package org.apache.lucene.index;

/**
 * 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.
 */

import java.io.IOException;

import org.apache.lucene.util.AttributeSource;

/** Enumerates indexed fields.  You must first call {@link
 *  #next} before calling {@link #terms}.
 *
 * @lucene.experimental */

public abstract class FieldsEnum {

  // TODO: maybe allow retrieving FieldInfo for current
  // field, as optional method?

  private AttributeSource atts = null;

  /**
   * Returns the related attributes.
   */
  public AttributeSource attributes() {
    if (atts == null) {
      atts = new AttributeSource();
    }
    return atts;
  }
  
  /** Increments the enumeration to the next field.  The
   *  returned field is always interned, so simple ==
   *  comparison is allowed.  Returns null when there are no
   *  more fields.*/
  public abstract String next() throws IOException;

  /** Get {@link TermsEnum} for the current field.  You
   *  should not call {@link #next} until you're done using
   *  this {@link TermsEnum}.  After {@link #next} returns
   *  null this method should not be called. This method
   *  will not return null. */
  public abstract TermsEnum terms() throws IOException;

  public final static FieldsEnum[] EMPTY_ARRAY = new FieldsEnum[0];

  /** Provides zero fields */
  public final static FieldsEnum EMPTY = new FieldsEnum() {

    @Override
    public String next() {
      return null;
    }

    @Override
    public TermsEnum terms() {
      throw new IllegalStateException("this method should never be called");
    }
  };
}