changed flood fill function to prevent stack overflow crashes

The previous implementation of the flood fill function (as used in the
level editor) used recursion, which could cause the program to crash
on some systems when using it with maximum level size. This was caused
by a stack overflow on systems that use a stack size not large enough
for the deep recursion involved in such cases. (Such crashes happened
on Windows systems, which usually have a stack size of 1 MB, while it
did not happen on Linux systems, which usually have 8 MB stack size.)

The new flood fill function which uses an iterative algorithm does not
have this problem (as the memory used for it is not on the stack).

diff --git a/src/tools.c b/src/tools.c
index 1e7fef67ff45883163b177bdc6ad061889f50df6..60d1072cb852046864b5ff0597785ce84ae1756e 100644 (file)
--- a/src/tools.c
+++ b/src/tools.c
@@ -1416,39 +1416,42 @@ void SetBorderElement(void)
   }
 }
 
-void FloodFillLevelExt(int from_x, int from_y, int fill_element,
+void FloodFillLevelExt(int start_x, int start_y, int fill_element,
                       int max_array_fieldx, int max_array_fieldy,
                       short field[max_array_fieldx][max_array_fieldy],
                       int max_fieldx, int max_fieldy)
 {
-  int i,x,y;
-  int old_element;
-  static int check[4][2] = { { -1, 0 }, { 0, -1 }, { 1, 0 }, { 0, 1 } };
-  static int safety = 0;
+  static struct XY stack_buffer[MAX_LEV_FIELDX * MAX_LEV_FIELDY];
+  static struct XY check[4] = { { -1, 0 }, { 0, -1 }, { 1, 0 }, { 0, 1 } };
+  int old_element = field[start_x][start_y];
+  int stack_pos = 0;
 
-  // check if starting field still has the desired content
-  if (field[from_x][from_y] == fill_element)
+  // do nothing if start field already has the desired content
+  if (old_element == fill_element)
     return;
 
-  safety++;
+  stack_buffer[stack_pos++] = (struct XY){ start_x, start_y };
 
-  if (safety > max_fieldx * max_fieldy)
-    Fail("Something went wrong in 'FloodFill()'. Please debug.");
+  while (stack_pos > 0)
+  {
+    struct XY current = stack_buffer[--stack_pos];
+    int i;
 
-  old_element = field[from_x][from_y];
-  field[from_x][from_y] = fill_element;
+    field[current.x][current.y] = fill_element;
 
-  for (i = 0; i < 4; i++)
-  {
-    x = from_x + check[i][0];
-    y = from_y + check[i][1];
+    for (i = 0; i < 4; i++)
+    {
+      int x = current.x + check[i].x;
+      int y = current.y + check[i].y;
 
-    if (IN_FIELD(x, y, max_fieldx, max_fieldy) && field[x][y] == old_element)
-      FloodFillLevelExt(x, y, fill_element, max_array_fieldx, max_array_fieldy,
-                       field, max_fieldx, max_fieldy);
-  }
+      // check for stack buffer overflow (should not happen)
+      if (stack_pos >= MAX_LEV_FIELDX * MAX_LEV_FIELDY)
+       Fail("Stack buffer overflow in 'FloodFillLevelExt()'. Please debug.");
 
-  safety--;
+      if (IN_FIELD(x, y, max_fieldx, max_fieldy) && field[x][y] == old_element)
+       stack_buffer[stack_pos++] = (struct XY){ x, y };
+    }
+  }
 }
 
 void FloodFillLevel(int from_x, int from_y, int fill_element,