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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
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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.cassandra.cql3.selection;

import java.nio.ByteBuffer;
import java.util.List;

import org.apache.cassandra.config.CFMetaData;
import org.apache.cassandra.cql3.AssignmentTestable;
import org.apache.cassandra.cql3.ColumnIdentifier;
import org.apache.cassandra.cql3.ColumnSpecification;
import org.apache.cassandra.cql3.functions.Function;
import org.apache.cassandra.cql3.selection.Selection.ResultSetBuilder;
import org.apache.cassandra.db.marshal.AbstractType;
import org.apache.cassandra.db.marshal.ReversedType;
import org.apache.cassandra.exceptions.InvalidRequestException;

/**
 * A Selector is used to convert the data returned by the storage engine into the data requested by the 
 * user. They correspond to the <selector> elements from the select clause.
 * Since the introduction of aggregation, Selectors cannot be called anymore by multiple threads 
 * as they have an internal state.
 */
public abstract class Selector implements AssignmentTestable
{
    /**
     * A factory for Selector instances.
     */
    public static abstract class Factory
    {
        public void addFunctionsTo(List functions)
        {
        }

        /**
         * Returns the column specification corresponding to the output value of the selector instances created by
         * this factory.
         *
         * @param cfm the column family meta data
         * @return a column specification
         */
        public final ColumnSpecification getColumnSpecification(CFMetaData cfm)
        {
            return new ColumnSpecification(cfm.ksName,
                                           cfm.cfName,
                                           ColumnIdentifier.getInterned(getColumnName(), true),
                                           getReturnType());
        }

        /**
         * Creates a new Selector instance.
         *
         * @return a new Selector instance
         */
        public abstract Selector newInstance() throws InvalidRequestException;

        /**
         * Checks if this factory creates selectors instances that creates aggregates.
         *
         * @return true if this factory creates selectors instances that creates aggregates,
         * false otherwise
         */
        public boolean isAggregateSelectorFactory()
        {
            return false;
        }

        /**
         * Checks if this factory creates writetime selectors instances.
         *
         * @return true if this factory creates writetime selectors instances,
         * false otherwise
         */
        public boolean isWritetimeSelectorFactory()
        {
            return false;
        }

        /**
         * Checks if this factory creates TTL selectors instances.
         *
         * @return true if this factory creates TTL selectors instances,
         * false otherwise
         */
        public boolean isTTLSelectorFactory()
        {
            return false;
        }

        /**
         * Checks if this factory creates Selectors that simply return the specified column.
         *
         * @param index the column index
         * @return true if this factory creates Selectors that simply return
         * the specified column, false otherwise.
         */
        public boolean isSimpleSelectorFactory(int index)
        {
            return false;
        }

        /**
         * Returns the name of the column corresponding to the output value of the selector instances created by
         * this factory.
         *
         * @return a column name
         */
        protected abstract String getColumnName();

        /**
         * Returns the type of the values returned by the selector instances created by this factory.
         *
         * @return the selector output type
         */
        protected abstract AbstractType getReturnType();

        /**
         * Record a mapping between the ColumnDefinitions that are used by the selector
         * instances created by this factory and a column in the ResultSet.Metadata
         * returned with a query. In most cases, this is likely to be a 1:1 mapping,
         * but some selector instances may utilise multiple columns (or none at all)
         * to produce a value (i.e. functions).
         *
         * @param mapping the instance of the column mapping belonging to the current query's Selection
         * @param resultsColumn the column in the ResultSet.Metadata to which the ColumnDefinitions used
         *                      by the Selector are to be mapped
         */
        protected abstract void addColumnMapping(SelectionColumnMapping mapping, ColumnSpecification resultsColumn);
    }

    /**
     * Add the current value from the specified ResultSetBuilder.
     *
     * @param protocolVersion protocol version used for serialization
     * @param rs the ResultSetBuilder
     * @throws InvalidRequestException if a problem occurs while add the input value
     */
    public abstract void addInput(int protocolVersion, ResultSetBuilder rs) throws InvalidRequestException;

    /**
     * Returns the selector output.
     *
     * @param protocolVersion protocol version used for serialization
     * @return the selector output
     * @throws InvalidRequestException if a problem occurs while computing the output value
     */
    public abstract ByteBuffer getOutput(int protocolVersion) throws InvalidRequestException;

    /**
     * Returns the Selector output type.
     *
     * @return the Selector output type.
     */
    public abstract AbstractType getType();

    /**
     * Checks if this Selector is creating aggregates.
     *
     * @return true if this Selector is creating aggregates false
     * otherwise.
     */
    public boolean isAggregate()
    {
        return false;
    }

    /**
     * Reset the internal state of this Selector.
     */
    public abstract void reset();

    public final AssignmentTestable.TestResult testAssignment(String keyspace, ColumnSpecification receiver)
    {
        // We should ignore the fact that the output type is frozen in our comparison as functions do not support
        // frozen types for arguments
        AbstractType receiverType = receiver.type;
        if (getType().isFrozenCollection())
            receiverType = receiverType.freeze();

        if (getType().isReversed())
            receiverType = ReversedType.getInstance(receiverType);

        if (receiverType.equals(getType()))
            return AssignmentTestable.TestResult.EXACT_MATCH;

        if (receiverType.isValueCompatibleWith(getType()))
            return AssignmentTestable.TestResult.WEAKLY_ASSIGNABLE;

        return AssignmentTestable.TestResult.NOT_ASSIGNABLE;
    }
}