Asking whether an edge already exists scanned the whole edge list, so
building a graph of E edges cost O(E^2). A visibility sweep adds a quadratic
number of edges, so the whole construction was cubic: 600 satellites took
twelve seconds. An index of outgoing edge positions makes the membership
test constant time, and the same index answers neighbours and out-degree
without a scan. Removal rebuilds the index in one pass, since it renumbers
every later position. Six hundred satellites now take 130 ms, a factor of
ninety five.
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