Loading...
Searching...
No Matches
lfsr.h
Go to the documentation of this file.
/* -*- c++ -*- */
/*
* Copyright 2008,2010,2012 Free Software Foundation, Inc.
*
* This file is part of GNU Radio
*
* SPDX-License-Identifier: GPL-3.0-or-later
*
*/
#ifndef INCLUDED_DIGITAL_LFSR_H
#define INCLUDED_DIGITAL_LFSR_H
#include <gnuradio/digital/api.h>
#ifndef HAVE_BUILTIN_PARITYL
#include <volk/volk.h>
#endif
#include <stdint.h>
#include <cstdint>
#include <stdexcept>
namespace gr {
namespace digital {
/*!
* \brief Fibonacci Linear Feedback Shift Register using specified
* polynomial mask
* \ingroup misc
*
* \details
* Generates a maximal length pseudo-random sequence of length
* 2^degree-1, if supplied with a primitive polynomial.
*
* Constructor: digital::lfsr(int mask, int seed, int reg_len);
*
* \param 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, K=3
* x^5 + x^3 + x^0 = 0x29, K=4
* x^6 + x^5 + x^0 = 0x61, K=5
*
* \param seed - the initialization vector placed into the
* register during initialization. Low order bit
* corresponds to x^0 coefficient -- the first to be
* shifted as output.
*
* \param reg_len - specifies the length of the feedback shift
* register to be used. During 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 lfsr
{
private:
uint64_t d_shift_register;
uint64_t d_mask;
uint64_t d_seed;
uint8_t d_shift_register_length; // less than 64
public:
lfsr(uint64_t mask, uint64_t seed, uint8_t reg_len)
: d_shift_register(seed),
d_mask(mask),
d_seed(seed),
d_shift_register_length(reg_len)
{
if (reg_len > 63)
throw std::invalid_argument("reg_len must be <= 63");
}
unsigned char next_bit()
{
unsigned char output = d_shift_register & 1;
uint64_t newbit;
#ifdef HAVE_BUILTIN_PARITYL
newbit = __builtin_parityl(d_shift_register & d_mask);
#else
volk_64u_popcnt(&newbit, d_shift_register & d_mask);
newbit %= 2;
#endif
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;
uint64_t newbit;
#ifdef HAVE_BUILTIN_PARITYL
newbit = __builtin_parityl(d_shift_register & d_mask) ^ (input & 1);
#else
volk_64u_popcnt(&newbit, d_shift_register & d_mask);
newbit = (newbit ^ input) & 1;
#endif
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;
#ifdef HAVE_BUILTIN_PARITYL
output = __builtin_parityl(d_shift_register & d_mask) ^ (input & 1);
#else
uint64_t _tmp;
volk_64u_popcnt(&_tmp, d_shift_register & d_mask);
output = (_tmp ^ input) & 1;
#endif
uint64_t newbit = input & 1;
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();
}
}
uint64_t mask() const { return d_mask; }
};
} /* namespace digital */
} /* namespace gr */
#endif /* INCLUDED_DIGITAL_LFSR_H */
gr::digital::lfsr::pre_shift
void pre_shift(int num)
Definition lfsr.h:157
gr::digital::lfsr::reset
void reset()
Definition lfsr.h:151
gr::digital::lfsr::next_bit_descramble
unsigned char next_bit_descramble(unsigned char input)
Definition lfsr.h:132
gr::digital::lfsr::next_bit
unsigned char next_bit()
Definition lfsr.h:101
gr::digital::lfsr::lfsr
lfsr(uint64_t mask, uint64_t seed, uint8_t reg_len)
Definition lfsr.h:91
gr::digital::lfsr::mask
uint64_t mask() const
Definition lfsr.h:164
gr::digital::lfsr::next_bit_scramble
unsigned char next_bit_scramble(unsigned char input)
Definition lfsr.h:117
api.h
gr::digital
Definition adaptive_algorithm.h:22
gr
GNU Radio logging wrapper.
Definition basic_block.h:29