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swiftcoder

Strange array overflow (aka WTF I missing here?)

3 posts in this topic

So I have this fun little function, which reliably crashes at line D (commented below), only if TILE_SIZE is set to an odd number:
const int TILE_SIZE = 7;

Tile *create(const std::array<vector3, 4> &corners) {
   std::vector<vector3> verts; // line A
   std::array<vector3, TILE_SIZE/2> past; // line B

   for (int i = 0; i < TILE_SIZE; ++i) {
      vector3 hi = cml::lerp(corners[0], corners[1], i / float(TILE_SIZE-1));
      vector3 lo = cml::lerp(corners[3], corners[2], i / float(TILE_SIZE-1));

      for (int j = 0; j < TILE_SIZE; ++j) {
         vector3 v = cml::lerp(lo, hi, j / float(TILE_SIZE-1)) );

         if (((i & 1) == 0 || i == TILE_SIZE-1) && ((j & 1) == 0 || j == TILE_SIZE-1)) {
            past[j/2] = v; // line C
         }

         verts.push_back(v); // line D
         verts.push_back(past[j/2]);
      }
   }

// ...
}
The debugger tells me that this is because the [tt]verts[/tt] array has an impossibly large size, which can be fixed by increasing the size of the [tt]past[/tt] array by 1, commenting line C, or swapping lines A and B.

"Array overflow", you say, "silly dev has an off-by-one error in line C", you say. And I'd agree with you, except I can't see it for the life of me. With TILE_SIZE=7, the [tt]past[/tt] array should have a length of 7/2=3, and the inner for loop counts up to (7-1)/2=3, which seems fine, and has been validated with much printf-debugging...

Any ideas to relieve my current anxiety?
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If TILE_SIZE is 7, then the loop goes from j=0 to j=6. When j is equal to 6, it tries to push past[j/2] onto verts. Since past has a length of TILE_SIZE/2 or 3, its valid indices go from 0 to 2. I am not sure at the moment how to fix this though.

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its valid indices go from 0 to 2.

Crap. After all these years, zero-based indexing still occasionally trips me up. And another pair of eyes always helps smile.png
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Try setting the past array size to ((TILE_SIZE + 1) / 2)? Though, I'm sure solutions are forthcoming now that the problem is pointed out.

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