Add function to approximate minimum Steiner tree #392
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mtreinish
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This commit adds a function to find an approximation of the minimum Steiner tree using the algorithm described in "A fast algorithm for Steiner tree" Kou, Markowsky & Berman (1981). https://link.springer.com/article/10.1007/BF00288961 This algorithm produces a tree whose weight is within a (2 - (2 / t)) factor of the weight of the optimal Steiner tree where t is the number of terminal nodes. Closes Qiskit#389
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hmm, the test I borrowed form networkx seems to have something non-deterministic in it: https://github.com/Qiskit/retworkx/pull/392/checks?check_run_id=3251676024#step:7:1251 it's failing in some cases. I'll have to dig into that. |
In Qiskit#390 one of the last changes made to the PR before merging was: Qiskit@3353b4b which changed the final loop constructing the metric closure edges from iterating over node indices and pulling the path from the hashmap for that index to just iterating over the hashmap. That however had the unintended side effect of introducing non-determinism to the output as the iteration order over a hashmap isn't guaranteed. This was causing failures in the steiner tree function as the metric closure edge order can affect the computed tree. This commit fixes this by switching back to using the node id order for the final output generation and adds a comment as to why.
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eb2a0aa should fix the test non-determinism and I think this is ready now |
georgios-ts
reviewed
Aug 11, 2021
georgios-ts
reviewed
Aug 12, 2021
Co-authored-by: georgios-ts <[email protected]>
Co-authored-by: georgios-ts <[email protected]>
georgios-ts
approved these changes
Aug 13, 2021
This adds a test case with an input graph that has equal distances. It is a test based on a review comment [1] that was previously producing an incorrect output (that wasn't even a tree). After fixing the underlying issue it would be good to have this tested to ensure we don't regress on it in the future. Co-authored-by: georgios-ts <[email protected]> [1] Qiskit#392 (comment)
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This commit adds a function to find an approximation of the minimum
Steiner tree using the algorithm described in "A fast algorithm for
Steiner tree" Kou, Markowsky & Berman (1981).
https://link.springer.com/article/10.1007/BF00288961
This algorithm produces a tree whose weight is within a (2 - (2 / t))
factor of the weight of the optimal Steiner tree where t is the number
of terminal nodes.
Closes #389
TODO: