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https://issues.apache.org/jira/browse/MAHOUT-207?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12781618#action_12781618
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Jake Mannix commented on MAHOUT-207:
------------------------------------
Hmm, in fact, SparseVector and AbstractVector are not currently consistent vis
a vis hashCode() / equals on trunk:
{code}
double[] gold = { 0, 1.1, 2.2, 3.3, 0 };
public void testEqualsHashcodeCompatibility() {
SparseVector sparse = new SparseVector(gold.length);
SparseVector pretendSparse = new SparseVector(gold.length);
for(int i=0; i<gold.length; i++) {
if(gold[i] != 0d) {
sparse.set(i, gold[i]);
}
pretendSparse.set(i, gold[i]);
}
assertEquals("vectors with zeroes", sparse, pretendSparse);
assertEquals("hashcodes with zeroes", sparse.hashCode(),
pretendSparse.hashCode());
}
{code}
fails. Submitting patch to fix shortly.
> AbstractVector.hashCode() should not care about the order of iteration over
> elements
> ------------------------------------------------------------------------------------
>
> Key: MAHOUT-207
> URL: https://issues.apache.org/jira/browse/MAHOUT-207
> Project: Mahout
> Issue Type: Improvement
> Components: Matrix
> Affects Versions: 0.2
> Environment: all
> Reporter: Jake Mannix
> Priority: Minor
> Fix For: 0.3
>
>
> As was discussed in MAHOUT-165, hashCode can be implemented simply like this:
> {code}
> public int hashCode() {
> final int prime = 31;
> int result = prime + ((name == null) ? 0 : name.hashCode());
> result = prime * result + size();
> Iterator<Element> iter = iterateNonZero();
> while (iter.hasNext()) {
> Element ele = iter.next();
> long v = Double.doubleToLongBits(ele.get());
> result += (ele.index() * (int)(v^(v>>32)));
> }
> return result;
> }
> {code}
> which obviates the need to sort the elements in the case of a random access
> hash-based implementation. Also, (ele.index() * (int)(v^(v>>32)) ) == 0 when
> v = Double.doubleToLongBits(0d), which avoids the wrong hashCode() for sparse
> vectors which have zero elements returned from the iterateNonZero() iterator.
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