/* -*- c++ -*- */
/*
 * Copyright 2010,2012 Free Software Foundation, Inc.
 *
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 *
 * GNU Radio is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

// Calculate the taps for the CPM phase responses

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <cmath>
#include <cfloat>
#include <analog/cpm.h>

//gives us erf on compilers without it
#include <boost/math/special_functions/erf.hpp>
namespace bm = boost::math;

namespace gr {
  namespace analog {

#ifndef M_TWOPI
#  define M_TWOPI (2*M_PI)
#endif

    //! Normalised sinc function, sinc(x)=sin(pi*x)/pi*x
    inline double
    sinc(double x)
    {
      if(x == 0) {
	return 1.0;
      }
      return sin(M_PI * x) / (M_PI * x);
    }


    //! Taps for L-RC CPM (Raised cosine of length L symbols)
    std::vector<float>
    generate_cpm_lrc_taps(unsigned samples_per_sym, unsigned L)
    {
      std::vector<float> taps(samples_per_sym * L, 1.0/L/samples_per_sym);
      for(unsigned i = 0; i < samples_per_sym * L; i++) {
	taps[i] *= 1 - cos(M_TWOPI * i / L / samples_per_sym);
      }

      return taps;
    }


    /*! Taps for L-SRC CPM (Spectral raised cosine of length L symbols).
     *
     * L-SRC has a time-continuous phase response function of
     *
     * g(t) = 1/LT * sinc(2t/LT) * cos(beta * 2pi t / LT) / (1 - (4beta / LT * t)^2)
     *
     * which is the Fourier transform of a cos-rolloff function with rolloff
     * beta, and looks like a sinc-function, multiplied with a rolloff term.
     * We return the main lobe of the sinc, i.e., everything between the
     * zero crossings.
     * The time-discrete IR is thus
     *
     * g(k) = 1/Ls * sinc(2k/Ls) * cos(beta * pi k / Ls) / (1 - (4beta / Ls * k)^2)
     * where k = 0...Ls-1
     * and s = samples per symbol.
     */
    std::vector<float>
    generate_cpm_lsrc_taps(unsigned samples_per_sym, unsigned L, double beta)
    {
      double Ls = (double) L * samples_per_sym;
      std::vector<double> taps_d(L * samples_per_sym, 0.0);
      std::vector<float> taps(L * samples_per_sym, 0.0);

      double sum = 0;
      for(unsigned i = 0; i < samples_per_sym * L; i++) {
	double k =  i - Ls/2; // Causal to acausal

	taps_d[i] = 1.0 / Ls * sinc(2.0 * k / Ls);

	// For k = +/-Ls/4*beta, the rolloff term's cos-function becomes zero
	// and the whole thing converges to PI/4 (to prove this, use de
	// l'hopital's rule).
	if(fabs(fabs(k) - Ls/4/beta) < 2*DBL_EPSILON) {
	  taps_d[i] *= M_PI_4;
	}
	else {
	  double tmp = 4.0 * beta * k / Ls;
	  taps_d[i] *= cos(beta * M_TWOPI * k / Ls) / (1 - tmp * tmp);
	}
	sum += taps_d[i];
      }

      for(unsigned i = 0; i < samples_per_sym * L; i++) {
	taps[i] = (float) taps_d[i] / sum;
      }

      return taps;
    }

    //! Taps for L-REC CPM (Rectangular pulse shape of length L symbols)
    std::vector<float>
    generate_cpm_lrec_taps(unsigned samples_per_sym, unsigned L)
    {
      return std::vector<float>(samples_per_sym * L, 1.0/L/samples_per_sym);
    }

    //! Helper function for TFM
    double tfm_g0(double k, double sps)
    {
      if(fabs(k) < 2 * DBL_EPSILON) {
	return 1.145393004159143; // 1 + pi^2/48 / sqrt(2)
      }

      const double pi2_24 = 0.411233516712057; // pi^2/24
      double f = M_PI * k / sps;
      return sinc(k/sps) - pi2_24 * (2 * sin(f) - 2*f*cos(f) - f*f*sin(f)) / (f*f*f);
    }

    //! Taps for TFM CPM (Tamed frequency modulation)
    //
    // See [2, Chapter 2.7.2].
    //
    // [2]: Anderson, Aulin and Sundberg; Digital Phase Modulation
    std::vector<float>
    generate_cpm_tfm_taps(unsigned sps, unsigned L)
    {
      unsigned causal_shift = sps * L / 2;
      std::vector<double> taps_d(sps * L, 0.0);
      std::vector<float> taps(sps * L, 0.0);

      double sum = 0;
      for(unsigned i = 0; i < sps * L; i++) {
	double k = (double)(((int)i) - ((int)causal_shift)); // Causal to acausal

	taps_d[i] = tfm_g0(k - sps, sps) +
	  2 * tfm_g0(k,       sps) +
	  tfm_g0(k + sps, sps);
	sum += taps_d[i];
      }

      for(unsigned i = 0; i < sps * L; i++) {
	taps[i] = (float) taps_d[i] / sum;
      }

      return taps;
    }

    //! Taps for Gaussian CPM. Phase response is truncated after \p L symbols.
    //  \p bt sets the 3dB-time-bandwidth product.
    //
    // Note: for h = 0.5, this is the phase response for GMSK.
    //
    // This C99-compatible formula for the taps is taken straight
    // from [1, Chapter 9.2.3].
    // A version in Q-notation can be found in [2, Chapter 2.7.2].
    //
    // [1]: Karl-Dirk Kammeyer; Nachrichtenübertragung, 4th Edition.
    // [2]: Anderson, Aulin and Sundberg; Digital Phase Modulation
    //
    std::vector<float>
    generate_cpm_gaussian_taps(unsigned samples_per_sym, unsigned L, double bt)
    {
      double Ls = (double) L * samples_per_sym;
      std::vector<double> taps_d(L * samples_per_sym, 0.0);
      std::vector<float> taps(L * samples_per_sym, 0.0);

      // alpha = sqrt(2/ln(2)) * pi * BT
      double alpha = 5.336446256636997 * bt;
      for(unsigned i = 0; i < samples_per_sym * L; i++) {
	double k =  i - Ls/2; // Causal to acausal
	taps_d[i] = (bm::erf(alpha * (k / samples_per_sym + 0.5)) -
		     bm::erf(alpha * (k / samples_per_sym - 0.5)))
	  * 0.5 / samples_per_sym;
	taps[i] = (float) taps_d[i];
      }

      return taps;
    }

    std::vector<float>
    cpm::phase_response(cpm_type type, unsigned samples_per_sym, unsigned L, double beta)
    {
      switch(type) {
      case LRC:
	return generate_cpm_lrc_taps(samples_per_sym, L);

      case LSRC:
	return generate_cpm_lsrc_taps(samples_per_sym, L, beta);

      case LREC:
	return generate_cpm_lrec_taps(samples_per_sym, L);

      case TFM:
	return generate_cpm_tfm_taps(samples_per_sym, L);

      case GAUSSIAN:
	return generate_cpm_gaussian_taps(samples_per_sym, L, beta);

      default:
	return generate_cpm_lrec_taps(samples_per_sym, 1);
      }
    }

  } // namespace analog
} // namespace gr