Diffusion-limited aggregation.pic.twitter.com/orBCZ0hQAG
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I may be missing something, but particles that get "reset" don't keep their positions and this makes the growth take much longer at the end. See this cell as well as the "alone" cell https://beta.observablehq.com/compare/6476797a385e8d05...8c95ed9efeef0aa7#updateSingleParticle … (I couldn't resist adding some other changes too...)
Yup, had noticed that myself, and @tmcw has already fixed it. Neat thing is, with that fix in place you can add a bias to the movement to direct the growth. :)
I just noticed a bigger issue, which is that Michael Fogleman's name was misspelled. 
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