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Galaxies: disallow placing an edge touching a dot.

Trivially, no edge of this kind can be part of any legal solution, so
it's a minor UI affordance that doesn't spoil any of the puzzly
thinking to prevent the user accidentally placing them in the first
place.

Suggestion from Larry Hastings, although this is not his patch.

--- a/galaxies.c

+++ b/galaxies.c

@@ -2473,6 +2473,21 @@

     return NULL;

 }

 #else

+static bool edge_placement_legal(const game_state *state, int x, int y)

+{

+    space *sp = &SPACE(state, x, y);

+    if (sp->type != s_edge)

+        return false;   /* this is a face-centre or a grid vertex */

+

+    /* Check this line doesn't actually intersect a dot */

+    unsigned int flags = (GRID(state, grid, x, y).flags |

+                          GRID(state, grid, x & ~1U, y & ~1U).flags |

+                          GRID(state, grid, (x+1) & ~1U, (y+1) & ~1U).flags);

+    if (flags & F_DOT)

+        return false;

+    return true;

+}

+

 static char *interpret_move(const game_state *state, game_ui *ui,

                             const game_drawstate *ds,

                             int x, int y, int button)

@@ -2510,13 +2525,11 @@

                             &px, &py);

         if (!INUI(state, px, py)) return NULL;

+        if (!edge_placement_legal(state, px, py))

+            return NULL;

-        sp = &SPACE(state, px, py);

-        assert(sp->type == s_edge);

-        {

-            sprintf(buf, "E%d,%d", px, py);

-            return dupstr(buf);

-        }

+        sprintf(buf, "E%d,%d", px, py);

+        return dupstr(buf);

     } else if (button == RIGHT_BUTTON) {

         int px1, py1;

@@ -2677,7 +2690,8 @@

             ui->srcx = ui->cur_x;

             ui->srcy = ui->cur_y;

             return UI_UPDATE;

-        } else if (sp->type == s_edge) {

+        } else if (sp->type == s_edge &&

+                   edge_placement_legal(state, ui->cur_x, ui->cur_y)) {

             sprintf(buf, "E%d,%d", ui->cur_x, ui->cur_y);

             return dupstr(buf);

         }