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/* -*- c++ -*- */
/*
* Copyright 2008 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_GRI_LFSR_H
#define INCLUDED_GRI_LFSR_H
#include <stdexcept>
#include <stdint.h>
/*!
* \brief Fibonacci Linear Feedback Shift Register using specified polynomial mask
* \ingroup math
*
* 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
*
*
* see http://en.wikipedia.org/wiki/Linear_feedback_shift_register
* for more explanation.
*
*
*
* 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.
*
* next_bit_scramble(unsigned char input) - Scramble an input stream
*
* 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 and the input LSB.
*
* next_bit_descramble(unsigned char input) - Descramble an input stream
*
* Perform one cycle of the LFSR. The output bit is taken from
* the modulo 2 sum of the masked shift register and the input LSB.
* The shift register MSB is assigned from the LSB of the input.
*
* See http://en.wikipedia.org/wiki/Scrambler for operation of these
* last two functions (see multiplicative scrambler.)
*
*/
class gri_lfsr
{
private:
uint32_t d_shift_register;
uint32_t d_mask;
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_lfsr(uint32_t mask, uint32_t seed, uint32_t reg_len)
: d_shift_register(seed), d_mask(mask), d_shift_register_length(reg_len)
{
if (reg_len > 31)
throw std::invalid_argument("reg_len must be <= 31");
}
unsigned char next_bit() {
unsigned char output = d_shift_register & 1;
unsigned char newbit = popCount( d_shift_register & d_mask )%2;
d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
return output;
}
unsigned char next_bit_scramble(unsigned char input) {
unsigned char output = d_shift_register & 1;
unsigned char newbit = (popCount( d_shift_register & d_mask )%2)^(input & 1);
d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
return output;
}
unsigned char next_bit_descramble(unsigned char input) {
unsigned char output = (popCount( d_shift_register & d_mask )%2)^(input & 1);
unsigned char newbit = input & 1;
d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
return output;
}
/*!
* 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_LFSR_H */
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