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A module that is everything required to understands Druid Segments
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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.druid.segment.join;

import org.apache.druid.java.util.common.IAE;
import org.apache.druid.segment.column.ColumnHolder;

import javax.annotation.Nullable;
import java.util.Comparator;
import java.util.List;

/**
 * Utility class for working with prefixes in join operations
 */
public class JoinPrefixUtils
{
  private static final Comparator DESCENDING_LENGTH_STRING_COMPARATOR = (s1, s2) ->
      Integer.compare(s2.length(), s1.length());

  /**
   * Checks that "prefix" is a valid prefix for a join clause (see {@link JoinableClause#getPrefix()}) and, if so,
   * returns it. Otherwise, throws an exception.
   */
  public static String validatePrefix(@Nullable final String prefix)
  {
    if (prefix == null || prefix.isEmpty()) {
      throw new IAE("Join clause cannot have null or empty prefix");
    } else if (isPrefixedBy(ColumnHolder.TIME_COLUMN_NAME, prefix) || ColumnHolder.TIME_COLUMN_NAME.equals(prefix)) {
      throw new IAE(
          "Join clause cannot have prefix[%s], since it would shadow %s",
          prefix,
          ColumnHolder.TIME_COLUMN_NAME
      );
    } else {
      return prefix;
    }
  }

  public static boolean isPrefixedBy(final String columnName, final String prefix)
  {
    return columnName.length() > prefix.length() && columnName.startsWith(prefix);
  }

  /**
   * Removes the prefix on {@code columnName}. Must only be called if the column name is actually prefixed; i.e.,
   * if {@link #isPrefixedBy(String, String)} would return true on the same arguments.
   *
   * @throws IllegalArgumentException if columnName does not start with prefix
   */
  public static String unprefix(final String columnName, final String prefix)
  {
    if (isPrefixedBy(columnName, prefix)) {
      return columnName.substring(prefix.length());
    } else {
      throw new IAE("Column[%s] does not start with prefix[%s]", columnName, prefix);
    }
  }

  /**
   * Check if any prefixes in the provided list duplicate or shadow each other.
   *
   * @param prefixes A mutable list containing the prefixes to check. This list will be sorted by descending
   *                 string length.
   */
  public static void checkPrefixesForDuplicatesAndShadowing(
      final List prefixes
  )
  {
    // this is a naive approach that assumes we'll typically handle only a small number of prefixes
    prefixes.sort(DESCENDING_LENGTH_STRING_COMPARATOR);
    for (int i = 0; i < prefixes.size(); i++) {
      String prefix = prefixes.get(i);
      for (int k = i + 1; k < prefixes.size(); k++) {
        String otherPrefix = prefixes.get(k);
        if (prefix.equals(otherPrefix)) {
          throw new IAE("Detected duplicate prefix in join clauses: [%s]", prefix);
        }
        if (isPrefixedBy(prefix, otherPrefix)) {
          throw new IAE("Detected conflicting prefixes in join clauses: [%s, %s]", prefix, otherPrefix);
        }
      }
    }
  }
}