
Hey, I've been looking for a hashing function for PixelBased types... with no succes, so I created one. Does this have any value for users? Regards,

Do you mean that unordered_map can automatically instantiate a hashing function for any type of pixel? I have tried this: boost::hash<pixel_rgb8_t>() And VC11 will spit: error C2664: 'boost::hash_value' : cannot convert parameter 1 from 'const boost::gil::rgb8_pixel_t' to 'std::type_index' Thanks. 2013/3/26 Christian Henning <chhenning@gmail.com>
I've been looking for a hashing function for PixelBased types... with no succes, so I created one. Does this have any value for users?
How about the one supplied by unordered_map?
Christian
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On Tue, Mar 26, 2013 at 9:51 AM, Harg Tholan <hargtholan@gmail.com> wrote:
Do you mean that unordered_map can automatically instantiate a hashing function for any type of pixel? I have tried this:
Ok, I see I was wrong. I simple solution is provided here: http://stackoverflow.com/questions/10405030/c-unordered-map-fail-when-used-w... Taking that you can create the following: template< typename Pixel > struct pixel_hasher { std::size_t operator()( const Pixel& p ) const { typedef channel_type<Pixel>::type channel_t; vector<channel_t> v; static_for_each( p, [&] ( const channel_t& c ) { v.push_back( c ); }); return boost::hash_range( v.begin(), v.end() ); } }; typedef rgb8_pixel_t pixel_t; pixel_t p( 1,2,3); unordered_map< pixel_t, int, pixel_hasher< pixel_t > > n; n[p] = 99; Hope this helps. Christian

Ok, I guess you solution is better for it uses "static_for_each" which would turn into statically generated code. But we may skip pushing stuff into a vector by doing std::size_t hash = 0; static_for_each( p, [&] ( const channel_t& c ) { boost::hash_combine(hash, c); }); return hash; Do we agree with this? Regards. 2013/3/26 Christian Henning <chhenning@gmail.com>
On Tue, Mar 26, 2013 at 9:51 AM, Harg Tholan <hargtholan@gmail.com> wrote:
Do you mean that unordered_map can automatically instantiate a hashing function for any type of pixel? I have tried this:
Ok, I see I was wrong. I simple solution is provided here:
http://stackoverflow.com/questions/10405030/c-unordered-map-fail-when-used-w...
Taking that you can create the following:
template< typename Pixel > struct pixel_hasher { std::size_t operator()( const Pixel& p ) const { typedef channel_type<Pixel>::type channel_t;
vector<channel_t> v; static_for_each( p, [&] ( const channel_t& c ) { v.push_back( c ); });
return boost::hash_range( v.begin(), v.end() ); } };
typedef rgb8_pixel_t pixel_t; pixel_t p( 1,2,3); unordered_map< pixel_t, int, pixel_hasher< pixel_t > > n; n[p] = 99;
Hope this helps.
Christian
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On Tue, Mar 26, 2013 at 10:25 AM, Harg Tholan <hargtholan@gmail.com> wrote:
Ok, I guess you solution is better for it uses "static_for_each" which would turn into statically generated code. But we may skip pushing stuff into a vector by doing
std::size_t hash = 0; static_for_each( p, [&] ( const channel_t& c ) { boost::hash_combine(hash, c); });
return hash;
Do we agree with this?
Looks good to me. Christian
participants (2)
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Christian Henning
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Harg Tholan