maybe if you create a ton of variables (within a type) holding the x values of each vertical line, and y value of each horizontal line, and say that if this line is complete then put it in the run for inter-line collisions. All inter-line collision runners will have their x and y (depending if they're vertical or horizontal) checked with the incomplete lines. It's kind of hard to explain. If the x of a vertical line is equal to the x of a horizontal line, then they are touching eachother. If the y of a horizontal line is equal to the y of a vertical line, then they are touching, and growth should stop. I didn't take time to look at your code, so this is all theoretical.