/**
* 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 java.util.Random;
import com.google.common.collect.AbstractIterator;
import org.apache.mahout.cf.taste.impl.common.SkippingIterator;
import org.apache.mahout.common.RandomUtils;
/**
* 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 Random random;
private final Iterator<? extends T> delegate;
private final double samplingRate;
public SamplingIterator(Iterator<? extends T> delegate, double samplingRate) {
random = RandomUtils.getRandom();
this.delegate = delegate;
this.samplingRate = samplingRate;
}
@Override
protected T computeNext() {
if (delegate instanceof SkippingIterator<?>) {
SkippingIterator<? extends T> skippingDelegate = (SkippingIterator<? extends T>) delegate;
int toSkip = 0;
while (random.nextDouble() >= samplingRate) {
toSkip++;
}
// Really, would be nicer to select value from geometric distribution, for small values of samplingRate
if (toSkip > 0) {
skippingDelegate.skip(toSkip);
}
if (skippingDelegate.hasNext()) {
return skippingDelegate.next();
}
} else {
while (delegate.hasNext()) {
T delegateNext = delegate.next();
if (random.nextDouble() < samplingRate) {
return delegateNext;
}
}
}
return endOfData();
}
}