Rounding error fix on WeightedIndex::update_weights()

[facorread]

WeightedIndex::update_weights() uses subtraction on old_w. Rounding errors, particularly on f32, may bring total_weight to below 0. This PR makes sure a negative total_weight results in an error.

[vks]

Would it be possible to add a test demonstrating this rounding error?

[dhardy]

Looks like a sensible change to me. I don't see the need for an extra test.

[vks]

Looks like a sensible change to me. I don't see the need for an extra test.

I think this would be nice to have, but the change certainly can be merged as is.

This needs to be rebased on master, because #945 was merged.

[facorread]

If you want, you can just enter the change by hand and commit to master, it might amount to less work than rebasing.

Thanks for your approval!

[dhardy]

@facorread you can probably just do:

git fetch origin
git rebase origin/master
# fix if necessary
git push --force

[facorread]

I am not authorized, sorry

[facorread]

Would it be possible to add a test demonstrating this rounding error?

I wanted to leave this example for future reference, for anyone who needs to understand this rounding error. Thank you.