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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* 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.cassandra.utils;
import java.util.Arrays;
/**
* Simple class for constructing an EsimtatedHistogram from a set of predetermined values
*/
public class HistogramBuilder
{
public static final long[] EMPTY_LONG_ARRAY = new long[]{};
public static final long[] ZERO = new long[]{ 0 };
public HistogramBuilder() {}
public HistogramBuilder(long[] values)
{
for (long value : values)
{
add(value);
}
}
private long[] values = new long[10];
int count = 0;
public void add(long value)
{
if (count == values.length)
values = Arrays.copyOf(values, values.length << 1);
values[count++] = value;
}
/**
* See {@link #buildWithStdevRangesAroundMean(int)}
* @return buildWithStdevRangesAroundMean(3)
*/
public EstimatedHistogram buildWithStdevRangesAroundMean()
{
return buildWithStdevRangesAroundMean(3);
}
/**
* Calculate the min, mean, max and standard deviation of the items in the builder, and
* generate an EstimatedHistogram with upto maxdev stdev size ranges either
* side of the mean, until min/max are hit; if either min/max are not reached a further range is
* inserted at the relevant ends. e.g., with a maxdevs of 3, there may be up to 8 ranges
* (between 9 boundaries, the middle being the mean); the middle 6 will have the same size (stdev)
* with the outermost two stretching out to min and max.
*
* @param maxdevs
* @return
*/
public EstimatedHistogram buildWithStdevRangesAroundMean(int maxdevs)
{
if (maxdevs < 0)
throw new IllegalArgumentException("maxdevs must be greater than or equal to zero");
final int count = this.count;
final long[] values = this.values;
if (count == 0)
return new EstimatedHistogram(EMPTY_LONG_ARRAY, ZERO);
long min = Long.MAX_VALUE, max = Long.MIN_VALUE;
double sum = 0, sumsq = 0;
for (int i = 0 ; i < count ; i++)
{
final long value = values[i];
sum += value;
sumsq += value * value;
if (value < min)
min = value;
if (value > max)
max = value;
}
final long mean = Math.round(sum / count);
final double stdev =
Math.sqrt((sumsq / count) - (mean * (double) mean));
// build the ranges either side of the mean
final long[] lowhalf = buildRange(mean, min, true, stdev, maxdevs);
final long[] highhalf = buildRange(mean, max, false, stdev, maxdevs);
// combine the ranges
final long[] ranges = new long[lowhalf.length + highhalf.length + 1];
System.arraycopy(lowhalf, 0, ranges, 0, lowhalf.length);
ranges[lowhalf.length] = mean;
System.arraycopy(highhalf, 0, ranges, lowhalf.length + 1, highhalf.length);
final EstimatedHistogram hist = new EstimatedHistogram(ranges, new long[ranges.length + 1]);
for (int i = 0 ; i < count ; i++)
hist.add(values[i]);
return hist;
}
private static long[] buildRange(long mean, long minormax, boolean ismin, double stdev, int maxdevs)
{
if (minormax == mean)
// minormax == mean we have no range to produce, but given the exclusive starts
// that begin at zero by default (or -Inf) in EstimatedHistogram we have to generate a min range
// to indicate where we start from
return ismin ? new long[] { mean - 1 } : EMPTY_LONG_ARRAY;
if (stdev < 1)
{
// deal with stdevs too small to generate sensible ranges
return ismin ? new long[] { minormax - 1, mean - 1 } :
new long[] { minormax };
}
long larger, smaller;
if (ismin) { larger = mean; smaller = minormax; }
else { larger = minormax; smaller = mean; }
double stdevsTo = (larger - smaller) / stdev;
if (stdevsTo > 0 && stdevsTo < 1)
// always round up if there's just one non-empty range
stdevsTo = 1;
else
// otherwise round to the nearest half stdev, to avoid tiny ranges at the start/end
stdevsTo = Math.round(stdevsTo);
// limit to 4 stdev ranges - last range will contain everything to boundary
final int len = Math.min(maxdevs + 1, (int) stdevsTo);
final long[] range = new long[len];
long next = ismin ? minormax - 1 : minormax;
for (int i = 0 ; i < range.length ; i++)
{
long delta = (range.length - (i + 1)) * (long) stdev;
if (ismin)
{
range[i] = next;
next = mean - delta;
}
else
{
range[len - 1 - i] = next;
next = mean + delta;
}
}
return range;
}
}