/**
* 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.distance;
import java.util.Iterator;
import org.apache.mahout.math.Vector;
/**
* Tanimoto coefficient implementation.
*
* http://en.wikipedia.org/wiki/Jaccard_index
*/
public class TanimotoDistanceMeasure extends WeightedDistanceMeasure {
/**
* Calculates the distance between two vectors.
*
* The coefficient (a measure of similarity) is: T(a, b) = a.b / (|a|^2 + |b|^2 - a.b)
*
* The distance d(a,b) = 1 - T(a,b)
*
* @return 0 for perfect match, > 0 for greater distance
*/
@Override
public double distance(Vector a, Vector b) {
double ab;
double denominator;
if (getWeights() != null) {
ab = dot(b, a); // b is SequentialAccess
denominator = dot(a, a) + dot(b, b) - ab;
} else {
ab = b.dot(a); // b is SequentialAccess
denominator = a.getLengthSquared() + b.getLengthSquared() - ab;
}
if (denominator < ab) { // correct for fp round-off: distance >= 0
denominator = ab;
}
if (denominator > 0) {
// denom == 0 only when dot(a,a) == dot(b,b) == dot(a,b) == 0
return 1.0 - ab / denominator;
} else {
return 0.0;
}
}
public double dot(Vector a, Vector b) {
boolean sameVector = a == b;
Iterator<Vector.Element> it = a.iterateNonZero();
Vector.Element el;
Vector weights = getWeights();
double dot = 0.0;
while (it.hasNext() && (el = it.next()) != null) {
double elementValue = el.get();
double value = elementValue * (sameVector ? elementValue : b.getQuick(el.index()));
value *= weights.getQuick(el.index());
dot += value;
}
return dot;
}
@Override
public double distance(double centroidLengthSquare, Vector centroid, Vector v) {
return distance(centroid, v); // TODO
}
}