[hilbert_space] Simplify algorithm of space_partition::merge_subspaces()
#961
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…es()` The new non-recursive algorithm is simpler, faster and safe from stack overflows.
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The current implementation of
space_partition::merge_subspaces()uses recursion,std::functionand repeatedly deletes elements from astd::multimap, which results in a significat runtime overhead.More concerningly, I experienced a stack oveflow caused by the recursion while trying to run the autopartition algorithm for a bigger system.
The new algorithm simply iterates over subspace connections$i\mapsto f$ generated by a single operator $c^\dagger/c$ , and merges all subspaces $f$ corresponding to the same $i$ .