Introduce a new game backend function (there seem to have been a lot

of these recently) whose job is to update a game_ui to be consistent
with a new game_state. This is called by midend.c in every situation
where the current game_state changes _other_ than as a result of
make_move (Undo, Redo, Restart, Solve).

The introduction of this function allows a game_ui to contain
information about selections or highlights within a game_state which
simply wouldn't make sense when transferred to another game_state.
In particular, I've used it to fix a subtle bug in Solo whereby,
although you couldn't right-click to pencil-mode highlight a filled
square, you could _get_ a pencil-mode highlight in a filled square
if you used Undo and Redo. (Undo to before the square was filled,
right-click to highlight it, then Redo. Alternatively, left-click
and clear the square, right-click to highlight it, then Undo.)

[originally from svn r5912]

cube.c

@@ -1003,6 +1003,11 @@ static void free_ui(game_ui *ui)
 {
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 struct game_drawstate {
     int ox, oy;                        /* pixel position of float origin */
 };
@@ -1637,6 +1642,7 @@ const struct game thegame = {
     FALSE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

fifteen.c

@@ -451,6 +451,11 @@ static void free_ui(game_ui *ui)
 {
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button) {
     int gx, gy, dx, dy, ux, uy, up, p;
@@ -835,6 +840,7 @@ const struct game thegame = {
     TRUE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

midend.c

@@ -228,6 +228,10 @@ void midend_new_game(midend_data *me)
 static int midend_undo(midend_data *me)
 {
     if (me->statepos > 1) {
+        if (me->ui)
+            me->ourgame->changed_state(me->ui,
+                                       me->states[me->statepos-1].state,
+                                       me->states[me->statepos-2].state);
 	me->statepos--;
         me->dir = -1;
         return 1;
@@ -238,6 +242,10 @@ static int midend_undo(midend_data *me)
 static int midend_redo(midend_data *me)
 {
     if (me->statepos < me->nstates) {
+        if (me->ui)
+            me->ourgame->changed_state(me->ui,
+                                       me->states[me->statepos-1].state,
+                                       me->states[me->statepos].state);
 	me->statepos++;
         me->dir = +1;
         return 1;
@@ -308,6 +316,10 @@ void midend_restart_game(midend_data *me)
     me->states[me->nstates].state = s;
     me->states[me->nstates].special = TRUE;   /* we just restarted */
     me->statepos = ++me->nstates;
+    if (me->ui)
+        me->ourgame->changed_state(me->ui,
+                                   me->states[me->statepos-2].state,
+                                   me->states[me->statepos-1].state);
     me->anim_time = 0.0;
     midend_finish_move(me);
     midend_redraw(me);
@@ -936,6 +948,10 @@ char *midend_solve(midend_data *me)
     me->states[me->nstates].state = s;
     me->states[me->nstates].special = TRUE;   /* created using solve */
     me->statepos = ++me->nstates;
+    if (me->ui)
+        me->ourgame->changed_state(me->ui,
+                                   me->states[me->statepos-2].state,
+                                   me->states[me->statepos-1].state);
     me->anim_time = 0.0;
     midend_finish_move(me);
     midend_redraw(me);

mines.c

@@ -2475,6 +2475,11 @@ static void free_ui(game_ui *ui)
     sfree(ui);
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button)
 {
@@ -3044,6 +3049,7 @@ const struct game thegame = {
     TRUE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

net.c

@@ -1786,6 +1786,11 @@ static void free_ui(game_ui *ui)
     sfree(ui);
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 /* ----------------------------------------------------------------------
  * Process a move.
  */
@@ -2591,6 +2596,7 @@ const struct game thegame = {
     FALSE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

netslide.c

@@ -1045,6 +1045,11 @@ static void slide_col(game_state *state, int dir, int col)
     slide_col_int(state->width, state->height, state->tiles, dir, col);
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *state, game_ui *ui,
                              game_drawstate *ds, int x, int y, int button)
 {
@@ -1749,6 +1754,7 @@ const struct game thegame = {
     FALSE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

nullgame.c

@@ -142,6 +142,11 @@ static void free_ui(game_ui *ui)
 {
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button)
 {
@@ -244,6 +249,7 @@ const struct game thegame = {
     FALSE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

pattern.c

@@ -758,6 +758,11 @@ static void free_ui(game_ui *ui)
     sfree(ui);
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button) {
     game_state *ret;
@@ -1127,6 +1132,7 @@ const struct game thegame = {
     FALSE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

puzzles.h

@@ -252,6 +252,8 @@ struct game {
     char *(*text_format)(game_state *state);
     game_ui *(*new_ui)(game_state *state);
     void (*free_ui)(game_ui *ui);
+    void (*changed_state)(game_ui *ui, game_state *oldstate,
+                          game_state *newstate);
     game_state *(*make_move)(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button);
     void (*size)(game_params *params, int *x, int *y);

rect.c

@@ -2183,6 +2183,11 @@ static void ui_draw_rect(game_state *state, game_ui *ui,
             }
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button) {
     int xc, yc;
@@ -2549,6 +2554,7 @@ const struct game thegame = {
     TRUE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

sixteen.c

@@ -578,6 +578,11 @@ static void free_ui(game_ui *ui)
 {
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button) {
     int cx, cy;
@@ -1006,6 +1011,7 @@ const struct game thegame = {
     TRUE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

solo.c

@@ -1853,6 +1853,22 @@ static void free_ui(game_ui *ui)
     sfree(ui);
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+    int c = newstate->c, r = newstate->r, cr = c*r;
+    /*
+     * We prevent pencil-mode highlighting of a filled square. So
+     * if the user has just filled in a square which we had a
+     * pencil-mode highlight in (by Undo, or by Redo, or by Solve),
+     * then we cancel the highlight.
+     */
+    if (ui->hx >= 0 && ui->hy >= 0 && ui->hpencil &&
+        newstate->grid[ui->hy * cr + ui->hx] != 0) {
+        ui->hx = ui->hy = -1;
+    }
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button)
 {
@@ -2278,6 +2294,7 @@ const struct game thegame = {
     TRUE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,

twiddle.c

@@ -616,6 +616,11 @@ static void free_ui(game_ui *ui)
 {
 }
 
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
 static game_state *make_move(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button)
 {
@@ -1174,6 +1179,7 @@ const struct game thegame = {
     TRUE, game_text_format,
     new_ui,
     free_ui,
+    game_changed_state,
     make_move,
     game_size,
     game_colours,