Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/117#discussion_r10863319
--- Diff: mllib/src/main/scala/org/apache/spark/mllib/util/MLUtils.scala ---
@@ -106,18 +111,46 @@ object MLUtils {
}
/**
- * Return the squared Euclidean distance between two vectors.
+ * Returns the squared Euclidean distance between two vectors. The
following formula will be used
+ * if it does not introduce too much numerical error:
+ * <pre>
+ * \|a - b\|_2^2 = \|a\|_2^2 + \|b\|_2^2 - 2 a^T b.
+ * </pre>
+ * When both vector norms are given, this is faster than computing the
squared distance directly,
+ * especially when one of the vectors is a sparse vector.
--- End diff --
I don't have a reference for this formula...
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