sparsity.hpp

#ifndef PASTELGFX_SPARSITY_HPP
#define PASTELGFX_SPARSITY_HPP

#include "pastel/gfx/sparsity.h"

#include "pastel/sys/ensure.h"

namespace Pastel
{

    template <typename Real_Range>
    typename boost::range_value<Real_Range>::type
        sparsity2(const Real_Range& input)
    {
        typedef typename boost::range_value<Real_Range>::type
            Real;

        // This is the squared 2-sparsity measure.
        // See 'sparsity.txt'.

        Real mean = 0;
        Real squareMean = 0;
        integer n = 0;

        // Compute mean and square mean.
        {
            Real_Iterator iter = input.begin();
            Real_Iterator end = input.end();
            while(iter != end)
            {

                const Real value = *iter;

                mean += value;
                squareMean += square(value);
                ++n;

                ++iter;
            }
        }

        ENSURE_OP(n, >=, 2);

        Real variance = 0;

        // Compute variance.
        {
            Real_Iterator iter = input.begin();
            Real_Iterator end = input.end();
            while(iter != end)
            {

                const Real value = *iter;
                variance += square(value - mean);
                ++iter;
            }
        }

        return ((Real)n / (n - 1)) * (variance / squareMean);
    }

}

#endif