/**
* 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.mahout.common.iterator;
import java.util.Iterator;
import com.google.common.base.Preconditions;
import com.google.common.collect.AbstractIterator;
import org.apache.commons.math3.distribution.PascalDistribution;
import org.apache.mahout.cf.taste.impl.common.SkippingIterator;
import org.apache.mahout.common.RandomUtils;
import org.apache.mahout.common.RandomWrapper;
/**
* Wraps an {@link Iterator} and returns only some subset of the elements that it would, as determined by a
* iterator rate parameter.
*/
public final class SamplingIterator<T> extends AbstractIterator<T> {
private final PascalDistribution geometricDistribution;
private final Iterator<? extends T> delegate;
public SamplingIterator(Iterator<? extends T> delegate, double samplingRate) {
this(RandomUtils.getRandom(), delegate, samplingRate);
}
public SamplingIterator(RandomWrapper random, Iterator<? extends T> delegate, double samplingRate) {
Preconditions.checkNotNull(delegate);
Preconditions.checkArgument(samplingRate > 0.0 && samplingRate <= 1.0);
// Geometric distribution is special case of negative binomial (aka Pascal) with r=1:
geometricDistribution = new PascalDistribution(random.getRandomGenerator(), 1, samplingRate);
this.delegate = delegate;
}
@Override
protected T computeNext() {
int toSkip = geometricDistribution.sample();
if (delegate instanceof SkippingIterator<?>) {
SkippingIterator<? extends T> skippingDelegate = (SkippingIterator<? extends T>) delegate;
skippingDelegate.skip(toSkip);
if (skippingDelegate.hasNext()) {
return skippingDelegate.next();
}
} else {
for (int i = 0; i < toSkip && delegate.hasNext(); i++) {
delegate.next();
}
if (delegate.hasNext()) {
return delegate.next();
}
}
return endOfData();
}
}