tile map bounds checking

If any one can help me with the bounds checking for my tilemap I would greatly appreciate it.

My tilemap engine works perfectly untill it tries to scroll to the very horizontal or vertical limit. If I scroll by the size of the tiles used then it gets to the xMaxPos (or yMaxPos) position without a hitch. I am therefore assuming that these two values are being calculated correctly.

The problem occurs when I try to scroll to the boundary by a unit less than the tile size or try to scroll away from the boundary when it's at the max position. The progam crashes very ungracefully.

I searched for threads on similar problems and an answer that comes up occasonally is to just add an extra row and column of tiles. I can't figure this out because surely as I approach the max coordinates or leave the max cordinates it disects the last visible tile, which can be displayed with no problems...

Code:

void TileMapLayer::draw(BITMAP *dest) {
   int xSrc, ySrc;
   int xDest, yDest;
   int topTile = xPos / xTileSize;
   int leftTile = yPos / yTileSize;
   int topTileOffset = xPos % xTileSize;
   int leftTileOffset = yPos % yTileSize;
   int bottomTile  = (int)((xPos + xScreenSize) / (float)xTileSize + 0.5) + ((topTileOffset == 0) ? 0 : 1);
   int rightTile = (int)((yPos + yScreenSize) / (float)yTileSize + 0.5) + ((leftTileOffset == 0) ? 0 : 1);
   for(xSrc = topTile; xSrc < bottomTile; xSrc++ ) {
      for(ySrc = leftTile; ySrc < rightTile; ySrc++ ) {
         xDest = (xSrc - topTile) * xTileSize - topTileOffset;
         yDest = (ySrc - leftTile) * yTileSize - leftTileOffset;
         blit(cell[xSrc][ySrc].image, dest, 0, 0, xDest, yDest, xTileSize, yTileSize);
      }
   }
}

bool TileMapLayer::scrollUp(int yOffset) {
   yPos -= yOffset;
   if (yPos < 0) {
      yPos = 0;
      return false;
   }
   return true;
}

bool TileMapLayer::scrollDown(int yOffset) {
   yPos += yOffset;
   if (yPos > yMaxPos - yTileSize) {
      yPos = yMaxPos - yTileSize;
      return false;
   }
   return true;
}

bool TileMapLayer::scrollLeft(int xOffset) {
   xPos -= xOffset;
   if (xPos < 0) {
      xPos = 0;
      return false;
   }
   return true;	
}

bool TileMapLayer::scrollRight(int xOffset) {
   xPos += xOffset;
   if (xPos > xMaxPos - xTileSize) {
      xPos = xMaxPos - xTileSize;
      return false;
   }
   return true;	
}

Have you tried walking this through a debugger and monitoring your values and pointers?

this is a classic range check problem. Because you're not aligned, your exceeding a boundary you're just not checking for because you've assumed it's taken care of.

1) Flow chart your program and test values
2) Walk it with a debugger and see specifically where it breaks.

This is how you solve this problem and learn by it.

Someone else won't always be around to help you.

Good luck.