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1   /*
2    * Copyright 2010 The Apache Software Foundation
3    *
4    * Licensed to the Apache Software Foundation (ASF) under one
5    * or more contributor license agreements.  See the NOTICE file
6    * distributed with this work for additional information
7    * regarding copyright ownership.  The ASF licenses this file
8    * to you under the Apache License, Version 2.0 (the
9    * "License"); you may not use this file except in compliance
10   * with the License.  You may obtain a copy of the License at
11   *
12   *     http://www.apache.org/licenses/LICENSE-2.0
13   *
14   * Unless required by applicable law or agreed to in writing, software
15   * distributed under the License is distributed on an "AS IS" BASIS,
16   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
17   * See the License for the specific language governing permissions and
18   * limitations under the License.
19   */
20  package org.apache.hadoop.hbase.filter;
21  
22  import org.apache.hadoop.hbase.KeyValue;
23  
24  import java.io.DataOutput;
25  import java.io.IOException;
26  import java.io.DataInput;
27  import java.util.List;
28  import java.util.ArrayList;
29  
30  import com.google.common.base.Preconditions;
31  
32  /**
33   * A filter that will only return the first KV from each row.
34   * <p>
35   * This filter can be used to more efficiently perform row count operations.
36   */
37  public class FirstKeyOnlyFilter extends FilterBase {
38    private boolean foundKV = false;
39  
40    public FirstKeyOnlyFilter() {
41    }
42  
43    public void reset() {
44      foundKV = false;
45    }
46  
47    public ReturnCode filterKeyValue(KeyValue v) {
48      if(foundKV) return ReturnCode.NEXT_ROW;
49      foundKV = true;
50      return ReturnCode.INCLUDE;
51    }
52  
53    public static Filter createFilterFromArguments(ArrayList<byte []> filterArguments) {
54      Preconditions.checkArgument(filterArguments.size() == 0,
55                                  "Expected 0 but got: %s", filterArguments.size());
56      return new FirstKeyOnlyFilter();
57    }
58  
59    public void write(DataOutput out) throws IOException {
60    }
61  
62    public void readFields(DataInput in) throws IOException {
63    }
64  }