[jira] [Commented] (HADOOP-11221) JAVA specification for hashcode does not enforce it to be non-negative, but IdentityHashStore assumes System.identityHashCode() is non-negative

[Colin Patrick McCabe (JIRA)]

    [ 
https://issues.apache.org/jira/browse/HADOOP-11221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14185346#comment-14185346
 ] 

Colin Patrick McCabe commented on HADOOP-11221:
-----------------------------------------------

Thanks, Jinghui.  +1

Can you add a comment to the code stating that numEntries must be a power of 2 
for this hashing code to work?

> JAVA specification for hashcode does not enforce it to be non-negative, but 
> IdentityHashStore assumes System.identityHashCode() is non-negative
> -----------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-11221
>                 URL: https://issues.apache.org/jira/browse/HADOOP-11221
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: util
>    Affects Versions: 2.4.1
>            Reporter: Jinghui Wang
>            Assignee: Jinghui Wang
>         Attachments: HADOOP-11221.patch, HADOOP-11221.v1.patch, 
> HADOOP-11221.v2.patch
>
>
> The following code snippet shows that IdentityHashStore assumes the hashCode 
> is always non-negative.
> {code:borderStyle=solid}
>    private void putInternal(Object k, Object v) {
>      int hash = System.identityHashCode(k);
>      final int numEntries = buffer.length / 2;
>      int index = hash % numEntries;
>        ...
>    }
>    
>   private int getElementIndex(K k) {
>      ...
>      final int numEntries = buffer.length / 2;
>      int hash = System.identityHashCode(k);
>      int index = hash % numEntries;
>      int firstIndex = index;
>      ...
>   }
> {code}



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