/*
 * 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.misc.document;

import java.io.IOException;
import java.io.Reader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.InvertableType;
import org.apache.lucene.document.StoredValue;
import org.apache.lucene.index.FieldInfo;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexableField;
import org.apache.lucene.index.IndexableFieldType;
import org.apache.lucene.util.BytesRef;

/**
 * Defers actually loading a field's value until you ask for it. You must not use the returned Field
 * instances after the provided reader has been closed.
 *
 * @see #getField
 */
public class LazyDocument {
  private final IndexReader reader;
  private final int docID;

  // null until first field is loaded
  private Document doc;

  private Map<Integer, List<LazyField>> fields = new HashMap<>();
  private Set<String> fieldNames = new HashSet<>();

  public LazyDocument(IndexReader reader, int docID) {
    this.reader = reader;
    this.docID = docID;
  }

  /**
   * Creates a StorableField whose value will be lazy loaded if and when it is used.
   *
   * <p><b>NOTE:</b> This method must be called once for each value of the field name specified in
   * sequence that the values exist. This method may not be used to generate multiple, lazy,
   * StorableField instances referring to the same underlying StorableField instance.
   *
   * <p>The lazy loading of field values from all instances of StorableField objects returned by
   * this method are all backed by a single StoredDocument per LazyDocument instance.
   */
  public IndexableField getField(FieldInfo fieldInfo) {

    fieldNames.add(fieldInfo.name);
    List<LazyField> values = fields.get(fieldInfo.number);
    if (null == values) {
      values = new ArrayList<>();
      fields.put(fieldInfo.number, values);
    }

    LazyField value = new LazyField(fieldInfo.name, fieldInfo.number);
    values.add(value);

    synchronized (this) {
      // edge case: if someone asks this LazyDoc for more LazyFields
      // after other LazyFields from the same LazyDoc have been
      // actuallized, we need to force the doc to be re-fetched
      // so the new LazyFields are also populated.
      doc = null;
    }
    return value;
  }

  /**
   * non-private for test only access
   *
   * @lucene.internal
   */
  synchronized Document getDocument() {
    if (doc == null) {
      try {
        doc = reader.storedFields().document(docID, fieldNames);
      } catch (IOException ioe) {
        throw new IllegalStateException("unable to load document", ioe);
      }
    }
    return doc;
  }

  // :TODO: synchronize to prevent redundent copying? (sync per field name?)
  private void fetchRealValues(String name, int fieldNum) {
    Document d = getDocument();

    List<LazyField> lazyValues = fields.get(fieldNum);
    IndexableField[] realValues = d.getFields(name);

    assert realValues.length <= lazyValues.size()
        : "More lazy values then real values for field: " + name;

    for (int i = 0; i < lazyValues.size(); i++) {
      LazyField f = lazyValues.get(i);
      if (null != f) {
        f.realValue = realValues[i];
      }
    }
  }

  /**
   * Lazy-loaded field
   *
   * @lucene.internal
   */
  public class LazyField implements IndexableField {
    private String name;
    private int fieldNum;
    volatile IndexableField realValue = null;

    private LazyField(String name, int fieldNum) {
      this.name = name;
      this.fieldNum = fieldNum;
    }

    /**
     * non-private for test only access
     *
     * @lucene.internal
     */
    public boolean hasBeenLoaded() {
      return null != realValue;
    }

    private IndexableField getRealValue() {
      if (null == realValue) {
        fetchRealValues(name, fieldNum);
      }
      assert hasBeenLoaded() : "field value was not lazy loaded";
      assert realValue.name().equals(name())
          : "realvalue name != name: " + realValue.name() + " != " + name();

      return realValue;
    }

    @Override
    public String name() {
      return name;
    }

    @Override
    public BytesRef binaryValue() {
      return getRealValue().binaryValue();
    }

    @Override
    public String stringValue() {
      return getRealValue().stringValue();
    }

    @Override
    public Reader readerValue() {
      return getRealValue().readerValue();
    }

    @Override
    public Number numericValue() {
      return getRealValue().numericValue();
    }

    @Override
    public IndexableFieldType fieldType() {
      return getRealValue().fieldType();
    }

    @Override
    public TokenStream tokenStream(Analyzer analyzer, TokenStream reuse) {
      return getRealValue().tokenStream(analyzer, reuse);
    }

    @Override
    public StoredValue storedValue() {
      return getRealValue().storedValue();
    }

    @Override
    public InvertableType invertableType() {
      return getRealValue().invertableType();
    }
  }
}
