eigen/doc/examples/make_circulant.cpp.evaluator

namespace Eigen {
  namespace internal {
    template<typename ArgType>
    struct evaluator<Circulant<ArgType> >
      : evaluator_base<Circulant<ArgType> >
    {
      typedef Circulant<ArgType> XprType;
      typedef typename nested_eval<ArgType, XprType::ColsAtCompileTime>::type ArgTypeNested;
      typedef typename remove_all<ArgTypeNested>::type ArgTypeNestedCleaned;
      typedef typename XprType::CoeffReturnType CoeffReturnType;

      enum { 
        CoeffReadCost = evaluator<ArgTypeNestedCleaned>::CoeffReadCost,
        Flags = Eigen::ColMajor 
      };
      
      evaluator(const XprType& xpr)
        : m_argImpl(xpr.m_arg), m_rows(xpr.rows())
      { }

      CoeffReturnType coeff(Index row, Index col) const
      {
        Index index = row - col;
        if (index < 0) index += m_rows;
        return m_argImpl.coeff(index);
      }

      evaluator<ArgTypeNestedCleaned> m_argImpl;
      const Index m_rows;
    };
  }
}