Boost C++ Libraries: Ticket #2841: boost::hash to support boost::dynamic_bitset

owner, component changed; cc set

Daniel James — Mon, 09 Mar 2009 21:46:53 GMT

cc daniel_james@… added
owner changed from Daniel James to Gennaro Prota
component hash → dynamic_bitset

I don't know anything about dynamic_bitset, but I suspect the hash function will also need to include the length of the bitset so that '1', '10' and '01' all have different hash values.

Anyway, this needs to be implemented as part of dynamic_bitset so I'm passing it to it's maintainer. But I'm adding myself as a CC, so he can ask for assistance if needed.

anonymous — Mon, 09 Mar 2009 22:01:22 GMT

Replying to danieljames:

I don't know anything about dynamic_bitset, but I suspect the hash function will also need to include the length of the bitset so that '1', '10' and '01' all have different hash values.

Well, yes, adding length may improve hashing quality. On the other hand, the length of m_bits array directly correlates with the length of bitset, so it's (kind of) already included.

Daniel James — Mon, 09 Mar 2009 22:17:29 GMT

'0', '00', '000' and '0000' should have different hash values, but unless I've misunderstood dynamic_bitset m_bits will be the same for all of them.

astukalov@… — Mon, 09 Mar 2009 22:36:20 GMT

Replying to danieljames:

'0', '00', '000' and '0000' should have different hash values, but unless I've misunderstood dynamic_bitset m_bits will be the same for all of them.

I think so too. It should be true for all zero bitsets with lengths <= bits per bitset block. For bigger lengths - two and more values would be hash_combined, resulting in different hash key.

Anyway, not including bitset length into hashing directly would have so small performance improvement, that I withdraw my previous comment. :)

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jim@… — Sun, 25 Aug 2013 07:28:16 GMT

It would also be great if dynamic_bitset supported std::hash, which exists beginning with C++11.

owner, status changed

acharles — Wed, 19 Feb 2014 04:49:05 GMT

owner changed from Gennaro Prota to acharles
status new → assigned

Proposed fix in: https://github.com/boostorg/dynamic_bitset/pull/1

milestone changed

acharles — Fri, 21 Feb 2014 09:24:55 GMT

milestone Boost 1.39.0 → Boost 1.56.0

paulo@… — Wed, 14 Jan 2015 00:48:08 GMT

What's needed to actually get this done?

version, milestone changed

XRumerTest — Sat, 14 Feb 2015 06:47:25 GMT

version Boost 1.38.0 → Boost 1.43.0
milestone Boost 1.56.0 → Boost 1.58.0

Hello. And Bye.

akim.demaille@… — Tue, 03 Mar 2015 09:59:04 GMT

Ping.

owner, status, severity changed; milestone deleted

James E. King, III — Thu, 06 Jun 2019 13:55:57 GMT

owner changed from acharles to James E. King, III
status assigned → new
severity Optimization → Not Applicable
milestone Boost 1.58.0

James E. King, III — Thu, 06 Jun 2019 13:56:16 GMT

https://github.com/boostorg/dynamic_bitset/pull/45