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From chiwanpark <...@git.apache.org>
Date Thu, 07 May 2015 09:41:22 GMT
```Github user chiwanpark commented on a diff in the pull request:

---
@@ -0,0 +1,45 @@
+/*
+ * 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
+ * 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
+ *
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * 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.
+ */
+
+
+
+/** This class implements a cosine distance metric. The class calculates the distance
between
+  * the given vectors by dividing the dot product of two vectors by the product of their
lengths.
+  * We convert the result of division to a usable distance. So, 1 - cos(angle) is actually
returned.
+  *
+  * @see http://en.wikipedia.org/wiki/Cosine_similarity
+  */
+class CosineDistanceMeasure extends DistanceMeasure {
+  override def distance(a: Vector, b: Vector): Double = {
+    checkValidArguments(a, b)
+
+    val dotProd: Double = a.dot(b)
+    val denominator: Double = a.magnitude * b.magnitude
+    if (dotProd == 0 && denominator == 0) {
--- End diff --

@tillrohrmann That is for case with zero-vector. Without the code dealing the zero-vector
case, we got a `DivisionByZeroError` in the case. I followed [the result of Wolfram Alpha](http://goo.gl/NXGLgo)
and [implementation of Mahout](https://github.com/apache/mahout/blob/master/mr/src/main/java/org/apache/mahout/common/distance/CosineDistanceMeasure.java#L66)
for resolving this case.

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