fe/src/main/java/org/apache/impala/common/AliasGenerator.java

// Licensed to the Apache Software Foundation (ASF) under one
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// distributed with this work for additional information
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// 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
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package org.apache.impala.common;

import java.util.Set;

import com.google.common.base.Preconditions;
import com.google.common.collect.Sets;

/**
 * Abstract class representing an alias generator. It uses a prefix and a
 * monotonically increasing counter to generate new aliases. Classes extending
 * this class are responsible for initializing the prefix.
 */
public abstract class AliasGenerator {
  private int numGeneratedAliases_ = 1;
  protected String aliasPrefix_ = null;
  protected Set<String> usedAliases_ = Sets.newHashSet();

  /**
   * Return the next available alias.
   */
  public String getNextAlias() {
    Preconditions.checkNotNull(aliasPrefix_);
    while (true) {
      String candidateAlias = aliasPrefix_ + Integer.toString(numGeneratedAliases_++);
      if (usedAliases_.add(candidateAlias)) return candidateAlias;
      if (numGeneratedAliases_ < 0) {
        throw new IllegalStateException("Overflow occured during alias generation.");
      }
    }
  }
}