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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.1
/*
 * 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.druid.math.expr;

import com.google.common.annotations.VisibleForTesting;
import com.google.inject.Inject;

/**
 * Like {@link org.apache.druid.common.config.NullHandling}, except for expressions processing configs
 */
public class ExpressionProcessing
{
  /**
   * INSTANCE is injected using static injection to avoid adding JacksonInject annotations all over the code.
   * @see {@link ExpressionProcessingModule} for details.
   *
   * It does not take effect in all unit tests since we don't use Guice Injection. Use {@link #initializeForTests}
   * when modules are not available.
   */
  @Inject
  private static ExpressionProcessingConfig INSTANCE;


  /**
   * Many unit tests do not setup modules for this value to be injected, this method provides a manual way to initialize
   * {@link #INSTANCE}
   */
  @VisibleForTesting
  public static void initializeForTests()
  {
    INSTANCE = new ExpressionProcessingConfig(null, null, null);
  }

  @VisibleForTesting
  public static void initializeForStrictBooleansTests(boolean useStrict)
  {
    INSTANCE = new ExpressionProcessingConfig(useStrict, null, null);
  }

  @VisibleForTesting
  public static void initializeForHomogenizeNullMultiValueStrings()
  {
    INSTANCE = new ExpressionProcessingConfig(null, null, true);
  }

  /**
   * All boolean expressions are {@link ExpressionType#LONG}
   */
  public static boolean useStrictBooleans()
  {
    checkInitialized();
    return INSTANCE.isUseStrictBooleans();
  }

  /**
   * All {@link ExprType#ARRAY} values will be converted to {@link ExpressionType#STRING} by their column selectors
   * (not within expression processing) to be treated as multi-value strings instead of native arrays.
   */
  public static boolean processArraysAsMultiValueStrings()
  {
    checkInitialized();
    return INSTANCE.processArraysAsMultiValueStrings();
  }

  /**
   * All multi-value string expression input values of 'null', '[]', and '[null]' will be coerced to '[null]'. If false,
   * (the default) this will only be done when single value expressions are implicitly mapped across multi-value rows,
   * so that the single valued expression will always be evaluated with an input value of 'null'
   */
  public static boolean isHomogenizeNullMultiValueStringArrays()
  {
    checkInitialized();
    return INSTANCE.isHomogenizeNullMultiValueStringArrays();
  }

  private static void checkInitialized()
  {
    // this should only be null in a unit test context, in production this will be injected by the null handling module
    if (INSTANCE == null) {
      throw new IllegalStateException(
          "ExpressionProcessing module not initialized, call ExpressionProcessing.initializeForTests() or one of its variants"
      );
    }
  }
}