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https://issues.apache.org/jira/browse/CALCITE-4096?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17148824#comment-17148824
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Julian Hyde commented on CALCITE-4096:
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As long as {{Pair}} implements {{Map.Entry}} we have to use {{Map.Entry}}'s
hashCode algorithm. So in effect you are proposing to make {{Pair}} no longer
implement {{Map.Entry}}.
For pairs where both entries have the same type, consider using
{{FlatLists.Flat2List}}. It uses List's {{hashCode}} algorithm, {{(31 +
left.HashCode()) * 31 + right.hashCode()}}. The space overhead is the same as
Pair - a struct with two pointers.
> Change Pair.hashCode() not to use XOR
> -------------------------------------
>
> Key: CALCITE-4096
> URL: https://issues.apache.org/jira/browse/CALCITE-4096
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Haisheng Yuan
> Priority: Major
>
> JDK Map Entry uses XOR maybe because it is unlikely to use Map Entry as the
> HashMap key.
> But Pair in Calcite is a general data structure, it is used in several places
> as the key of HashMap/HashSet. XOR is not a good candidate for hash
> algorithm, it is more likely to generate hash collision than simple prime
> multiplication, especially when pair.left equals pair.right.
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