Faster union find #415
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Faster union find #415
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This speeds up
single_linkageby about two orders of magnitude by using direct array access infast_findwhich Cython can compile into a pure C loop.I saw in the commit history that this was attempted before but abandoned due to unexplained crashes. The reason is that the current code is actually buggy and only works because python's wraparound indexing hides the problem. When using raw pointer access the bug leads to heap corruption and subsequent random crashes. A simple check to only label if we are not at the root already fixes this.
Also fixes #321 which should further improve overall performance.