Speedup VariableOrdering.append from O(N^2) to O(N)

[ahojnnes]

Motivation and Context

When specifying the ordering for larger problems with many thousands of variables, the current implementation becomes very slow due to the O(N^2) complexity caused by the O(N) complexity of checking existence of var in the _var_order list. For small problems, the new implementation is a little slower but the difference is negligible. For large problems, the speed goes down from multiple 10s of seconds to

How Has This Been Tested

The runtime has been tested on various bundle adjustment problems. The functionality should be covered by existing unit tests with unchanged behavior.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

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[ahojnnes]

CI failures seem unrelated to the changes in this PR.

[luisenp]

Thanks so much for this fix. It's unfortunate how such a careless but small oversight had so large of an impact. I'll make another PR first to fix the CI errors. Then we can merge.

[luisenp]

@ahojnnes Ci should be fixed now if you rebase on top of latest main.

[ahojnnes]

@luisenp Thanks for fixing the CI. I rebased on latest master. Cheers.

[ahojnnes]

@luisenp Another (seemingly transient) CI failure. Might need a retrigger of the build, which I don't have permission for.

[luisenp]

LGTM. Thanks for the fix!