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/*
 * Copyright (c) 2006, 2021 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0,
 * or the Eclipse Distribution License v. 1.0 which is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
 */

// Contributors:
//     Oracle - initial API and implementation
//
package org.eclipse.persistence.jpa.jpql.parser;

import java.util.List;
import org.eclipse.persistence.jpa.jpql.ExpressionTools;
import org.eclipse.persistence.jpa.jpql.WordParser;

/**
 * In the SELECT clause the result of a query may be the result of an aggregate function
 * applied to a path expression. The following aggregate functions can be used in the SELECT
 * clause of a query: AVG, COUNT, MAX, MIN, SUM.
 * 
 * A single_valued_association_field is designated by the name of an association-field
 * in a one-to-one or many-to-one relationship. The type of a single_valued_association_field
 * and thus a single_valued_association_path_expression is the abstract schema type of
 * the related entity.
 * 

 * The argument to an aggregate function may be preceded by the keyword DISTINCT to specify
 * that duplicate values are to be eliminated before the aggregate function is applied. Null values
 * are eliminated before the aggregate function is applied, regardless of whether the keyword
 * DISTINCT
 * is specified.
 *
 * 
BNF: aggregate_expression ::= { AVG | MAX | MIN | SUM } ([DISTINCT] state_field_path_expression) |
 *                          COUNT ([DISTINCT] identification_variable |
 *                                            state_field_path_expression |
 *                                            single_valued_object_path_expression)
 *
 * @see AvgFunction
 * @see CountFunction
 * @see MaxFunction
 * @see MinFunction
 * @see SumFunction
 *
 * @version 2.5
 * @since 2.3
 * @author Pascal Filion
 */
public abstract class AggregateFunction extends AbstractSingleEncapsulatedExpression {

    /**
     * The actual DISTINCT identifier found in the string representation of the JPQL query.
     */
    private String distinctIdentifier;

    /**
     * Determines whether whitespace was found after the identifier DISTINCT.
     */
    private boolean hasSpaceAfterDistinct;

    /**
     * Creates a new AggregateFunction.
     *
     * @param parent The parent of this expression
     * @param identifier The JPQL identifier that starts this expression
     */
    protected AggregateFunction(AbstractExpression parent, String identifier) {
        super(parent, identifier);
    }

    @Override
    protected void addOrderedEncapsulatedExpressionTo(List children) {

        if (distinctIdentifier != null) {
            children.add(buildStringExpression(DISTINCT));
        }

        if (hasSpaceAfterDistinct) {
            children.add(buildStringExpression(SPACE));
        }

        super.addOrderedEncapsulatedExpressionTo(children);
    }

    /**
     * Creates the {@link AbstractExpression} to represent the given word.
     *
     * @param word The word that was parsed
     * @return The encapsulated {@link AbstractExpression}
     */
    protected AbstractExpression buildEncapsulatedExpression(WordParser wordParser, String word) {
        return new StateFieldPathExpression(this, word);
    }

    @Override
    public String getEncapsulatedExpressionQueryBNFId() {
        return InternalAggregateFunctionBNF.ID;
    }

    /**
     * Returns the actual DISTINCT identifier found in the string representation of the JPQL
     * query, which has the actual case that was used.
     *
     * @return The DISTINCT identifier that was actually parsed, or an empty string if it was
     * not parsed
     */
    public String getActualDistinctIdentifier() {
        return (distinctIdentifier != null) ?distinctIdentifier : ExpressionTools.EMPTY_STRING;
    }

    @Override
    public JPQLQueryBNF getQueryBNF() {
        return getQueryBNF(AggregateExpressionBNF.ID);
    }

    /**
     * Determines whether the DISTINCT identifier was specified in the query.
     *
     * @return true if the query has DISTINCT; false otherwise
     */
    public final boolean hasDistinct() {
        return distinctIdentifier != null;
    }

    /**
     * Determines whether a whitespace was parsed after DISTINCT.
     *
     * @return true if there was a whitespace after DISTINCT; false
     * otherwise
     */
    public final boolean hasSpaceAfterDistinct() {
        return hasSpaceAfterDistinct;
    }

    @Override
    protected final void parseEncapsulatedExpression(WordParser wordParser,
                                                     int whitespaceCount,
                                                     boolean tolerant) {

        // Parse 'DISTINCT'
        if (wordParser.startsWithIdentifier(DISTINCT)) {
            distinctIdentifier = wordParser.moveForward(DISTINCT);
            hasSpaceAfterDistinct = wordParser.skipLeadingWhitespace() > 0;
        }

        // Parse the rest
        super.parseEncapsulatedExpression(wordParser, whitespaceCount, tolerant);
    }

    @Override
    protected void toParsedTextEncapsulatedExpression(StringBuilder writer, boolean actual) {

        if (distinctIdentifier != null) {
            writer.append(actual ? distinctIdentifier : DISTINCT);
        }

        if (hasSpaceAfterDistinct) {
            writer.append(SPACE);
        }

        super.toParsedTextEncapsulatedExpression(writer, actual);
    }
}