1 /**
2 *
3 * Licensed to the Apache Software Foundation (ASF) under one
4 * or more contributor license agreements. See the NOTICE file
5 * distributed with this work for additional information
6 * regarding copyright ownership. The ASF licenses this file
7 * to you under the Apache License, Version 2.0 (the
8 * "License"); you may not use this file except in compliance
9 * with the License. You may obtain a copy of the License at
10 *
11 * http://www.apache.org/licenses/LICENSE-2.0
12 *
13 * Unless required by applicable law or agreed to in writing, software
14 * distributed under the License is distributed on an "AS IS" BASIS,
15 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16 * See the License for the specific language governing permissions and
17 * limitations under the License.
18 */
19
20 package org.apache.hadoop.hbase.util;
21
22 /**
23 * This class maintains mean and variation for any sequence of input provided to it.
24 * It is initialized with number of rolling periods which basically means the number of past
25 * inputs whose data will be considered to maintain mean and variation.
26 * It will use O(N) memory to maintain these statistics, where N is number of look up periods it
27 * was initialized with.
28 * If zero is passed during initialization then it will maintain mean and variance from the
29 * start. It will use O(1) memory only. But note that since it will maintain mean / variance
30 * from the start the statistics may behave like constants and may ignore short trends.
31 * All operations are O(1) except the initialization which is O(N).
32 */
33 public class RollingStatCalculator {
34 private double currentSum;
35 private double currentSqrSum;
36 // Total number of data values whose statistic is currently present
37 private long numberOfDataValues;
38 private int rollingPeriod;
39 private int currentIndexPosition;
40 // to be used only if we have non-zero rolling period
41 private long [] dataValues;
42
43 /**
44 * Creates a RollingStatCalculator with given number of rolling periods.
45 * @param rollingPeriod
46 */
47 public RollingStatCalculator(int rollingPeriod) {
48 this.rollingPeriod = rollingPeriod;
49 this.dataValues = fillWithZeros(rollingPeriod);
50 this.currentSum = 0.0;
51 this.currentSqrSum = 0.0;
52 this.currentIndexPosition = 0;
53 this.numberOfDataValues = 0;
54 }
55
56 /**
57 * Inserts given data value to array of data values to be considered for statistics calculation
58 * @param data
59 */
60 public void insertDataValue(long data) {
61 // if current number of data points already equals rolling period and rolling period is
62 // non-zero then remove one data and update the statistics
63 if(numberOfDataValues >= rollingPeriod && rollingPeriod > 0) {
64 this.removeData(dataValues[currentIndexPosition]);
65 }
66 numberOfDataValues++;
67 currentSum = currentSum + (double)data;
68 currentSqrSum = currentSqrSum + ((double)data * data);
69 if (rollingPeriod >0)
70 {
71 dataValues[currentIndexPosition] = data;
72 currentIndexPosition = (currentIndexPosition + 1) % rollingPeriod;
73 }
74 }
75
76 /**
77 * Update the statistics after removing the given data value
78 * @param data
79 */
80 private void removeData(long data) {
81 currentSum = currentSum - (double)data;
82 currentSqrSum = currentSqrSum - ((double)data * data);
83 numberOfDataValues--;
84 }
85
86 /**
87 * @return mean of the data values that are in the current list of data values
88 */
89 public double getMean() {
90 return this.currentSum / (double)numberOfDataValues;
91 }
92
93 /**
94 * @return deviation of the data values that are in the current list of data values
95 */
96 public double getDeviation() {
97 double variance = (currentSqrSum - (currentSum*currentSum)/(double)(numberOfDataValues))/
98 numberOfDataValues;
99 return Math.sqrt(variance);
100 }
101
102 /**
103 * @param size
104 * @return an array of given size initialized with zeros
105 */
106 private long [] fillWithZeros(int size) {
107 long [] zeros = new long [size];
108 for (int i=0; i<size; i++) {
109 zeros[i] = 0L;
110 }
111 return zeros;
112 }
113 }