gri_float_lfsr.h


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/* -*- c++ -*- */
//This is original code for Gnuradio
#ifndef INCLUDED_GRI_FLOAT_LFSR_H
#define INCLUDED_GRI_FLOAT_LFSR_H 

#include <stdexcept>
#include <stdint.h>
/*
#include <bitset>
#include <iostream>
*/

/*!
 * \brief Fibonacci Linear Feedback Shift Register using specified polynomial mask
 * \ingroup misc
 *
 * Generates a maximal length pseudo-random sequence of length 2^degree-1
 * 
 * Constructor: gri_lfsr(int mask, int seed, int reg_len);
 *  
 *      mask - polynomial coefficients representing the locations
 *             of feedback taps from a shift register which are xor'ed
 *             together to form the new high order bit.
 *
 *             Some common masks might be:
 *              x^4 + x^3 + x^0 = 0x19
 *              x^5 + x^3 + x^0 = 0x29
 *              x^6 + x^5 + x^0 = 0x61
 *
 *      seed - the initialization vector placed into the register
 *             durring initialization.   Low order bit corresponds
 *             to x^0 coefficient -- the first to be shifted as output.
 *
 *   reg_len - specifies the length of the feedback shift register 
 *             to be used.   Durring each iteration, the register
 *             is rightshifted one and the new bit is placed in bit reg_len.
 *             reg_len should generally be at least order(mask) + 1
 *
 *
 *
 *  next_bit() - Standard LFSR operation
 * 
 *      Perform one cycle of the LFSR.  The output bit is taken from
 *      the shift register LSB.  The shift register MSB is assigned from
 *      the modulo 2 sum of the masked shift register.
 *
 * 
 */

class gri_float_lfsr
{
 private:
  uint32_t d_shift_register;
  uint32_t d_mask;
  uint32_t d_seed;
  uint32_t d_shift_register_length;	// less than 32 

  static uint32_t
  popCount(uint32_t x)
  {
    uint32_t r = x - ((x >> 1) & 033333333333)
                   - ((x >> 2) & 011111111111);
    return ((r + (r >> 3)) & 030707070707) % 63;
  } 

 public:

  gri_float_lfsr(uint32_t mask, uint32_t seed, uint32_t reg_len)
    : d_shift_register(seed), 
      d_mask(mask), 
      d_seed(seed),
      d_shift_register_length(reg_len)
  {
    if (reg_len > 31)
      throw std::invalid_argument("reg_len must be <= 31");
  } 

  //output 1 or -1 for the direct spreading code
  float next_bit() {
  	/*
  	std::bitset<32> bs((long)d_shift_register);
  	for( int i = (int) bs.size(); i >= 0; i-- )
  		std::cout << bs[i] << " ";
	printf("\n"); 
*/
 	float output = (float)(d_shift_register & 1) * 2.0 - 1.0;
       unsigned char newbit = popCount( d_shift_register & d_mask )%2;
       d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
       return output;
  }

  /*!
   * Reset shift register to initial seed value
   */
  void reset() { d_shift_register = d_seed; } 

  /*!
   * Rotate the register through x number of bits
   * where we are just throwing away the results to get queued up correctly
   */
  void pre_shift(int num){
    for(int i=0; i<num; i++){
      next_bit();
    }
  } 

  int mask() const { return d_mask; }
};

#endif /* INCLUDED_GRI_FLOAT_LFSR_H */