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https://issues.apache.org/jira/browse/MAHOUT-1117?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13498966#comment-13498966
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Ted Dunning commented on MAHOUT-1117:
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The current hash code is fine for vectors that just have fancy implementations
(like DenseVector). It is fairly perverse for sparse vectors, but that is hard
to avoid.
The place where the current hashcode implementation breaks down is with
WeightedVector and friends. My feeling is that simply combining
super.hashCode() with hashes of the index and weight should give us a decent
hash code here. Even without doing that, the base hashcode is fine since
a.equals(b) implies a.hashCode() == b.hashCode even without providing class
specific hashes.
> Vectors are not hashable
> ------------------------
>
> Key: MAHOUT-1117
> URL: https://issues.apache.org/jira/browse/MAHOUT-1117
> Project: Mahout
> Issue Type: Improvement
> Affects Versions: 1.0
> Reporter: Dan Filimon
> Priority: Minor
>
> No *Vector classes (DenseVector, WeightedVector, etc.) implement hashCode().
> In working on improving clustering in Mahout, Ted Dunning wrote prototype
> code for Streaming KMeans and Ball KMeans, that I'm working with him on.
> These need to be used together in the MapReduce version.
> However, in Ball KMeans, we initialize the clusters using a probabilistic
> approach similar to k-means++. This however requires a
> Multinomial<WeightedVector> distribution of the points we want to cluster to
> pick the centroids.
> Internally, the Multinomial<T> uses a HashMap to keep track of the values it
> can sample from.
> Since Vectors don't override Object's hashCode(), it is possible to get the
> same value multiple times in the map (as long as the references differ).
> This is less of an issue because of how we're adding the vectors to the
> multinomial (we can guarantee that the references will be unique) and once
> MAHOUT-1116 is resolved the hashing will work okay for our needs.
> It still seems that it would be useful to have hashable vectors.
> What do you think? And what would a hash function look like?
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