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package overflowdb.tinkerpop.optimizations;

import org.apache.tinkerpop.gremlin.process.traversal.Step;
import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
import org.apache.tinkerpop.gremlin.process.traversal.TraversalStrategy;
import org.apache.tinkerpop.gremlin.process.traversal.step.TraversalParent;
import org.apache.tinkerpop.gremlin.process.traversal.step.map.CountGlobalStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.map.GraphStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.map.NoOpBarrierStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.sideEffect.AggregateStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.sideEffect.IdentityStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.sideEffect.SideEffectStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.util.CollectingBarrierStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.util.EmptyStep;
import org.apache.tinkerpop.gremlin.process.traversal.strategy.AbstractTraversalStrategy;
import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalHelper;
import org.apache.tinkerpop.gremlin.structure.Element;

import java.util.Collections;
import java.util.List;
import java.util.Set;

/**
 * This strategy will do a direct index size call if the traversal is a count of the vertices and edges of the
 * graph or a one-to-one map chain thereof.
 *
 * @author Marko A. Rodriguez (http://markorodriguez.com)
 * example:  * g.V().count()               // is replaced by TinkerCountGlobalStep
 * g.V().map(out()).count()    // is replaced by TinkerCountGlobalStep
 * g.E().label().count()       // is replaced by TinkerCountGlobalStep
 * 
 */
public final class CountStrategy extends AbstractTraversalStrategy implements TraversalStrategy.ProviderOptimizationStrategy {

  private static final CountStrategy INSTANCE = new CountStrategy();

  private CountStrategy() {
  }

  @Override
  public void apply(final Traversal.Admin traversal) {
    if (!(traversal.getParent() instanceof EmptyStep) || TraversalHelper.onGraphComputer(traversal))
      return;
    final List steps = traversal.getSteps();
    if (steps.size() < 2 ||
        !(steps.get(0) instanceof GraphStep) ||
        0 != ((GraphStep) steps.get(0)).getIds().length ||
        !(steps.get(steps.size() - 1) instanceof CountGlobalStep))
      return;
    for (int i = 1; i < steps.size() - 1; i++) {
      final Step current = steps.get(i);
      if (!(//current instanceof MapStep ||  // MapSteps will not necessarily emit an element as demonstrated in https://issues.apache.org/jira/browse/TINKERPOP-1958
          current instanceof IdentityStep ||
              current instanceof NoOpBarrierStep ||
              current instanceof CollectingBarrierStep) ||
          (current instanceof TraversalParent &&
              TraversalHelper.anyStepRecursively(s -> (s instanceof SideEffectStep || s instanceof AggregateStep), (TraversalParent) current)))
        return;
    }
    final Class elementClass = ((GraphStep) steps.get(0)).getReturnClass();
    TraversalHelper.removeAllSteps(traversal);
    traversal.addStep(new OdbCountStep<>(traversal, elementClass));
  }

  @Override
  public Set> applyPost() {
    return Collections.singleton(OdbGraphStepStrategy.class);
  }

  public static CountStrategy instance() {
    return INSTANCE;
  }
}