/*
 * 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.rel.rules;

import org.apache.calcite.plan.RelOptRule;
import org.apache.calcite.plan.RelOptRuleCall;
import org.apache.calcite.plan.RelOptUtil;
import org.apache.calcite.rel.RelNode;
import org.apache.calcite.rel.core.Aggregate;
import org.apache.calcite.rel.core.Aggregate.Group;
import org.apache.calcite.rel.core.AggregateCall;
import org.apache.calcite.rel.core.Project;
import org.apache.calcite.rel.core.RelFactories;
import org.apache.calcite.rex.RexInputRef;
import org.apache.calcite.rex.RexNode;
import org.apache.calcite.tools.RelBuilder;
import org.apache.calcite.tools.RelBuilderFactory;
import org.apache.calcite.util.ImmutableBitSet;
import org.apache.calcite.util.mapping.Mappings;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * Planner rule that recognizes a {@link org.apache.calcite.rel.core.Aggregate}
 * on top of a {@link org.apache.calcite.rel.core.Project} and if possible
 * aggregate through the project or removes the project.
 *
 * <p>This is only possible when the grouping expressions and arguments to
 * the aggregate functions are field references (i.e. not expressions).
 *
 * <p>In some cases, this rule has the effect of trimming: the aggregate will
 * use fewer columns than the project did.
 */
public class AggregateProjectMergeRule extends RelOptRule implements TransformationRule {
  public static final AggregateProjectMergeRule INSTANCE =
      new AggregateProjectMergeRule(Aggregate.class, Project.class, RelFactories.LOGICAL_BUILDER);

  public AggregateProjectMergeRule(
      Class<? extends Aggregate> aggregateClass,
      Class<? extends Project> projectClass,
      RelBuilderFactory relBuilderFactory) {
    super(
        operand(aggregateClass,
            operand(projectClass, any())),
        relBuilderFactory, null);
  }

  public void onMatch(RelOptRuleCall call) {
    final Aggregate aggregate = call.rel(0);
    final Project project = call.rel(1);
    RelNode x = apply(call, aggregate, project);
    if (x != null) {
      call.transformTo(x);
    }
  }

  public static RelNode apply(RelOptRuleCall call, Aggregate aggregate,
      Project project) {
    // Find all fields which we need to be straightforward field projections.
    final Set<Integer> interestingFields = RelOptUtil.getAllFields(aggregate);

    // Build the map from old to new; abort if any entry is not a
    // straightforward field projection.
    final Map<Integer, Integer> map = new HashMap<>();
    for (int source : interestingFields) {
      final RexNode rex = project.getProjects().get(source);
      if (!(rex instanceof RexInputRef)) {
        return null;
      }
      map.put(source, ((RexInputRef) rex).getIndex());
    }

    final ImmutableBitSet newGroupSet = aggregate.getGroupSet().permute(map);
    ImmutableList<ImmutableBitSet> newGroupingSets = null;
    if (aggregate.getGroupType() != Group.SIMPLE) {
      newGroupingSets =
          ImmutableBitSet.ORDERING.immutableSortedCopy(
              ImmutableBitSet.permute(aggregate.getGroupSets(), map));
    }

    final ImmutableList.Builder<AggregateCall> aggCalls =
        ImmutableList.builder();
    final int sourceCount = aggregate.getInput().getRowType().getFieldCount();
    final int targetCount = project.getInput().getRowType().getFieldCount();
    final Mappings.TargetMapping targetMapping =
        Mappings.target(map, sourceCount, targetCount);
    for (AggregateCall aggregateCall : aggregate.getAggCallList()) {
      aggCalls.add(aggregateCall.transform(targetMapping));
    }

    final Aggregate newAggregate =
        aggregate.copy(aggregate.getTraitSet(), project.getInput(),
            newGroupSet, newGroupingSets, aggCalls.build());

    // Add a project if the group set is not in the same order or
    // contains duplicates.
    final RelBuilder relBuilder = call.builder();
    relBuilder.push(newAggregate);
    final List<Integer> newKeys =
        Lists.transform(aggregate.getGroupSet().asList(), map::get);
    if (!newKeys.equals(newGroupSet.asList())) {
      final List<Integer> posList = new ArrayList<>();
      for (int newKey : newKeys) {
        posList.add(newGroupSet.indexOf(newKey));
      }
      for (int i = newAggregate.getGroupCount();
           i < newAggregate.getRowType().getFieldCount(); i++) {
        posList.add(i);
      }
      relBuilder.project(relBuilder.fields(posList));
    }

    return relBuilder.build();
  }
}