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The plugin that powers ESQL for Elasticsearch

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/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */

package org.elasticsearch.xpack.esql.optimizer.rules;

import org.elasticsearch.common.util.set.Sets;
import org.elasticsearch.xpack.esql.core.analyzer.AnalyzerRules;
import org.elasticsearch.xpack.esql.core.expression.Expression;
import org.elasticsearch.xpack.esql.core.expression.Expressions;
import org.elasticsearch.xpack.esql.core.plan.logical.LogicalPlan;
import org.elasticsearch.xpack.esql.plan.logical.Aggregate;

import java.util.ArrayList;
import java.util.List;

/**
 * Rule that removes Aggregate overrides in grouping, aggregates and across them inside.
 * The overrides appear when the same alias is used multiple times in aggregations and/or groupings:
 * STATS x = COUNT(*), x = MIN(a) BY x = b + 1, x = c + 10
 * becomes
 * STATS BY x = c + 10
 * That is the last declaration for a given alias, overrides all the other declarations, with
 * groups having priority vs aggregates.
 * Separately, it replaces expressions used as group keys inside the aggregates with references:
 * STATS max(a + b + 1) BY a + b
 * becomes
 * STATS max($x + 1) BY $x = a + b
 */
public final class RemoveStatsOverride extends AnalyzerRules.AnalyzerRule {

    @Override
    protected boolean skipResolved() {
        return false;
    }

    @Override
    protected LogicalPlan rule(Aggregate agg) {
        return agg.resolved() ? removeAggDuplicates(agg) : agg;
    }

    private static Aggregate removeAggDuplicates(Aggregate agg) {
        var groupings = agg.groupings();
        var aggregates = agg.aggregates();

        groupings = removeDuplicateNames(groupings);
        aggregates = removeDuplicateNames(aggregates);

        // replace EsqlAggregate with Aggregate
        return new Aggregate(agg.source(), agg.child(), agg.aggregateType(), groupings, aggregates);
    }

    private static  List removeDuplicateNames(List list) {
        var newList = new ArrayList<>(list);
        var nameSet = Sets.newHashSetWithExpectedSize(list.size());

        // remove duplicates
        for (int i = list.size() - 1; i >= 0; i--) {
            var element = list.get(i);
            var name = Expressions.name(element);
            if (nameSet.add(name) == false) {
                newList.remove(i);
            }
        }
        return newList.size() == list.size() ? list : newList;
    }
}