Interpolate.h

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/**
   Interpolationn helpers.

   See test_interpolate.cxx.
 */

#ifndef WIRECELLUTIL_INTERPOLATE
#define WIRECELLUTIL_INTERPOLATE

#include <vector>

namespace WireCell {



    /**
       Use like:
       
          linterp<double> lin(f.begin(), f.end(), x0, xstep);
          ...
          double y = lin(42.0);

       where "f" is some kind of collection of doubles.

     */
    template<class Real>
    class linterp {
    public:
        template<class BidiIterator>
        linterp(BidiIterator f, BidiIterator end_p, Real left_endpoint, Real step)
            : m_dat(f,end_p), m_le(left_endpoint), m_step(step) {
            m_re = m_le + m_step * (m_dat.size()-1);
        }

        Real operator()(Real x) const {
            if (x <= m_le) return m_dat.front();
            if (x >= m_re) return m_dat.back();

            int ind = int((x-m_le)/m_step);
            Real y0 = m_dat[ind];
            Real y1 = m_dat[ind+1];
            Real x0 = m_le + ind * m_step;

            return y0 + (x-x0) * (y1-y0) / m_step;
        }

    private:
        std::vector<Real> m_dat;
        Real m_le, m_re, m_step;
    };



    /** You may also want to use Boost for fancier interpolation.
     * They have similar calling interface:
     #include <boost/math/interpolators/cubic_b_spline.hpp>
     #include <iostream>
     ...
     boost::math::cubic_b_spline<double> spline(f.begin(), f.end(), x0, xstep);
     spline(42);
     *
     * More info here:
     https://www.boost.org/doc/libs/1_65_0/libs/math/doc/html/math_toolkit/interpolate/cubic_b.html
    */


}

#endif