The pruned nodes (in red) represent entire regions of space that the algorithm never examines. The points inside those regions are never checked. Compare the "Nodes Visited" count to the total number of points. The quadtree is doing far less work than a brute-force scan.
Jonathan Head,South East Asia correspondentand
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For implementers, backpressure adds complexity without providing guarantees. The machinery to track queue sizes, compute desiredSize, and invoke pull() at the right times must all be implemented correctly. However, since these signals are advisory, all that work doesn't actually prevent the problems backpressure is supposed to solve.
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