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This module contains the Table/SQL API for writing table programs within the table ecosystem using the Java programming language.
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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.flink.table.operations.utils;

import org.apache.flink.annotation.Internal;
import org.apache.flink.table.api.ValidationException;
import org.apache.flink.table.catalog.ResolvedSchema;
import org.apache.flink.table.expressions.Expression;
import org.apache.flink.table.expressions.ExpressionUtils;
import org.apache.flink.table.expressions.UnresolvedReferenceExpression;
import org.apache.flink.table.expressions.ValueLiteralExpression;
import org.apache.flink.table.expressions.utils.ApiExpressionDefaultVisitor;
import org.apache.flink.table.functions.BuiltInFunctionDefinitions;
import org.apache.flink.table.operations.QueryOperation;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

import static org.apache.flink.table.expressions.ApiExpressionUtils.unresolvedCall;
import static org.apache.flink.table.expressions.ApiExpressionUtils.unresolvedRef;
import static org.apache.flink.table.expressions.ApiExpressionUtils.valueLiteral;

/** Utility class for creating valid alias expressions that can be later used as a projection. */
@Internal
final class AliasOperationUtils {

    private static final AliasLiteralValidator aliasLiteralValidator = new AliasLiteralValidator();
    private static final String ALL_REFERENCE = "*";

    /**
     * Creates a list of valid alias expressions. Resulting expression might still contain {@link
     * UnresolvedReferenceExpression}.
     *
     * @param aliases aliases to validate
     * @param child relational operation on top of which to apply the aliases
     * @return validated list of aliases
     */
    static List createAliasList(List aliases, QueryOperation child) {
        ResolvedSchema childSchema = child.getResolvedSchema();

        if (aliases.size() > childSchema.getColumnCount()) {
            throw new ValidationException("Aliasing more fields than we actually have.");
        }

        List fieldAliases =
                aliases.stream()
                        .map(f -> f.accept(aliasLiteralValidator))
                        .collect(Collectors.toList());

        List childNames = childSchema.getColumnNames();
        return IntStream.range(0, childNames.size())
                .mapToObj(
                        idx -> {
                            UnresolvedReferenceExpression oldField =
                                    unresolvedRef(childNames.get(idx));
                            if (idx < fieldAliases.size()) {
                                ValueLiteralExpression alias = fieldAliases.get(idx);
                                return unresolvedCall(
                                        BuiltInFunctionDefinitions.AS, oldField, alias);
                            } else {
                                return oldField;
                            }
                        })
                .collect(Collectors.toList());
    }

    private static class AliasLiteralValidator
            extends ApiExpressionDefaultVisitor {

        @Override
        public ValueLiteralExpression visit(ValueLiteralExpression valueLiteral) {
            String name =
                    ExpressionUtils.extractValue(valueLiteral, String.class)
                            .orElseThrow(
                                    () ->
                                            new ValidationException(
                                                    "Alias accepts only names that are not '*' reference."));

            if (name.equals(ALL_REFERENCE)) {
                throw new ValidationException("Alias can not accept '*' as name.");
            }
            return valueLiteral;
        }

        @Override
        protected ValueLiteralExpression defaultMethod(Expression expression) {
            throw new ValidationException("Alias accepts only names that are not '*' reference.");
        }

        @Override
        public ValueLiteralExpression visit(UnresolvedReferenceExpression unresolvedReference) {

            if (unresolvedReference.getName().equals(ALL_REFERENCE)) {
                throw new ValidationException("Alias can not accept '*' as name.");
            }
            return valueLiteral(unresolvedReference.getName());
        }
    }

    private AliasOperationUtils() {}
}