public void apply()

in gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/traversal/strategy/optimization/MessagePassingReductionStrategy.java [78:135]
45 lines of code
21 McCabe index (conditional complexity)

    public void apply(final Traversal.Admin<?, ?> traversal) {
        // only process the first traversal step in an OLAP chain
        TraversalHelper.getFirstStepOfAssignableClass(TraversalVertexProgramStep.class, traversal).ifPresent(step -> {
            final Graph graph = traversal.getGraph().orElse(EmptyGraph.instance()); // best guess at what the graph will be as its dynamically determined
            final Traversal.Admin<?, ?> compiledComputerTraversal = step.generateProgram(graph, EmptyMemory.instance()).getTraversal().get().clone();
            if (!compiledComputerTraversal.isLocked())
                compiledComputerTraversal.applyStrategies();
            if (!TraversalHelper.hasStepOfAssignableClassRecursively(Arrays.asList(LocalStep.class, LambdaHolder.class), compiledComputerTraversal) && // don't do anything with lambdas or locals as this leads to unknown adjacencies
                    !compiledComputerTraversal.getTraverserRequirements().contains(TraverserRequirement.PATH) &&            // when dynamic detachment is provided in 3.3.0, remove this (TODO)
                    !compiledComputerTraversal.getTraverserRequirements().contains(TraverserRequirement.LABELED_PATH) &&    // when dynamic detachment is provided in 3.3.0, remove this (TODO)
                    !(TraversalHelper.getStepsOfAssignableClass(TraversalParent.class, compiledComputerTraversal).          // this is a strict precaution that could be loosed with deeper logic on barriers in global children
                            stream().
                            filter(parent -> !parent.getGlobalChildren().isEmpty()).findAny().isPresent())) {
                final Traversal.Admin<?, ?> computerTraversal = step.computerTraversal.get().clone();
                if (computerTraversal.getSteps().size() > 1 &&
                        !(computerTraversal.getStartStep().getNextStep() instanceof Barrier) &&
                        TraversalHelper.hasStepOfAssignableClassRecursively(Arrays.asList(VertexStep.class, EdgeVertexStep.class), computerTraversal) &&
                        TraversalHelper.isLocalStarGraph(computerTraversal)) {
                    Step barrier = (Step) TraversalHelper.getFirstStepOfAssignableClass(Barrier.class, computerTraversal).orElse(null);

                    // if the barrier isn't present gotta check for uncapped profile() which can happen if you do
                    // profile("metrics") - see below for more worries
                    if (null == barrier) {
                        final ProfileSideEffectStep pses = TraversalHelper.getFirstStepOfAssignableClass(ProfileSideEffectStep.class, computerTraversal).orElse(null);
                        if (pses != null)
                            barrier = pses.getPreviousStep();
                    }

                    // if the barrier is a profile() then we'll mess stuff up if we wrap that in a local() as in:
                    //    local(..., ProfileSideEffectStep)
                    // which won't compute right on OLAP (or anything??). By stepping back we cut things off at
                    // just before the ProfileSideEffectStep to go inside the local() so that ProfileSideEffectStep
                    // shows up just after it
                    //
                    // why does this strategy need to know so much about profile!?!
                    if (barrier != null && barrier.getPreviousStep() instanceof ProfileSideEffectStep)
                        barrier = barrier.getPreviousStep().getPreviousStep();

                    if (MessagePassingReductionStrategy.insertElementId(barrier)) // out().count() -> out().id().count()
                        TraversalHelper.insertBeforeStep(new IdStep(computerTraversal), barrier, computerTraversal);
                    if (!(MessagePassingReductionStrategy.endsWithElement(null == barrier ? computerTraversal.getEndStep() : barrier))) {
                        Traversal.Admin newChildTraversal = new DefaultGraphTraversal<>();
                        TraversalHelper.removeToTraversal(computerTraversal.getStartStep() instanceof GraphStep ?
                                computerTraversal.getStartStep().getNextStep() :
                                (Step) computerTraversal.getStartStep(), null == barrier ?
                                EmptyStep.instance() :
                                barrier, newChildTraversal);
                        newChildTraversal = newChildTraversal.getSteps().size() > 1 ? (Traversal.Admin) __.local(newChildTraversal) : newChildTraversal;
                        if (null == barrier)
                            TraversalHelper.insertTraversal(0, newChildTraversal, computerTraversal);
                        else
                            TraversalHelper.insertTraversal(barrier.getPreviousStep(), newChildTraversal, computerTraversal);
                    }
                }
                step.setComputerTraversal(computerTraversal);
            }
        });
    }