(state, 0);
+        debug(("%s game state: %dx%d:%s", what, state->w, state->h, desc));
+        sfree(desc);
+    }
+#endif
+}
+
+/* Expects a fully-numbered game_state on input, and makes sure
+ * FLAG_IMMUTABLE is only set on those numbers we need to solve
+ * (as for a real new-game); returns 1 if it managed
+ * this (such that it could solve it), or 0 if not. */
+static int new_game_strip(game_state *state, random_state *rs)
+{
+    int *scratch, i, j, ret = 1;
+    game_state *copy = dup_game(state);
+
+    debug(("new_game_strip."));
+
+    strip_nums(copy);
+    debug_desc("Stripped", copy);
+
+    if (solve_state(copy) > 0) {
+        debug(("new_game_strip: soluble immediately after strip."));
+        free_game(copy);
+        return 1;
+    }
+
+    scratch = snewn(state->n, int);
+    for (i = 0; i < state->n; i++) scratch[i] = i;
+    shuffle(scratch, state->n, sizeof(int), rs);
+
+    /* This is scungy. It might just be quick enough.
+     * It goes through, adding set numbers in empty squares
+     * until either we run out of empty squares (in the one
+     * we're half-solving) or else we solve it properly.
+     * NB that we run the entire solver each time, which
+     * strips the grid beforehand; we will save time if we
+     * avoid that. */
+    for (i = 0; i < state->n; i++) {
+        j = scratch[i];
+        if (copy->nums[j] > 0 && copy->nums[j] <= state->n)
+            continue; /* already solved to a real number here. */
+        assert(state->nums[j] <= state->n);
+        debug(("new_game_strip: testing add IMMUTABLE number %d at square (%d,%d).",
+               state->nums[j], j%state->w, j/state->w));
+        copy->nums[j] = state->nums[j];
+        copy->flags[j] |= FLAG_IMMUTABLE;
+        state->flags[j] |= FLAG_IMMUTABLE;
+        debug_state("Copy of state: ", copy);
+        if (solve_state(copy) > 0) goto solved;
+        assert(check_nums(state, copy, 1));
+    }
+    ret = 0;
+    goto done;
+
+solved:
+    debug(("new_game_strip: now solved."));
+    /* Since we added basically at random, try now to remove numbers
+     * and see if we can still solve it; if we can (still), really
+     * remove the number. Make sure we don't remove the anchor numbers
+     * 1 and N. */
+    for (i = 0; i < state->n; i++) {
+        j = scratch[i];
+        if ((state->flags[j] & FLAG_IMMUTABLE) &&
+            (state->nums[j] != 1 && state->nums[j] != state->n)) {
+            debug(("new_game_strip: testing remove IMMUTABLE number %d at square (%d,%d).",
+                  state->nums[j], j%state->w, j/state->w));
+            state->flags[j] &= ~FLAG_IMMUTABLE;
+            dup_game_to(copy, state);
+            strip_nums(copy);
+            if (solve_state(copy) > 0) {
+                assert(check_nums(state, copy, 0));
+                debug(("new_game_strip: OK, removing number"));
+            } else {
+                assert(state->nums[j] <= state->n);
+                debug(("new_game_strip: cannot solve, putting IMMUTABLE back."));
+                copy->nums[j] = state->nums[j];
+                state->flags[j] |= FLAG_IMMUTABLE;
+            }
+        }
+    }
+
+done:
+    debug(("new_game_strip: %ssuccessful.", ret ? "" : "not "));
+    sfree(scratch);
+    free_game(copy);
+    return ret;
+}
+
+static char *new_game_desc(game_params *params, random_state *rs,
+			   char **aux, int interactive)
+{
+    game_state *state = blank_game(params->w, params->h);
+    char *ret;
+    int headi, taili;
+
+generate:
+    blank_game_into(state);
+
+    /* keep trying until we fill successfully. */
+    do {
+        if (params->force_corner_start) {
+            headi = 0;
+            taili = state->n-1;
+        } else {
+            do {
+                headi = random_upto(rs, state->n);
+                taili = random_upto(rs, state->n);
+            } while (headi == taili);
+        }
+    } while (!new_game_fill(state, rs, headi, taili));
+
+    debug_state("Filled game:", state);
+
+    assert(state->nums[headi] <= state->n);
+    assert(state->nums[taili] <= state->n);
+
+    state->flags[headi] |= FLAG_IMMUTABLE;
+    state->flags[taili] |= FLAG_IMMUTABLE;
+
+    /* This will have filled in directions and _all_ numbers.
+     * Store the game definition for this, as the solved-state. */
+    if (!new_game_strip(state, rs)) {
+        goto generate;
+    }
+    strip_nums(state);
+    {
+        game_state *tosolve = dup_game(state);
+        assert(solve_state(tosolve) > 0);
+        free_game(tosolve);
+    }
+    ret = generate_desc(state, 0);
+    free_game(state);
+    return ret;
+}
+
+static char *validate_desc(game_params *params, char *desc)
+{
+    char *ret = NULL;
+
+    unpick_desc(params, desc, NULL, &ret);
+    return ret;
+}
+
+/* --- Linked-list and numbers array --- */
+
+/* Assuming numbers are always up-to-date, there are only four possibilities
+ * for regions changing:
+ *
+ * 1) two differently-coloured regions being combined (the resulting colouring
+ *     should be based on the larger of the two regions)
+ * 2) a numbered region having a single number added to the start (the
+ *     region's colour will remain, and the numbers will shift by 1)
+ * 3) a numbered region having a single number added to the end (the
+ *     region's colour and numbering remains as-is)
+ * 4) two unnumbered squares being joined (will pick the smallest unused set
+ *     of colours to use for the new region).
+ *
+ * There should never be any complications with regions containing 3 colours
+ * being combined, since two of those colours should have been merged on a
+ * previous move.
+ */
+
+/* New algorithm for working out numbering:
+ *
+ * At start, only remove numbers from cells with neither prev nor next.
+ * Search for all cells with !prev && next (head of chain); for each one:
+   * Search the group for a 'real' number: if we find one the num. for
+      the head of the chain is trivial.
+   * Otherwise, if we _don't_ have a number already:
+     * If head->next has a number, that number is the one we should use
+     * Otherwise pick the smallest unused colour set.
+   * and if we _do_ have a number already:
+     * Work out the size of this group (the dsf must already have been set up)
+     * Start enumerating through the group counting squares that have the
+        same colouring as us
+     * If we reach a square with a different colour, work out which set is
+        bigger (ncol1 vs ncol2 == sz-ncol1), and use that colour
+     * If we reached a square with no colour (or the end of the group, which
+        would be weird under the circumstances) just keep the existing colour.
+ */
+
+#define COLOUR(a) ((a) / (state->n+1))
+#define START(c) ((c) * (state->n+1))
+
+static int lowest_start(game_state *state, int *scratch)
+{
+    int i, c;
+
+    /* Fill in 'scratch' array with the currently-used colours... */
+    memset(scratch, 0, state->n * sizeof(int));
+    for (i = 0; i < state->n; i++) {
+        if (state->nums[i] != 0)
+            scratch[COLOUR(state->nums[i])] = 1;
+    }
+    /* ... and return the first one that was unused. */
+    for (c = 1; c < state->n; c++) { /* NB start at 1 */
+        if (scratch[c] == 0)
+            return START(c);
+    }
+    assert(!"shouldn't get here");
+    return -1; /* suyb */
+}
+
+static int used_colour(game_state *state, int i, int start)
+{
+    int j;
+    for (j = 0; j < i; j++) {
+        if (state->nums[j] == start)
+            return 1;
+    }
+    return 0;
+}
+
+static int head_number(game_state *state, int i, int *scratch)
+{
+    int off = 0, start = -1, ss, j = i, c, n, sz;
+    const char *why = NULL;
+
+    assert(state->prev[i] == -1 && state->next[i] != -1);
+
+    /* Search through this chain looking for real numbers, checking that
+     * they match up (if there are more than one). */
+    while (j != -1) {
+        if (state->flags[j] & FLAG_IMMUTABLE) {
+            ss = state->nums[j] - off;
+            if (start == -1) {
+                start = ss;
+                why = "contains cell with immutable number";
+            } else if (start != ss) {
+                debug(("head_number: chain with non-sequential numbers."));
+                state->impossible = 1;
+            }
+        }
+        off++;
+        j = state->next[j];
+        assert(j != i); /* we have created a loop, obviously wrong */
+    }
+    if (start != -1) goto found;
+
+    if (state->nums[i] == 0) {
+        if (state->nums[state->next[i]] != 0) {
+            /* make sure we start at a 0 offset. */
+            start = START(COLOUR(state->nums[state->next[i]]));
+            why = "adding blank cell to head of numbered region";
+        } else {
+            start = lowest_start(state, scratch);
+            why = "lowest available colour group";
+        }
+    } else {
+        c = COLOUR(state->nums[i]);
+        n = 1;
+        sz = dsf_size(state->dsf, i);
+        j = i;
+        while (state->next[j] != -1) {
+            j = state->next[j];
+            if (state->nums[j] == 0) {
+                start = START(c);
+                why = "adding blank cell to end of numbered region";
+                break;
+            }
+            if (COLOUR(state->nums[j]) == c)
+                n++;
+            else {
+                int start_alternate = START(COLOUR(state->nums[j]));
+                if (n < (sz - n) && !used_colour(state, i, start_alternate)) {
+                    start = start_alternate;
+                    why = "joining two coloured regions, swapping to larger colour";
+                } else {
+                    start = START(c);
+                    why = "joining two coloured regions, taking largest";
+                }
+                break;
+            }
+        }
+        /* If we got here then we may have split a region into
+         * two; make sure we don't assign a colour we've already used. */
+        if (start == -1) {
+            start = (c == 0) ? lowest_start(state, scratch) : START(c);
+            why = "got to end of coloured region";
+        }
+        if (used_colour(state, i, start)) {
+            start = lowest_start(state, scratch);
+            why = "split region in two, lowest available colour group";
+        }
+    }
+
+found:
+    assert(start != -1 && why != NULL);
+    debug(("Chain at (%d,%d) numbered at %d: %s.",
+           i%state->w, i/state->w, start, why));
+    return start;
+}
+
+#if 0
+static void debug_numbers(game_state *state)
+{
+    int i, w=state->w;
+
+    for (i = 0; i < state->n; i++) {
+        debug(("(%d,%d) --> (%d,%d) --> (%d,%d)",
+               state->prev[i]==-1 ? -1 : state->prev[i]%w,
+               state->prev[i]==-1 ? -1 : state->prev[i]/w,
+               i%w, i/w,
+               state->next[i]==-1 ? -1 : state->next[i]%w,
+               state->next[i]==-1 ? -1 : state->next[i]/w));
+    }
+    w = w+1;
+}
+#endif
+
+static void connect_numbers(game_state *state)
+{
+    int i, di, dni;
+
+    dsf_init(state->dsf, state->n);
+    for (i = 0; i < state->n; i++) {
+        if (state->next[i] != -1) {
+            assert(state->prev[state->next[i]] == i);
+            di = dsf_canonify(state->dsf, i);
+            dni = dsf_canonify(state->dsf, state->next[i]);
+            if (di == dni) {
+                debug(("connect_numbers: chain forms a loop."));
+                state->impossible = 1;
+            }
+            dsf_merge(state->dsf, di, dni);
+        }
+    }
+}
+
+static void update_numbers(game_state *state)
+{
+    int i, j, nnum;
+    int *scratch = snewn(state->n, int);
+
+    for (i = 0; i < state->n; i++) {
+        assert(state->nums[i] >= 0);
+        state->numsi[i] = -1;
+    }
+
+    for (i = 0; i < state->n; i++) {
+        if (state->flags[i] & FLAG_IMMUTABLE) {
+            assert(state->nums[i] >= 0);
+            assert(state->nums[i] <= state->n);
+            state->numsi[state->nums[i]] = i;
+        }
+        else if (state->prev[i] == -1 && state->next[i] == -1)
+            state->nums[i] = 0;
+    }
+    connect_numbers(state);
+
+    for (i = 0; i < state->n; i++) {
+        /* Look for a cell that is the start of a chain
+         * (has a next but no prev). */
+        if (state->prev[i] != -1 || state->next[i] == -1) continue;
+
+        nnum = head_number(state, i, scratch);
+        j = i;
+        while (j != -1) {
+            if (nnum > 0 && nnum <= state->n)
+                state->numsi[nnum] = j;
+            state->nums[j] = nnum++;
+            j = state->next[j];
+            assert(j != i); /* loop?! */
+        }
+    }
+    /*debug_numbers(state);*/
+    sfree(scratch);
+}
+
+static int check_completion(game_state *state, int mark_errors)
+{
+    int n, j, k, error = 0, complete;
+
+    /* NB This only marks errors that are possible to perpetrate with
+     * the current UI in interpret_move. Things like forming loops in
+     * linked sections and having numbers not add up should be forbidden
+     * by the code elsewhere, so we don't bother marking those (because
+     * it would add lots of tricky drawing code for very little gain). */
+    if (mark_errors) {
+        for (j = 0; j < state->n; j++)
+            state->flags[j] &= ~FLAG_ERROR;
+    }
+
+    /* Search for repeated numbers. */
+    for (j = 0; j < state->n; j++) {
+        if (state->nums[j] > 0 && state->nums[j] <= state->n) {
+            for (k = j+1; k < state->n; k++) {
+                if (state->nums[k] == state->nums[j]) {
+                    if (mark_errors) {
+                        state->flags[j] |= FLAG_ERROR;
+                        state->flags[k] |= FLAG_ERROR;
+                    }
+                    error = 1;
+                }
+            }
+        }
+    }
+
+    /* Search and mark numbers n not pointing to n+1; if any numbers
+     * are missing we know we've not completed. */
+    complete = 1;
+    for (n = 1; n < state->n; n++) {
+        if (state->numsi[n] == -1 || state->numsi[n+1] == -1)
+            complete = 0;
+        else if (!ispointingi(state, state->numsi[n], state->numsi[n+1])) {
+            if (mark_errors) {
+                state->flags[state->numsi[n]] |= FLAG_ERROR;
+                state->flags[state->numsi[n+1]] |= FLAG_ERROR;
+            }
+            error = 1;
+        } else {
+            /* make sure the link is explicitly made here; for instance, this
+             * is nice if the user drags from 2 out (making 3) and a 4 is also
+             * visible; this ensures that the link from 3 to 4 is also made. */
+            if (mark_errors)
+                makelink(state, state->numsi[n], state->numsi[n+1]);
+        }
+    }
+
+    if (error) return 0;
+    return complete;
+}
+static game_state *new_game(midend *me, game_params *params, char *desc)
+{
+    game_state *state = NULL;
+
+    unpick_desc(params, desc, &state, NULL);
+    if (!state) assert(!"new_game failed to unpick");
+
+    update_numbers(state);
+    check_completion(state, 1); /* update any auto-links */
+
+    return state;
+}
+
+/* --- Solver --- */
+
+/* If a tile has a single tile it can link _to_, or there's only a single
+ * location that can link to a given tile, fill that link in. */
+static int solve_single(game_state *state, game_state *copy, int *from)
+{
+    int i, j, sx, sy, x, y, d, poss, w=state->w, nlinks = 0;
+
+    /* The from array is a list of 'which square can link _to_ us';
+     * we start off with from as '-1' (meaning 'not found'); if we find
+     * something that can link to us it is set to that index, and then if
+     * we find another we set it to -2. */
+
+    memset(from, -1, state->n*sizeof(int));
+
+    /* poss is 'can I link to anything' with the same meanings. */
+
+    for (i = 0; i < state->n; i++) {
+        if (state->next[i] != -1) continue;
+        if (state->nums[i] == state->n) continue; /* no next from last no. */
+
+        d = state->dirs[i];
+        poss = -1;
+        sx = x = i%w; sy = y = i/w;
+        while (1) {
+            x += dxs[d]; y += dys[d];
+            if (!INGRID(state, x, y)) break;
+            if (!isvalidmove(state, 1, sx, sy, x, y)) continue;
+
+            /* can't link to somewhere with a back-link we would have to
+             * break (the solver just doesn't work like this). */
+            j = y*w+x;
+            if (state->prev[j] != -1) continue;
+
+            if (state->nums[i] > 0 && state->nums[j] > 0 &&
+                state->nums[i] <= state->n && state->nums[j] <= state->n &&
+                state->nums[j] == state->nums[i]+1) {
+                debug(("Solver: forcing link through existing consecutive numbers."));
+                poss = j;
+                from[j] = i;
+                break;
+            }
+
+            /* if there's been a valid move already, we have to move on;
+             * we can't make any deductions here. */
+            poss = (poss == -1) ? j : -2;
+
+            /* Modify the from array as described above (which is enumerating
+             * what points to 'j' in a similar way). */
+            from[j] = (from[j] == -1) ? i : -2;
+        }
+        if (poss == -2) {
+            /*debug(("Solver: (%d,%d) has multiple possible next squares.", sx, sy));*/
+            ;
+        } else if (poss == -1) {
+            debug(("Solver: nowhere possible for (%d,%d) to link to.", sx, sy));
+            copy->impossible = 1;
+            return -1;
+        } else {
+            debug(("Solver: linking (%d,%d) to only possible next (%d,%d).",
+                   sx, sy, poss%w, poss/w));
+            makelink(copy, i, poss);
+            nlinks++;
+        }
+    }
+
+    for (i = 0; i < state->n; i++) {
+        if (state->prev[i] != -1) continue;
+        if (state->nums[i] == 1) continue; /* no prev from 1st no. */
+
+        x = i%w; y = i/w;
+        if (from[i] == -1) {
+            debug(("Solver: nowhere possible to link to (%d,%d)", x, y));
+            copy->impossible = 1;
+            return -1;
+        } else if (from[i] == -2) {
+            /*debug(("Solver: (%d,%d) has multiple possible prev squares.", x, y));*/
+            ;
+        } else {
+            debug(("Solver: linking only possible prev (%d,%d) to (%d,%d).",
+                   from[i]%w, from[i]/w, x, y));
+            makelink(copy, from[i], i);
+            nlinks++;
+        }
+    }
+
+    return nlinks;
+}
+
+/* Returns 1 if we managed to solve it, 0 otherwise. */
+static int solve_state(game_state *state)
+{
+    game_state *copy = dup_game(state);
+    int *scratch = snewn(state->n, int), ret;
+
+    debug_state("Before solver: ", state);
+
+    while (1) {
+        update_numbers(state);
+
+        if (solve_single(state, copy, scratch)) {
+            dup_game_to(state, copy);
+            if (state->impossible) break; else continue;
+        }
+        break;
+    }
+    free_game(copy);
+    sfree(scratch);
+
+    update_numbers(state);
+    ret = state->impossible ? -1 : check_completion(state, 0);
+    debug(("Solver finished: %s",
+           ret < 0 ? "impossible" : ret > 0 ? "solved" : "not solved"));
+    debug_state("After solver: ", state);
+    return ret;
+}
+
+static char *solve_game(game_state *state, game_state *currstate,
+			char *aux, char **error)
+{
+    game_state *tosolve;
+    char *ret = NULL;
+    int result;
+
+    tosolve = dup_game(currstate);
+    result = solve_state(tosolve);
+    if (result > 0)
+        ret = generate_desc(tosolve, 1);
+    free_game(tosolve);
+    if (ret) return ret;
+
+    tosolve = dup_game(state);
+    result = solve_state(tosolve);
+    if (result < 0)
+        *error = "Puzzle is impossible.";
+    else if (result == 0)
+        *error = "Unable to solve puzzle.";
+    else
+        ret = generate_desc(tosolve, 1);
+
+    free_game(tosolve);
+
+    return ret;
+}
+
+/* --- UI and move routines. --- */
+
+
+struct game_ui {
+    int cx, cy, cshow;
+
+    int dragging, drag_is_from;
+    int sx, sy;         /* grid coords of start cell */
+    int dx, dy;         /* pixel coords of drag posn */
+};
+
+static game_ui *new_ui(game_state *state)
+{
+    game_ui *ui = snew(game_ui);
+
+    /* NB: if this is ever changed to as to require more than a structure
+     * copy to clone, there's code that needs fixing in game_redraw too. */
+
+    ui->cx = ui->cy = ui->cshow = 0;
+
+    ui->dragging = 0;
+    ui->sx = ui->sy = ui->dx = ui->dy = 0;
+
+    return ui;
+}
+
+static void free_ui(game_ui *ui)
+{
+    sfree(ui);
+}
+
+static char *encode_ui(game_ui *ui)
+{
+    return NULL;
+}
+
+static void decode_ui(game_ui *ui, char *encoding)
+{
+}
+
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+    if (!oldstate->completed && newstate->completed)
+        ui->cshow = ui->dragging = 0;
+}
+
+struct game_drawstate {
+    int tilesize, started, solved;
+    int w, h, n;
+    int *nums, *dirp;
+    unsigned int *f;
+    double angle_offset;
+
+    int dragging, dx, dy;
+    blitter *dragb;
+};
+
+static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
+			    int mx, int my, int button)
+{
+    int x = FROMCOORD(mx), y = FROMCOORD(my), w = state->w;
+    char buf[80];
+
+    if (IS_CURSOR_MOVE(button)) {
+        move_cursor(button, &ui->cx, &ui->cy, state->w, state->h, 0);
+        ui->cshow = 1;
+        if (ui->dragging) {
+            ui->dx = COORD(ui->cx) + TILE_SIZE/2;
+            ui->dy = COORD(ui->cy) + TILE_SIZE/2;
+        }
+        return "";
+    } else if (IS_CURSOR_SELECT(button)) {
+        if (!ui->cshow)
+            ui->cshow = 1;
+        else if (ui->dragging) {
+            ui->dragging = FALSE;
+            if (ui->sx == ui->cx && ui->sy == ui->cy) return "";
+            if (ui->drag_is_from) {
+                if (!isvalidmove(state, 0, ui->sx, ui->sy, ui->cx, ui->cy)) return "";
+                sprintf(buf, "L%d,%d-%d,%d", ui->sx, ui->sy, ui->cx, ui->cy);
+            } else {
+                if (!isvalidmove(state, 0, ui->cx, ui->cy, ui->sx, ui->sy)) return "";
+                sprintf(buf, "L%d,%d-%d,%d", ui->cx, ui->cy, ui->sx, ui->sy);
+            }
+            return dupstr(buf);
+        } else {
+            ui->dragging = TRUE;
+            ui->sx = ui->cx;
+            ui->sy = ui->cy;
+            ui->dx = COORD(ui->cx) + TILE_SIZE/2;
+            ui->dy = COORD(ui->cy) + TILE_SIZE/2;
+            ui->drag_is_from = (button == CURSOR_SELECT) ? 1 : 0;
+        }
+        return "";
+    }
+    if (IS_MOUSE_DOWN(button)) {
+        if (ui->cshow) {
+            ui->cshow = ui->dragging = 0;
+        }
+        assert(!ui->dragging);
+        if (!INGRID(state, x, y)) return NULL;
+
+        if (button == LEFT_BUTTON) {
+            /* disallow dragging from the final number. */
+            if (state->nums[y*w+x] == state->n) return NULL;
+        } else if (button == RIGHT_BUTTON) {
+            /* disallow dragging to the first number. */
+            if (state->nums[y*w+x] == 1) return NULL;
+        }
+
+        ui->dragging = TRUE;
+        ui->drag_is_from = (button == LEFT_BUTTON) ? 1 : 0;
+        ui->sx = x;
+        ui->sy = y;
+        ui->dx = mx;
+        ui->dy = my;
+        ui->cshow = 0;
+        return "";
+    } else if (IS_MOUSE_DRAG(button) && ui->dragging) {
+        ui->dx = mx;
+        ui->dy = my;
+        return "";
+    } else if (IS_MOUSE_RELEASE(button) && ui->dragging) {
+        ui->dragging = FALSE;
+        if (ui->sx == x && ui->sy == y) return ""; /* single click */
+
+        if (!INGRID(state, x, y)) {
+            int si = ui->sy*w+ui->sx;
+            if (state->prev[si] == -1 && state->next[si] == -1)
+                return "";
+            sprintf(buf, "%c%d,%d",
+                    ui->drag_is_from ? 'C' : 'X', ui->sx, ui->sy);
+            return dupstr(buf);
+        }
+
+        if (ui->drag_is_from) {
+            if (!isvalidmove(state, 0, ui->sx, ui->sy, x, y)) return "";
+            sprintf(buf, "L%d,%d-%d,%d", ui->sx, ui->sy, x, y);
+        } else {
+            if (!isvalidmove(state, 0, x, y, ui->sx, ui->sy)) return "";
+            sprintf(buf, "L%d,%d-%d,%d", x, y, ui->sx, ui->sy);
+        }
+        return dupstr(buf);
+    } /* else if (button == 'H' || button == 'h')
+        return dupstr("H"); */
+    else if ((button == 'x' || button == 'X') && ui->cshow) {
+        int si = ui->cy*w + ui->cx;
+        if (state->prev[si] == -1 && state->next[si] == -1)
+            return "";
+        sprintf(buf, "%c%d,%d",
+                (button == 'x') ? 'C' : 'X', ui->cx, ui->cy);
+        return dupstr(buf);
+    }
+
+    return NULL;
+}
+
+static void unlink_cell(game_state *state, int si)
+{
+    debug(("Unlinking (%d,%d).", si%state->w, si/state->w));
+    if (state->prev[si] != -1) {
+        debug((" ... removing prev link from (%d,%d).",
+               state->prev[si]%state->w, state->prev[si]/state->w));
+        state->next[state->prev[si]] = -1;
+        state->prev[si] = -1;
+    }
+    if (state->next[si] != -1) {
+        debug((" ... removing next link to (%d,%d).",
+               state->next[si]%state->w, state->next[si]/state->w));
+        state->prev[state->next[si]] = -1;
+        state->next[si] = -1;
+    }
+}
+
+static game_state *execute_move(game_state *state, char *move)
+{
+    game_state *ret = NULL;
+    int sx, sy, ex, ey, si, ei, w = state->w;
+    char c;
+
+    debug(("move: %s", move));
+
+    if (move[0] == 'S') {
+        game_params p;
+	game_state *tmp;
+        char *valid;
+	int i;
+
+        p.w = state->w; p.h = state->h;
+        valid = validate_desc(&p, move+1);
+        if (valid) {
+            debug(("execute_move: move not valid: %s", valid));
+            return NULL;
+        }
+	ret = dup_game(state);
+        tmp = new_game(NULL, &p, move+1);
+	for (i = 0; i < state->n; i++) {
+	    ret->prev[i] = tmp->prev[i];
+	    ret->next[i] = tmp->next[i];
+	}
+	free_game(tmp);
+        ret->used_solve = 1;
+    } else if (sscanf(move, "L%d,%d-%d,%d", &sx, &sy, &ex, &ey) == 4) {
+        if (!isvalidmove(state, 0, sx, sy, ex, ey)) return NULL;
+
+        ret = dup_game(state);
+
+        si = sy*w+sx; ei = ey*w+ex;
+        makelink(ret, si, ei);
+    } else if (sscanf(move, "%c%d,%d", &c, &sx, &sy) == 3) {
+        if (c != 'C' && c != 'X') return NULL;
+        if (!INGRID(state, sx, sy)) return NULL;
+        si = sy*w+sx;
+        if (state->prev[si] == -1 && state->next[si] == -1)
+            return NULL;
+
+        ret = dup_game(state);
+
+        if (c == 'C') {
+            /* Unlink the single cell we dragged from the board. */
+            unlink_cell(ret, si);
+        } else {
+            int i, set, sset = state->nums[si] / (state->n+1);
+            for (i = 0; i < state->n; i++) {
+                /* Unlink all cells in the same set as the one we dragged
+                 * from the board. */
+
+                if (state->nums[i] == 0) continue;
+                set = state->nums[i] / (state->n+1);
+                if (set != sset) continue;
+
+                unlink_cell(ret, i);
+            }
+        }
+    } else if (strcmp(move, "H") == 0) {
+        ret = dup_game(state);
+        solve_state(ret);
+    }
+    if (ret) {
+        update_numbers(ret);
+        if (check_completion(ret, 1)) ret->completed = 1;
+    }
+
+    return ret;
+}
+
+/* ----------------------------------------------------------------------
+ * Drawing routines.
+ */
+
+static void game_compute_size(game_params *params, int tilesize,
+			      int *x, int *y)
+{
+    /* Ick: fake up `ds->tilesize' for macro expansion purposes */
+    struct { int tilesize, order; } ads, *ds = &ads;
+    ads.tilesize = tilesize;
+
+    *x = TILE_SIZE * params->w + 2 * BORDER;
+    *y = TILE_SIZE * params->h + 2 * BORDER;
+}
+
+static void game_set_size(drawing *dr, game_drawstate *ds,
+			  game_params *params, int tilesize)
+{
+    ds->tilesize = tilesize;
+    assert(TILE_SIZE > 0);
+
+    assert(!ds->dragb);
+    ds->dragb = blitter_new(dr, BLITTER_SIZE, BLITTER_SIZE);
+}
+
+/* Colours chosen from the webby palette to work as a background to black text,
+ * W then some plausible approximation to pastelly ROYGBIV; we then interpolate
+ * between consecutive pairs to give another 8 (and then the drawing routine
+ * will reuse backgrounds). */
+static const unsigned long bgcols[8] = {
+    0xffffff, /* white */
+    0xffa07a, /* lightsalmon */
+    0x98fb98, /* green */
+    0x7fffd4, /* aquamarine */
+    0x9370db, /* medium purple */
+    0xffa500, /* orange */
+    0x87cefa, /* lightskyblue */
+    0xffff00, /* yellow */
+};
+
+static float *game_colours(frontend *fe, int *ncolours)
+{
+    float *ret = snewn(3 * NCOLOURS, float);
+    int c, i;
+
+    game_mkhighlight(fe, ret, COL_BACKGROUND, COL_HIGHLIGHT, COL_LOWLIGHT);
+
+    for (i = 0; i < 3; i++) {
+        ret[COL_NUMBER * 3 + i] = 0.0F;
+        ret[COL_ARROW * 3 + i] = 0.0F;
+        ret[COL_CURSOR * 3 + i] = ret[COL_BACKGROUND * 3 + i] / 2.0F;
+        ret[COL_GRID * 3 + i] = ret[COL_BACKGROUND * 3 + i] / 1.3F;
+    }
+    ret[COL_NUMBER_SET * 3 + 0] = 0.0F;
+    ret[COL_NUMBER_SET * 3 + 1] = 0.0F;
+    ret[COL_NUMBER_SET * 3 + 2] = 0.9F;
+
+    ret[COL_ERROR * 3 + 0] = 1.0F;
+    ret[COL_ERROR * 3 + 1] = 0.0F;
+    ret[COL_ERROR * 3 + 2] = 0.0F;
+
+    ret[COL_DRAG_ORIGIN * 3 + 0] = 0.2F;
+    ret[COL_DRAG_ORIGIN * 3 + 1] = 1.0F;
+    ret[COL_DRAG_ORIGIN * 3 + 2] = 0.2F;
+
+    for (c = 0; c < 8; c++) {
+         ret[(COL_B0 + c) * 3 + 0] = (float)((bgcols[c] & 0xff0000) >> 16) / 256.0F;
+         ret[(COL_B0 + c) * 3 + 1] = (float)((bgcols[c] & 0xff00) >> 8) / 256.0F;
+         ret[(COL_B0 + c) * 3 + 2] = (float)((bgcols[c] & 0xff)) / 256.0F;
+    }
+    for (c = 0; c < 8; c++) {
+        for (i = 0; i < 3; i++) {
+           ret[(COL_B0 + 8 + c) * 3 + i] =
+               (ret[(COL_B0 + c) * 3 + i] + ret[(COL_B0 + c + 1) * 3 + i]) / 2.0F;
+        }
+    }
+
+#define average(r,a,b,w) do { \
+    for (i = 0; i < 3; i++) \
+	ret[(r)*3+i] = ret[(a)*3+i] + w * (ret[(b)*3+i] - ret[(a)*3+i]); \
+} while (0)
+    average(COL_ARROW_BG_DIM, COL_BACKGROUND, COL_ARROW, 0.1F);
+    average(COL_NUMBER_SET_MID, COL_B0, COL_NUMBER_SET, 0.3F);
+    for (c = 0; c < NBACKGROUNDS; c++) {
+	/* I assume here that COL_ARROW and COL_NUMBER are the same.
+	 * Otherwise I'd need two sets of COL_M*. */
+	average(COL_M0 + c, COL_B0 + c, COL_NUMBER, 0.3F);
+	average(COL_D0 + c, COL_B0 + c, COL_NUMBER, 0.1F);
+	average(COL_X0 + c, COL_BACKGROUND, COL_B0 + c, 0.5F);
+    }
+
+    *ncolours = NCOLOURS;
+    return ret;
+}
+
+static game_drawstate *game_new_drawstate(drawing *dr, game_state *state)
+{
+    struct game_drawstate *ds = snew(struct game_drawstate);
+    int i;
+
+    ds->tilesize = ds->started = ds->solved = 0;
+    ds->w = state->w;
+    ds->h = state->h;
+    ds->n = state->n;
+
+    ds->nums = snewn(state->n, int);
+    ds->dirp = snewn(state->n, int);
+    ds->f = snewn(state->n, unsigned int);
+    for (i = 0; i < state->n; i++) {
+        ds->nums[i] = 0;
+        ds->dirp[i] = -1;
+        ds->f[i] = 0;
+    }
+
+    ds->angle_offset = 0.0F;
+
+    ds->dragging = ds->dx = ds->dy = 0;
+    ds->dragb = NULL;
+
+    return ds;
+}
+
+static void game_free_drawstate(drawing *dr, game_drawstate *ds)
+{
+    sfree(ds->nums);
+    sfree(ds->dirp);
+    sfree(ds->f);
+    if (ds->dragb) blitter_free(dr, ds->dragb);
+
+    sfree(ds);
+}
+
+/* cx, cy are top-left corner. sz is the 'radius' of the arrow.
+ * ang is in radians, clockwise from 0 == straight up. */
+static void draw_arrow(drawing *dr, int cx, int cy, int sz, double ang,
+                       int cfill, int cout)
+{
+    int coords[14];
+    int xdx, ydx, xdy, ydy, xdx3, xdy3;
+    double s = sin(ang), c = cos(ang);
+
+    xdx3 = (int)(sz * (c/3 + 1) + 0.5) - sz;
+    xdy3 = (int)(sz * (s/3 + 1) + 0.5) - sz;
+    xdx = (int)(sz * (c + 1) + 0.5) - sz;
+    xdy = (int)(sz * (s + 1) + 0.5) - sz;
+    ydx = -xdy;
+    ydy = xdx;
+
+
+    coords[2*0 + 0] = cx - ydx;
+    coords[2*0 + 1] = cy - ydy;
+    coords[2*1 + 0] = cx + xdx;
+    coords[2*1 + 1] = cy + xdy;
+    coords[2*2 + 0] = cx + xdx3;
+    coords[2*2 + 1] = cy + xdy3;
+    coords[2*3 + 0] = cx + xdx3 + ydx;
+    coords[2*3 + 1] = cy + xdy3 + ydy;
+    coords[2*4 + 0] = cx - xdx3 + ydx;
+    coords[2*4 + 1] = cy - xdy3 + ydy;
+    coords[2*5 + 0] = cx - xdx3;
+    coords[2*5 + 1] = cy - xdy3;
+    coords[2*6 + 0] = cx - xdx;
+    coords[2*6 + 1] = cy - xdy;
+
+    draw_polygon(dr, coords, 7, cfill, cout);
+}
+
+static void draw_arrow_dir(drawing *dr, int cx, int cy, int sz, int dir,
+                           int cfill, int cout, double angle_offset)
+{
+    double ang = 2.0 * PI * (double)dir / 8.0 + angle_offset;
+    draw_arrow(dr, cx, cy, sz, ang, cfill, cout);
+}
+
+/* cx, cy are centre coordinates.. */
+static void draw_star(drawing *dr, int cx, int cy, int rad, int npoints,
+                      int cfill, int cout, double angle_offset)
+{
+    int *coords, n;
+    double a, r;
+
+    assert(npoints > 0);
+
+    coords = snewn(npoints * 2 * 2, int);
+
+    for (n = 0; n < npoints * 2; n++) {
+        a = 2.0 * PI * ((double)n / ((double)npoints * 2.0)) + angle_offset;
+        r = (n % 2) ? (double)rad/2.0 : (double)rad;
+
+        /* We're rotating the point at (0, -r) by a degrees */
+        coords[2*n+0] = cx + (int)( r * sin(a));
+        coords[2*n+1] = cy + (int)(-r * cos(a));
+    }
+    draw_polygon(dr, coords, npoints*2, cfill, cout);
+    sfree(coords);
+}
+
+static int num2col(game_drawstate *ds, int num)
+{
+    int set = num / (ds->n+1);
+
+    if (num <= 0) return COL_BACKGROUND;
+    return COL_B0 + (set % 16);
+}
+
+#define ARROW_HALFSZ (7 * TILE_SIZE / 32)
+
+#define F_CUR           0x001   /* Cursor on this tile. */
+#define F_DRAG_SRC      0x002   /* Tile is source of a drag. */
+#define F_ERROR         0x004   /* Tile marked in error. */
+#define F_IMMUTABLE     0x008   /* Tile (number) is immutable. */
+#define F_ARROW_POINT   0x010   /* Tile points to other tile */
+#define F_ARROW_INPOINT 0x020   /* Other tile points in here. */
+#define F_DIM           0x040   /* Tile is dim */
+
+static void tile_redraw(drawing *dr, game_drawstate *ds, int tx, int ty,
+                        int dir, int dirp, int num, unsigned int f,
+                        double angle_offset, int print_ink)
+{
+    int cb = TILE_SIZE / 16, textsz;
+    char buf[20];
+    int arrowcol, sarrowcol, setcol, textcol;
+    int n = num % (ds->n+1), set = num / (ds->n+1);
+    int acx, acy, asz;
+
+    /* Calculate colours. */
+
+    if (print_ink >= 0) {
+	/*
+	 * We're printing, so just do everything in black.
+	 */
+	arrowcol = textcol = print_ink;
+	setcol = sarrowcol = -1;       /* placate optimiser */
+    } else {
+
+	setcol = num2col(ds, num);
+
+#define dim(fg,bg) ( \
+      (bg)==COL_BACKGROUND ? COL_ARROW_BG_DIM : \
+      (bg) + COL_D0 - COL_B0 \
+    )
+
+#define mid(fg,bg) ( \
+      (fg)==COL_NUMBER_SET ? COL_NUMBER_SET_MID : \
+      (bg) + COL_M0 - COL_B0 \
+    )
+
+#define dimbg(bg) ( \
+      (bg)==COL_BACKGROUND ? COL_BACKGROUND : \
+      (bg) + COL_X0 - COL_B0 \
+    )
+
+	if (f & F_DRAG_SRC) arrowcol = COL_DRAG_ORIGIN;
+	else if (f & F_DIM) arrowcol = dim(COL_ARROW, setcol);
+	else if (f & F_ARROW_POINT) arrowcol = mid(COL_ARROW, setcol);
+	else arrowcol = COL_ARROW;
+
+	if (f & (F_ERROR)) textcol = COL_ERROR;
+	else {
+	    if (f & F_IMMUTABLE) textcol = COL_NUMBER_SET;
+	    else textcol = COL_NUMBER;
+
+	    if (f & F_DIM) textcol = dim(textcol, setcol);
+	    else if (((f & F_ARROW_POINT) || num==ds->n) &&
+		     ((f & F_ARROW_INPOINT) || num==1))
+		textcol = mid(textcol, setcol);
+	}
+
+	if (f & F_DIM) sarrowcol = dim(COL_ARROW, setcol);
+	else sarrowcol = COL_ARROW;
+    }
+
+    /* Clear tile background */
+
+    if (print_ink < 0) {
+	draw_rect(dr, tx, ty, TILE_SIZE, TILE_SIZE,
+		  (f & F_DIM) ? dimbg(setcol) : setcol);
+    }
+
+    /* Draw large (outwards-pointing) arrow. */
+
+    asz = ARROW_HALFSZ;         /* 'radius' of arrow/star. */
+    acx = tx+TILE_SIZE/2+asz;   /* centre x */
+    acy = ty+TILE_SIZE/2+asz;   /* centre y */
+
+    if (num == ds->n && (f & F_IMMUTABLE))
+        draw_star(dr, acx, acy, asz, 5, arrowcol, arrowcol, angle_offset);
+    else
+        draw_arrow_dir(dr, acx, acy, asz, dir, arrowcol, arrowcol, angle_offset);
+    if (print_ink < 0 && (f & F_CUR))
+        draw_rect_corners(dr, acx, acy, asz+1, COL_CURSOR);
+
+    /* Draw dot iff this tile requires a predecessor and doesn't have one. */
+
+    if (print_ink < 0) {
+	acx = tx+TILE_SIZE/2-asz;
+	acy = ty+TILE_SIZE/2+asz;
+
+	if (!(f & F_ARROW_INPOINT) && num != 1) {
+	    draw_circle(dr, acx, acy, asz / 4, sarrowcol, sarrowcol);
+	}
+    }
+
+    /* Draw text (number or set). */
+
+    if (num != 0) {
+        assert(num > 0);
+        if (set == 0) {
+            sprintf(buf, "%d", n);
+        } else {
+            if (n == 0)
+                sprintf(buf, "%c", (int)(set+'a'-1));
+            else
+                sprintf(buf, "%c+%d", (int)(set+'a'-1), n);
+        }
+        textsz = min(2*asz, (TILE_SIZE - 2 * cb) / (int)strlen(buf));
+        draw_text(dr, tx + cb, ty + TILE_SIZE/4, FONT_VARIABLE, textsz,
+                  ALIGN_VCENTRE | ALIGN_HLEFT, textcol, buf);
+    }
+
+    if (print_ink < 0) {
+	draw_rect_outline(dr, tx, ty, TILE_SIZE, TILE_SIZE, COL_GRID);
+	draw_update(dr, tx, ty, TILE_SIZE, TILE_SIZE);
+    }
+}
+
+static void draw_drag_indicator(drawing *dr, game_drawstate *ds,
+                                game_state *state, game_ui *ui, int validdrag)
+{
+    int dir, w = ds->w, acol = COL_ARROW;
+    int fx = FROMCOORD(ui->dx), fy = FROMCOORD(ui->dy);
+    double ang;
+
+    if (validdrag) {
+        /* If we could move here, lock the arrow to the appropriate direction. */
+        dir = ui->drag_is_from ? state->dirs[ui->sy*w+ui->sx] : state->dirs[fy*w+fx];
+
+        ang = (2.0 * PI * dir) / 8.0; /* similar to calculation in draw_arrow_dir. */
+    } else {
+        /* Draw an arrow pointing away from/towards the origin cell. */
+        int ox = COORD(ui->sx) + TILE_SIZE/2, oy = COORD(ui->sy) + TILE_SIZE/2;
+        double tana, offset;
+        double xdiff = fabs(ox - ui->dx), ydiff = fabs(oy - ui->dy);
+
+        if (xdiff == 0) {
+            ang = (oy > ui->dy) ? 0.0F : PI;
+        } else if (ydiff == 0) {
+            ang = (ox > ui->dx) ? 3.0F*PI/2.0F : PI/2.0F;
+        } else {
+            if (ui->dx > ox && ui->dy < oy) {
+                tana = xdiff / ydiff;
+                offset = 0.0F;
+            } else if (ui->dx > ox && ui->dy > oy) {
+                tana = ydiff / xdiff;
+                offset = PI/2.0F;
+            } else if (ui->dx < ox && ui->dy > oy) {
+                tana = xdiff / ydiff;
+                offset = PI;
+            } else {
+                tana = ydiff / xdiff;
+                offset = 3.0F * PI / 2.0F;
+            }
+            ang = atan(tana) + offset;
+        }
+
+        if (!ui->drag_is_from) ang += PI; /* point to origin, not away from. */
+
+    }
+    draw_arrow(dr, ui->dx, ui->dy, ARROW_HALFSZ, ang, acol, acol);
+}
+
+static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
+			game_state *state, int dir, game_ui *ui,
+			float animtime, float flashtime)
+{
+    int x, y, i, w = ds->w, dirp, force = 0;
+    unsigned int f;
+    double angle_offset = 0.0;
+    game_state *postdrop = NULL;
+
+    if (flashtime > 0.0F)
+        angle_offset = 2.0 * PI * (flashtime / FLASH_SPIN);
+    if (angle_offset != ds->angle_offset) {
+        ds->angle_offset = angle_offset;
+        force = 1;
+    }
+
+    if (ds->dragging) {
+        assert(ds->dragb);
+        blitter_load(dr, ds->dragb, ds->dx, ds->dy);
+        draw_update(dr, ds->dx, ds->dy, BLITTER_SIZE, BLITTER_SIZE);
+        ds->dragging = FALSE;
+    }
+
+    /* If an in-progress drag would make a valid move if finished, we
+     * reflect that move in the board display. We let interpret_move do
+     * most of the heavy lifting for us: we have to copy the game_ui so
+     * as not to stomp on the real UI's drag state. */
+    if (ui->dragging) {
+        game_ui uicopy = *ui;
+        char *movestr = interpret_move(state, &uicopy, ds, ui->dx, ui->dy, LEFT_RELEASE);
+
+        if (movestr != NULL && strcmp(movestr, "") != 0) {
+            postdrop = execute_move(state, movestr);
+            sfree(movestr);
+
+            state = postdrop;
+        }
+    }
+
+    if (!ds->started) {
+        int aw = TILE_SIZE * state->w;
+        int ah = TILE_SIZE * state->h;
+        draw_rect(dr, 0, 0, aw + 2 * BORDER, ah + 2 * BORDER, COL_BACKGROUND);
+        draw_rect_outline(dr, BORDER - 1, BORDER - 1, aw + 2, ah + 2, COL_GRID);
+        draw_update(dr, 0, 0, aw + 2 * BORDER, ah + 2 * BORDER);
+    }
+    for (x = 0; x < state->w; x++) {
+        for (y = 0; y < state->h; y++) {
+            i = y*w + x;
+            f = 0;
+            dirp = -1;
+
+            if (ui->cshow && x == ui->cx && y == ui->cy)
+                f |= F_CUR;
+
+            if (ui->dragging) {
+                if (x == ui->sx && y == ui->sy)
+                    f |= F_DRAG_SRC;
+                else if (ui->drag_is_from) {
+                    if (!ispointing(state, ui->sx, ui->sy, x, y))
+                        f |= F_DIM;
+                } else {
+                    if (!ispointing(state, x, y, ui->sx, ui->sy))
+                        f |= F_DIM;
+                }
+            }
+
+            if (state->impossible ||
+                state->nums[i] < 0 || state->flags[i] & FLAG_ERROR)
+                f |= F_ERROR;
+            if (state->flags[i] & FLAG_IMMUTABLE)
+                f |= F_IMMUTABLE;
+
+            if (state->next[i] != -1)
+                f |= F_ARROW_POINT;
+
+            if (state->prev[i] != -1) {
+                /* Currently the direction here is from our square _back_
+                 * to its previous. We could change this to give the opposite
+                 * sense to the direction. */
+                f |= F_ARROW_INPOINT;
+                dirp = whichdir(x, y, state->prev[i]%w, state->prev[i]/w);
+            }
+
+            if (state->nums[i] != ds->nums[i] ||
+                f != ds->f[i] || dirp != ds->dirp[i] ||
+                force || !ds->started) {
+                tile_redraw(dr, ds,
+                            BORDER + x * TILE_SIZE,
+                            BORDER + y * TILE_SIZE,
+                            state->dirs[i], dirp, state->nums[i], f,
+                            angle_offset, -1);
+                ds->nums[i] = state->nums[i];
+                ds->f[i] = f;
+                ds->dirp[i] = dirp;
+            }
+        }
+    }
+    if (ui->dragging) {
+        ds->dragging = TRUE;
+        ds->dx = ui->dx - BLITTER_SIZE/2;
+        ds->dy = ui->dy - BLITTER_SIZE/2;
+        blitter_save(dr, ds->dragb, ds->dx, ds->dy);
+
+        draw_drag_indicator(dr, ds, state, ui, postdrop ? 1 : 0);
+    }
+    if (postdrop) free_game(postdrop);
+    if (!ds->started) ds->started = TRUE;
+}
+
+static float game_anim_length(game_state *oldstate, game_state *newstate,
+			      int dir, game_ui *ui)
+{
+    return 0.0F;
+}
+
+static float game_flash_length(game_state *oldstate, game_state *newstate,
+			       int dir, game_ui *ui)
+{
+    if (!oldstate->completed &&
+        newstate->completed && !newstate->used_solve)
+        return FLASH_SPIN;
+    else
+        return 0.0F;
+}
+
+static int game_timing_state(game_state *state, game_ui *ui)
+{
+    return TRUE;
+}
+
+static void game_print_size(game_params *params, float *x, float *y)
+{
+    int pw, ph;
+
+    game_compute_size(params, 1300, &pw, &ph);
+    *x = pw / 100.0F;
+    *y = ph / 100.0F;
+}
+
+static void game_print(drawing *dr, game_state *state, int tilesize)
+{
+    int ink = print_mono_colour(dr, 0);
+    int x, y;
+
+    /* Fake up just enough of a drawstate */
+    game_drawstate ads, *ds = &ads;
+    ds->tilesize = tilesize;
+    ds->n = state->n;
+
+    /*
+     * Border and grid.
+     */
+    print_line_width(dr, TILE_SIZE / 40);
+    for (x = 1; x < state->w; x++)
+	draw_line(dr, COORD(x), COORD(0), COORD(x), COORD(state->h), ink);
+    for (y = 1; y < state->h; y++)
+	draw_line(dr, COORD(0), COORD(y), COORD(state->w), COORD(y), ink);
+    print_line_width(dr, 2*TILE_SIZE / 40);
+    draw_rect_outline(dr, COORD(0), COORD(0), TILE_SIZE*state->w,
+		      TILE_SIZE*state->h, ink);
+
+    /*
+     * Arrows and numbers.
+     */
+    print_line_width(dr, 0);
+    for (y = 0; y < state->h; y++)
+	for (x = 0; x < state->w; x++)
+	    tile_redraw(dr, ds, COORD(x), COORD(y), state->dirs[y*state->w+x],
+			0, state->nums[y*state->w+x], 0, 0.0, ink);
+}
+
+#ifdef COMBINED
+#define thegame signpost
+#endif
+
+const struct game thegame = {
+    "Signpost", "games.signpost", "signpost",
+    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_TILE_SIZE, game_compute_size, game_set_size,
+    game_colours,
+    game_new_drawstate,
+    game_free_drawstate,
+    game_redraw,
+    game_anim_length,
+    game_flash_length,
+    TRUE, FALSE, game_print_size, game_print,
+    FALSE,			       /* wants_statusbar */
+    FALSE, game_timing_state,
+    REQUIRE_RBUTTON | REQUIRE_NUMPAD,  /* flags */
+};
+
+#ifdef STANDALONE_SOLVER
+
+#include <time.h>
+#include <stdarg.h>
+
+const char *quis = NULL;
+int verbose = 0;
+
+void usage(FILE *out) {
+    fprintf(out, "usage: %s [--stdin] [--soak] [--seed SEED] <params>|<game id>\n", quis);
+}
+
+static void cycle_seed(char **seedstr, random_state *rs)
+{
+    char newseed[16];
+    int j;
+
+    newseed[15] = '\0';
+    newseed[0] = '1' + (char)random_upto(rs, 9);
+    for (j = 1; j < 15; j++)
+        newseed[j] = '0' + (char)random_upto(rs, 10);
+    sfree(*seedstr);
+    *seedstr = dupstr(newseed);
+}
+
+static void start_soak(game_params *p, char *seedstr)
+{
+    time_t tt_start, tt_now, tt_last;
+    char *desc, *aux;
+    random_state *rs;
+    long n = 0, nnums = 0, i;
+    game_state *state;
+
+    tt_start = tt_now = time(NULL);
+    printf("Soak-generating a %dx%d grid.\n", p->w, p->h);
+
+    while (1) {
+       rs = random_new(seedstr, strlen(seedstr));
+       desc = thegame.new_desc(p, rs, &aux, 0);
+
+       state = thegame.new_game(NULL, p, desc);
+       for (i = 0; i < state->n; i++) {
+           if (state->flags[i] & FLAG_IMMUTABLE)
+               nnums++;
+       }
+       thegame.free_game(state);
+
+       sfree(desc);
+       cycle_seed(&seedstr, rs);
+       random_free(rs);
+
+       n++;
+       tt_last = time(NULL);
+       if (tt_last > tt_now) {
+           tt_now = tt_last;
+           printf("%ld total, %3.1f/s, %3.1f nums/grid (%3.1f%%).\n",
+                  n,
+                  (double)n / ((double)tt_now - tt_start),
+                  (double)nnums / (double)n,
+                  ((double)nnums * 100.0) / ((double)n * (double)p->w * (double)p->h) );
+       }
+    }
+}
+
+static void process_desc(char *id)
+{
+    char *desc, *err, *solvestr;
+    game_params *p;
+    game_state *s;
+
+    printf("%s\n  ", id);
+
+    desc = strchr(id, ':');
+    if (!desc) {
+        fprintf(stderr, "%s: expecting game description.", quis);
+        exit(1);
+    }
+
+    *desc++ = '\0';
+
+    p = thegame.default_params();
+    thegame.decode_params(p, id);
+    err = thegame.validate_params(p, 1);
+    if (err) {
+        fprintf(stderr, "%s: %s", quis, err);
+        thegame.free_params(p);
+        return;
+    }
+
+    err = thegame.validate_desc(p, desc);
+    if (err) {
+        fprintf(stderr, "%s: %s\nDescription: %s\n", quis, err, desc);
+        thegame.free_params(p);
+        return;
+    }
+
+    s = thegame.new_game(NULL, p, desc);
+
+    solvestr = thegame.solve(s, s, NULL, &err);
+    if (!solvestr)
+        fprintf(stderr, "%s\n", err);
+    else
+        printf("Puzzle is soluble.\n");
+
+    thegame.free_game(s);
+    thegame.free_params(p);
+}
+
+int main(int argc, const char *argv[])
+{
+    char *id = NULL, *desc, *err, *aux = NULL;
+    int soak = 0, verbose = 0, stdin_desc = 0, n = 1, i;
+    char *seedstr = NULL, newseed[16];
+
+    setvbuf(stdout, NULL, _IONBF, 0);
+
+    quis = argv[0];
+    while (--argc > 0) {
+        char *p = (char*)(*++argv);
+        if (!strcmp(p, "-v") || !strcmp(p, "--verbose"))
+            verbose = 1;
+        else if (!strcmp(p, "--stdin"))
+            stdin_desc = 1;
+        else if (!strcmp(p, "-e") || !strcmp(p, "--seed")) {
+            seedstr = dupstr(*++argv);
+            argc--;
+        } else if (!strcmp(p, "-n") || !strcmp(p, "--number")) {
+            n = atoi(*++argv);
+            argc--;
+        } else if (!strcmp(p, "-s") || !strcmp(p, "--soak")) {
+            soak = 1;
+        } else if (*p == '-') {
+            fprintf(stderr, "%s: unrecognised option `%s'\n", argv[0], p);
+            usage(stderr);
+            exit(1);
+        } else {
+            id = p;
+        }
+    }
+
+    sprintf(newseed, "%lu", time(NULL));
+    seedstr = dupstr(newseed);
+
+    if (id || !stdin_desc) {
+        if (id && strchr(id, ':')) {
+            /* Parameters and description passed on cmd-line:
+             * try and solve it. */
+            process_desc(id);
+        } else {
+            /* No description passed on cmd-line: decode parameters
+             * (with optional seed too) */
+
+            game_params *p = thegame.default_params();
+
+            if (id) {
+                char *cmdseed = strchr(id, '#');
+                if (cmdseed) {
+                    *cmdseed++ = '\0';
+                    sfree(seedstr);
+                    seedstr = dupstr(cmdseed);
+                }
+
+                thegame.decode_params(p, id);
+            }
+
+            err = thegame.validate_params(p, 1);
+            if (err) {
+                fprintf(stderr, "%s: %s", quis, err);
+                thegame.free_params(p);
+                exit(1);
+            }
+
+            /* We have a set of valid parameters; either soak with it
+             * or generate a single game description and print to stdout. */
+            if (soak)
+                start_soak(p, seedstr);
+            else {
+                char *pstring = thegame.encode_params(p, 0);
+
+                for (i = 0; i < n; i++) {
+                    random_state *rs = random_new(seedstr, strlen(seedstr));
+
+                    if (verbose) printf("%s#%s\n", pstring, seedstr);
+                    desc = thegame.new_desc(p, rs, &aux, 0);
+                    printf("%s:%s\n", pstring, desc);
+                    sfree(desc);
+
+                    cycle_seed(&seedstr, rs);
+
+                    random_free(rs);
+                }
+
+                sfree(pstring);
+            }
+            thegame.free_params(p);
+        }
+    }
+
+    if (stdin_desc) {
+        char buf[4096];
+
+        while (fgets(buf, sizeof(buf), stdin)) {
+	    buf[strcspn(buf, "\r\n")] = '\0';
+            process_desc(buf);
+        }
+    }
+    sfree(seedstr);
+
+    return 0;
+}
+
+#endif
+
+
+/* vim: set shiftwidth=4 tabstop=8: */