/**
* 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.cf.taste.impl.common;
import java.util.NoSuchElementException;
import java.util.Random;
import org.apache.mahout.common.RandomUtils;
/**
* Wraps a {@link LongPrimitiveIterator} and returns only some subset of the elements that it would,
* as determined by a sampling rate parameter.
*/
public final class SamplingLongPrimitiveIterator extends AbstractLongPrimitiveIterator {
private final Random random;
private final LongPrimitiveIterator delegate;
private final double samplingRate;
private long next;
private boolean hasNext;
public SamplingLongPrimitiveIterator(LongPrimitiveIterator delegate, double samplingRate) {
random = RandomUtils.getRandom();
this.delegate = delegate;
this.samplingRate = samplingRate;
this.hasNext = true;
doNext();
}
@Override
public boolean hasNext() {
return hasNext;
}
@Override
public long nextLong() {
if (hasNext) {
long result = next;
doNext();
return result;
}
throw new NoSuchElementException();
}
@Override
public long peek() {
if (hasNext) {
return next;
}
throw new NoSuchElementException();
}
private void doNext() {
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) {
delegate.skip(toSkip);
}
if (delegate.hasNext()) {
next = delegate.next();
} else {
hasNext = false;
}
}
/**
* @throws UnsupportedOperationException
*/
@Override
public void remove() {
throw new UnsupportedOperationException();
}
@Override
public void skip(int n) {
delegate.skip((int) (n / samplingRate)); // Kind of an approximation, but this is expected skip
if (delegate.hasNext()) {
next = delegate.next();
} else {
hasNext = false;
}
}
public static LongPrimitiveIterator maybeWrapIterator(LongPrimitiveIterator delegate, double samplingRate) {
return samplingRate >= 1.0 ? delegate : new SamplingLongPrimitiveIterator(delegate, samplingRate);
}
}