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Search for all points in a specific radius of another point

I am making a simple game and stumbled upon this problem. Assume that there are several points in a 2D space. I want points close to each other to interact in some way.

Let me take a picture here for a better understanding of the problem: image of the problem

Now the problem is not calculating the distance . I know how to do this.

At first I had about 10 points, and I could just check each combination, but, as you can already assume, this is extremely ineffective with increasing the number of points. What if I had only a million points, but all of them would be very far from each other?

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You can also go to cells smaller than R. You will have fewer candidates who cannot be close enough. For example, with R / 2, you should check 25 link lists instead of 9, but have on average (if randomly distributed) the 25/36th as many points to check. This may be a minor gain.

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