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<div><span style="font-family:Calibri, Arial, Helvetica, sans-serif;font-size:12pt">To get 5 more bits:</span><br>
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<div>phash2(Term, 1 bsl 32)<span><br>
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You get less collisions, but it still does not get close to a uniform distribution for your example.</div>
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<span><span>Test(500) -> 249993<br>
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/Sverker</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> erlang-questions <erlang-questions-bounces@erlang.org> on behalf of Roger Lipscombe <roger@differentpla.net><br>
<b>Sent:</b> Wednesday, December 15, 2021 12:57 PM<br>
<b>To:</b> erlang-questions <erlang-questions@erlang.org><br>
<b>Subject:</b> phash2 isn't very resistant to collisions?</font>
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<div class="PlainText">I'm sure this has come up previously, but all I could find talked about atoms.<br>
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I know it's not intended to be cryptographically strong, but<br>
erlang:phash2 is woefully collidey with 2-tuples:<br>
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Test = fun(N) -><br>
L = [{X, Y} || X <- lists:seq(0, N - 1), Y <- lists:seq(0, N - 1)],<br>
Expected = N * N,<br>
Expected = length(L),<br>
Expected = length(lists:usort([erlang:phash2(Term) || Term <- L]))<br>
end.<br>
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1> Test(10).<br>
100<br>
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2> Test(100).<br>
** exception error: no match of right hand side value 9999<br>
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3> Test(99).<br>
** exception error: no match of right hand side value 9800<br>
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4> Test(500).<br>
** exception error: no match of right hand side value 249763<br>
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What alternatives are there to phash2, if I still care about performance?<br>
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