Fixed block collection with classical conditions in Collect2QBlocks #2956
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Aug 12, 2019
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Aug 5, 2020
…iskit#2956) * Updated calls to be the quantum versions * Added in a test case * Fixed linting
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Summary
As revealed in the random testing there was a niche bug in Collect2QBlocks that lead to blocks of 3 qubits being collected. This occured because the predeccessors/successors of the nodes were added using the calls
predecessors()andsuccessors()rather thanquantum_predecessors()andquantum_successors(). This meant that if there was a pred/succ node that shared the same classical register, but was not acxnode it could be added despite potentially not sharing the same qubits as the node currently being looked at.As it is blocks of quantum gates we are looking for, it makes sense to only look at the quantum successors/predecessors.
Details and comments
Solved by replacing all the relevant calls.
NB there are still some calls to
predecessors()andsuccessors()as these calls are looking at the ordering of the nodes, so they need to take into account any classical aspects as well as quantum.