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/**
 * Copyright 2012 Twitter, Inc.
 *
 * Licensed 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 parquet.hadoop.metadata;

import static java.util.Collections.unmodifiableMap;
import static parquet.Preconditions.checkNotNull;

import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

import parquet.schema.MessageType;

/**
 * Merged metadata when reading from multiple files.
 * THis is to allow schema evolution
 *
 * @author Julien Le Dem
 *
 */
public class GlobalMetaData implements Serializable {
  private static final long serialVersionUID = 1L;

  private final MessageType schema;

  private final Map> keyValueMetaData;

  private final Set createdBy;

  /**
   * @param schema the union of the schemas for all the files
   * @param keyValueMetaData the merged app specific metadata
   * @param createdBy the description of the library that created the file
   */
  public GlobalMetaData(MessageType schema, Map> keyValueMetaData, Set createdBy) {
    super();
    this.schema = checkNotNull(schema, "schema");
    this.keyValueMetaData = unmodifiableMap(checkNotNull(keyValueMetaData, "keyValueMetaData"));
    this.createdBy = createdBy;
  }

  /**
   * @return the schema for the file
   */
  public MessageType getSchema() {
    return schema;
  }

  @Override
  public String toString() {
    return "GlobalMetaData{schema: "+schema+ ", metadata: " + keyValueMetaData + "}";
  }

  /**
   * @return meta data for extensions
   */
  public Map> getKeyValueMetaData() {
    return keyValueMetaData;
  }

  /**
   * @return the description of the library that created the file
   */
  public Set getCreatedBy() {
    return createdBy;
  }

  /**
   * Will merge the metadata as if it was coming from a single file.
   * (for all part files written together this will always work)
   * If there are conflicting values an exception will be thrown
   * @return the merged version of this
   */
  public FileMetaData merge() {
    String createdByString = createdBy.size() == 1 ?
      createdBy.iterator().next() :
      createdBy.toString();
    Map mergedKeyValues = new HashMap();
    for (Entry> entry : keyValueMetaData.entrySet()) {
      if (entry.getValue().size() > 1) {
        throw new RuntimeException("could not merge metadata: key " + entry.getKey() + " has conflicting values: " + entry.getValue());
      }
      mergedKeyValues.put(entry.getKey(), entry.getValue().iterator().next());
    }
    return new FileMetaData(schema, mergedKeyValues, createdByString);
  }

}