use boost::boyer_moore with boost::gil

Question

I want to search for a small image from a large one, my algorithm is:

1. search for the first line
2. if first line matches, then compare the rest

I want to use boost::algorithm::boyer_moore to do the line searching, it works fine with std::string:

#include <string>
using namespace std;
#include "boost/algorithm/searching/boyer_moore.hpp"
using namespace boost::algorithm;

int main() {
    string s;

    boyer_moore<string::iterator> bm(s.begin(), s.end()); // it compiles
}

the code compiles, but this one not:

#include "boost/mpl/vector.hpp"
using namespace boost;
#include "boost/gil/gil_all.hpp"
using namespace boost::gil;

#include "boost/algorithm/searching/boyer_moore.hpp"
using namespace boost::algorithm;

int main() {
    typedef rgba8_image_t image_t;
    typedef image_t::view_t view_t;

    view_t vw;

    boyer_moore<view_t::x_iterator> bm(vw.row_begin(0), vw.row_end(0)); // compile error
}

Both of them are iterators, what's wrong with the second one?

Thanks.

Answer 1

According to the docs the algorithm uses an auxiliary data structure called skip_table. By default (when the value_type of the iterator is not a char or unsigned char) this table uses a tr1::unordered_map, and this requires that gil::pixel be hash-able. So you have two options: you either change the default skip_table by specializing BM_traits for your iterator (this is sadly undocumented), or you make gil::pixel hash-able. For the latter you can create a std::size_t hash_value(pixel<ChannelValue,Layout> const& val) inside the namespace boost::gil. The following compiles with g++ 4.9.0 and Visual Studio 2013 (and does nothing):

#include <boost/functional/hash.hpp> //ADDED
#include <boost/mpl/vector.hpp>
#include <boost/gil/gil_all.hpp>
#include <boost/algorithm/searching/boyer_moore.hpp>

using namespace boost;
using namespace boost::gil;
using namespace boost::algorithm;

namespace boost {
    namespace gil
    {
        template <typename ChannelValue, typename Layout>
        std::size_t hash_value(pixel<ChannelValue, Layout> const& b)
        {
            std::size_t seed = 0;
            for (int c = 0; c<num_channels<pixel<ChannelValue, Layout> >::value; ++c)
                hash_combine(seed, b[c]);
            return seed;
        }
    }
}

namespace std { //ADDED
    template <typename ChannelValue, typename Layout>
    struct hash<boost::gil::pixel<ChannelValue,Layout> > {
        size_t operator ()(boost::gil::pixel<ChannelValue, Layout> const& value) const {
            return hash_value(value);
        }
    };
}

int main() {
    typedef rgba8_image_t image_t;
    typedef image_t::view_t view_t;

    view_t vw;

    boyer_moore<view_t::x_iterator> bm(vw.row_begin(0), vw.row_end(0)); // compile error
}