/*
 * 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.lucene.codecs;


import java.io.IOException;
import java.util.ServiceLoader;
import java.util.Set;

import org.apache.lucene.index.SegmentReadState;
import org.apache.lucene.index.SegmentWriteState;
import org.apache.lucene.util.NamedSPILoader;

Encodes/decodes per-document values.
 Note, when extending this class, the name (getName) may written into the index in certain configurations. In order for the segment to be read, the name must resolve to your implementation via forName(String). This method uses Java's Service Provider Interface (SPI) to resolve format names. 

If you implement your own format, make sure that it has a no-arg constructor
so SPI can load it.
See Also:
ServiceLoader
@lucene.experimental
/** 
 * Encodes/decodes per-document values.
 * <p>
 * Note, when extending this class, the name ({@link #getName}) may
 * written into the index in certain configurations. In order for the segment 
 * to be read, the name must resolve to your implementation via {@link #forName(String)}.
 * This method uses Java's 
 * {@link ServiceLoader Service Provider Interface} (SPI) to resolve format names.
 * <p>
 * If you implement your own format, make sure that it has a no-arg constructor
 * so SPI can load it.
 * @see ServiceLoader
 * @lucene.experimental */
public abstract class DocValuesFormat implements NamedSPILoader.NamedSPI {
  
  
This static holder class prevents classloading deadlock by delaying
init of doc values formats until needed.
/**
   * This static holder class prevents classloading deadlock by delaying
   * init of doc values formats until needed.
   */
  private static final class Holder {
    private static final NamedSPILoader<DocValuesFormat> LOADER = new NamedSPILoader<>(DocValuesFormat.class);
    
    private Holder() {}
    
    static NamedSPILoader<DocValuesFormat> getLoader() {
      if (LOADER == null) {
        throw new IllegalStateException("You tried to lookup a DocValuesFormat by name before all formats could be initialized. "+
          "This likely happens if you call DocValuesFormat#forName from a DocValuesFormat's ctor.");
      }
      return LOADER;
    }
  }
  
  
Unique name that's used to retrieve this format when
 reading the index.
/** Unique name that's used to retrieve this format when
   *  reading the index.
   */
  private final String name;

  
Creates a new docvalues format.
 The provided name will be written into the index segment in some configurations (such as when using PerFieldDocValuesFormat): in such configurations, for the segment to be read this class should be registered with Java's SPI mechanism (registered in META-INF/ of your jar file, etc). 
Params:
name – must be all ascii alphanumeric, and less than 128 characters in length./**
   * Creates a new docvalues format.
   * <p>
   * The provided name will be written into the index segment in some configurations
   * (such as when using {@code PerFieldDocValuesFormat}): in such configurations,
   * for the segment to be read this class should be registered with Java's
   * SPI mechanism (registered in META-INF/ of your jar file, etc).
   * @param name must be all ascii alphanumeric, and less than 128 characters in length.
   */
  protected DocValuesFormat(String name) {
    NamedSPILoader.checkServiceName(name);
    this.name = name;
  }

  
Returns a DocValuesConsumer to write docvalues to the index. /** Returns a {@link DocValuesConsumer} to write docvalues to the
   *  index. */
  public abstract DocValuesConsumer fieldsConsumer(SegmentWriteState state) throws IOException;

  
Returns a DocValuesProducer to read docvalues from the index. 

NOTE: by the time this call returns, it must hold open any files it will 
need to use; else, those files may be deleted. Additionally, required files 
may be deleted during the execution of this call before there is a chance 
to open them. Under these circumstances an IOException should be thrown by 
the implementation. IOExceptions are expected and will automatically cause 
a retry of the segment opening logic with the newly revised segments.
/** 
   * Returns a {@link DocValuesProducer} to read docvalues from the index. 
   * <p>
   * NOTE: by the time this call returns, it must hold open any files it will 
   * need to use; else, those files may be deleted. Additionally, required files 
   * may be deleted during the execution of this call before there is a chance 
   * to open them. Under these circumstances an IOException should be thrown by 
   * the implementation. IOExceptions are expected and will automatically cause 
   * a retry of the segment opening logic with the newly revised segments.
   */
  public abstract DocValuesProducer fieldsProducer(SegmentReadState state) throws IOException;

  @Override
  public final String getName() {
    return name;
  }
  
  @Override
  public String toString() {
    return "DocValuesFormat(name=" + name + ")";
  }
  
  
looks up a format by name /** looks up a format by name */
  public static DocValuesFormat forName(String name) {
    return Holder.getLoader().lookup(name);
  }
  
  
returns a list of all available format names /** returns a list of all available format names */
  public static Set<String> availableDocValuesFormats() {
    return Holder.getLoader().availableServices();
  }
  
  
Reloads the DocValues format list from the given ClassLoader. Changes to the docvalues formats are visible after the method ends, all iterators (availableDocValuesFormats(),...) stay consistent. 
NOTE: Only new docvalues formats are added, existing ones are
never removed or replaced.
This method is expensive and should only be called for discovery
of new docvalues formats on the given classpath/classloader!
/** 
   * Reloads the DocValues format list from the given {@link ClassLoader}.
   * Changes to the docvalues formats are visible after the method ends, all
   * iterators ({@link #availableDocValuesFormats()},...) stay consistent. 
   * 
   * <p><b>NOTE:</b> Only new docvalues formats are added, existing ones are
   * never removed or replaced.
   * 
   * <p><em>This method is expensive and should only be called for discovery
   * of new docvalues formats on the given classpath/classloader!</em>
   */
  public static void reloadDocValuesFormats(ClassLoader classloader) {
    Holder.getLoader().reload(classloader);
  }
}