TINKERPOP-1349: RepeatUnrollStrategy should unroll loops while maintaining equivalent semantics.

[okram]

https://issues.apache.org/jira/browse/TINKERPOP-1349

RepeatUnrollStrategy is a Standard-only strategy that will unroll a repeat() into a linear form if and only if it has a known loop amount (e.g. times(2)). That is repeat(out()).times(2) becomes out().barrier().out().barrier(). The barrier() insertions are necessary as repeat() itself is a barrier step and also, the benefit of bulking is leveraged. RepeatUnrollStrategy removes the need for RepeatStep, RepeatEndStep, and LoopTraversal.

RepeatUnrollStrategyTest ensures that unrolling is faster than not-unrolling. Here are some runtimes. The first item in the Pair is the runtime and the second item is the amount of unique objects being processed. With more unique objects, the faster it gets. Note that at 10 unique elements (tiny traversal), sometimes rolled is faster than unrolled and vice versa (i.e. they are equal).

rolled:[0.22843720299999998, 10]    unrolled:[0.174451055, 10]
rolled:[0.514145509, 100]       unrolled:[0.212532769, 100]
rolled:[1.4944357259999999, 1000]   unrolled:[0.441714987, 1000]
rolled:[28.502658626, 10000]        unrolled:[12.936112649999998, 10000]

Also in this ticket, I fixed a bug in BranchStep where children were not being integrated and thus, susceptible to clone-problems. Moreover, I added a timing test to IdentityRemovalStrategyTest. @spmallette, do you know how to make a test run once and only once in a Parameterized test case? If so, can you update IdentityRemoveStrategyTest and RepeatUnrollStrategyTest accordingly?

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CHANGELOG

* Added `RepeatUnrollStrategy` to linearize a `repeat()`-traversal if loop amount is known at compile time.
* Fixed a bug in `BranchStep` around child integration during `clone()`.
* Fixed a bug in `AbstractStep` around label set cloning.

VOTE +1.

[okram]

NOTE: This can not go into tp31/ because NoOpBarrierStep is new to TinkerPop 3.2.0.

[dkuppitz]

VOTE: +1

[spmallette]

Travis is failing - looks like an issue from Rat - missing an Apache header in TraversalStrategyPerformanceTest.

@okram I don't think there's a way to get a Parameterized test to run a single test once. It pretty much treats the whole test case as parameterized when you use that feature. Using Enclosed as you did is the only way to make that work afaik.

@pietermartin do you intend to vote on this one? you had a fair number of comments/questions on the JIRA ticket....

[pietermartin]

@spmallette No I do not intend to vote. The questions were about understanding why the barrier steps were needed and then to understand why barrier/bulking has such a significant performance impact. I have not read the RepeatUnrollStrategy code nor ran any tests for it.

[spmallette]

All tests pass with docker/build.sh -t -n -i

VOTE +1

[dkuppitz]

Same here, docker/build.sh -t -i -n reported success over night. All changes after my first vote were included in this test run.