boost::hash to support boost::dynamic_bitset

[astukalov@…]

IIUC, there's no out-of-box support for hashing of dynamic_bitset.

Could something as simple as

    template <typename B, typename A>
    friend std::size_t hash_value(const dynamic_bitset<B, A>& v) {
        return hash_value( v.m_bits );
    }

be added to boost::dynamic_bitset class?

[Daniel James]

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]

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]

'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@…]

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@…]

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

[acharles]

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

[paulo@…]

What's needed to actually get this done?

[XRumerTest]

Hello. And Bye.

[akim.demaille@…]

Ping.

[James E. King, III]

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