atsc_randomize.h

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/* -*- c++ -*- */
/*
 * Copyright 2014 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * 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.
 */

#ifndef INCLUDED_ATSC_RANDOMIZE_H
#define INCLUDED_ATSC_RANDOMIZE_H

#include "atsc_types.h"

namespace gr {
  namespace dtv {

    class atsc_randomize
    {
    public:
      atsc_randomize();

      /*! \brief reset randomizer LFSR
       *
       * must be called during the Data Segment Sync interval prior to the
       * first data segment.  I.e., the LFSR is reset prior to the first
       * field of each VSB data frame.
       */
      void reset ();

      //! randomize (whiten) mpeg packet and remove leading MPEG-2 sync byte
      void randomize (atsc_mpeg_packet_no_sync &out, const atsc_mpeg_packet &in);

      //! derandomize (de-whiten) mpeg packet and add leading MPEG-2 sync byte
      void derandomize (atsc_mpeg_packet &out, const atsc_mpeg_packet_no_sync &in);

      unsigned int state() const { return d_state; }
    private:
      static void initialize_output_map ();
      static unsigned char slow_output_map (int st);

      static unsigned char fast_output_map (int st){
        return s_output_map[(st & 0xb23c) >> 2]; // Magic const with 8 bits set improves cache
                                                 // utilization.  The bits correspond to the taps
                                                 // used in output calculation.  Others may be
                                                 // safely ignored.
      }

      //! return current output value
      unsigned char output (){
        return fast_output_map (d_state);
      }

      //! clock LFSR; advance to next state.
      void clk (){
        if (d_state & 0x1)
          d_state = ((d_state ^ MASK) >> 1) | 0x8000;
        else
          d_state = d_state >> 1;
      }

      //! return current output value and advance to next state
      unsigned char output_and_clk (){
        unsigned char r = output ();
        clk ();
        return r;
      }

      unsigned int                  d_state;

      static const unsigned int PRELOAD_VALUE = 0x018f; /* 0xf180 bit reversed */
      static const unsigned int MASK = 0xa638;
      static unsigned char          s_output_map[1 << 14];
      static bool                   s_output_map_initialized_p;
    };
  } /* namespace dtv */
} /* namespace gr */

#endif /* INCLUDED_ATSC_RANDOMIZE_H */