Make turn restrictions faster and thread-safe by moving them into StreetEdge

[leonardehrenfried]

Summary

It moves the turn restrictions form a global map to the individual StreetEdge instances.

This has several advantages:

• It makes it easier to reason about
• Profiling has shown that the map access is comparatively slow
• It's easier to make a method on StreetEdge thread-safe than one on Graph

Thread safety

This is actually a bit trickier than one might suspect since we have the following constraints:

• not setting the turn restrictions to null (as this would make access be synchronized)
• not creating a separate empty set for every street edge
• synchronized or concurrent modification
• fast access
• being able to serialize the turn restriction collection

For these reasons I've chosen the following design:

• a global instance of an empty set of turn restrictions that is shared between all instances that have none
• synchronized modification with copy-on-write semantics
• fast access without extra copying or allocations by making the collection immutable after modification

Issue

Relates to #3884 but not confirmed that it solves the issue.

Unit tests

Unit tests adjusted

Code style

yes

Documentation

Plenty of Javadoc added.

[hannesj]

This seems to reduce the time spent in StreetRouter from 19.5 ms to 18.0 ms in the SpeedTest.

kryo_set_serialization.diff can be used to fix the serialization issue, with the java.util.ImmutableCollections Set

[t2gran]

nice

[t2gran]

There is probably a long discussion on the use of Set vs List here. And I would agree that we sometimes want to use Set just to communicate the "unique"-ness of the elements. But, Set have a bigger overhead to then (maybe not for 1 and 2 elements), but at least mentally - so wen memory footprint and performance is high priority I prefer to use list, and just make sure the construction of them enforce the "unique"-ness constraint. Also, in this case there is no need to expose the collection type outside the StreetEdge.

[leonardehrenfried]

There is probably a long discussion on the use of Set vs List here. And I would agree that we sometimes want to use Set just to communicate the "unique"-ness of the elements. But, Set have a bigger overhead to then (maybe not for 1 and 2 elements), but at least mentally - so wen memory footprint and performance is high priority I prefer to use list, and just make sure the construction of them enforce the "unique"-ness constraint. Also, in this case there is no need to expose the collection type outside the StreetEdge.

I'm happy to go along with this suggestion.

[leonardehrenfried]

🙏