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Core Calcite APIs and engine.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to you under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.calcite.plan;

import org.apache.calcite.rel.RelNode;

/**
 * Calling convention trait.
 */
public interface Convention extends RelTrait {
  /**
   * Convention that for a relational expression that does not support any
   * convention. It is not implementable, and has to be transformed to
   * something else in order to be implemented.
   *
   * Relational expressions generally start off in this form.
   *
   * Such expressions always have infinite cost.
   */
  Convention NONE = new Impl("NONE", RelNode.class);

  Class getInterface();

  String getName();

  /**
   * Returns whether we should convert from this convention to
   * {@code toConvention}. Used by {@link ConventionTraitDef}.
   *
   * @param toConvention Desired convention to convert to
   * @return Whether we should convert from this convention to toConvention
   */
  boolean canConvertConvention(Convention toConvention);

  /**
   * Returns whether we should convert from this trait set to the other trait
   * set.
   *
   * The convention decides whether it wants to handle other trait
   * conversions, e.g. collation, distribution, etc.  For a given convention, we
   * will only add abstract converters to handle the trait (convention,
   * collation, distribution, etc.) conversions if this function returns true.
   *
   * @param fromTraits Traits of the RelNode that we are converting from
   * @param toTraits Target traits
   * @return Whether we should add converters
   */
  boolean useAbstractConvertersForConversion(RelTraitSet fromTraits,
      RelTraitSet toTraits);

  /**
   * Default implementation.
   */
  class Impl implements Convention {
    private final String name;
    private final Class relClass;

    public Impl(String name, Class relClass) {
      this.name = name;
      this.relClass = relClass;
    }

    @Override public String toString() {
      return getName();
    }

    public void register(RelOptPlanner planner) {}

    public boolean satisfies(RelTrait trait) {
      return this == trait;
    }

    public Class getInterface() {
      return relClass;
    }

    public String getName() {
      return name;
    }

    public RelTraitDef getTraitDef() {
      return ConventionTraitDef.INSTANCE;
    }

    public boolean canConvertConvention(Convention toConvention) {
      return false;
    }

    public boolean useAbstractConvertersForConversion(RelTraitSet fromTraits,
        RelTraitSet toTraits) {
      return false;
    }
  }
}

// End Convention.java