3 * Copyright 2008,2010 Free Software Foundation, Inc.
5 * This file is part of GNU Radio
7 * GNU Radio is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License as published by
9 * the Free Software Foundation; either version 3, or (at your option)
12 * GNU Radio is distributed in the hope that it will be useful,
13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 * GNU General Public License for more details.
17 * You should have received a copy of the GNU General Public License
18 * along with GNU Radio; see the file COPYING. If not, write to
19 * the Free Software Foundation, Inc., 51 Franklin Street,
20 * Boston, MA 02110-1301, USA.
23 #ifndef INCLUDED_GRI_LFSR_H
24 #define INCLUDED_GRI_LFSR_H
30 * \brief Fibonacci Linear Feedback Shift Register using specified polynomial mask
33 * Generates a maximal length pseudo-random sequence of length 2^degree-1
35 * Constructor: gri_lfsr(int mask, int seed, int reg_len);
37 * mask - polynomial coefficients representing the locations
38 * of feedback taps from a shift register which are xor'ed
39 * together to form the new high order bit.
41 * Some common masks might be:
42 * x^4 + x^3 + x^0 = 0x19
43 * x^5 + x^3 + x^0 = 0x29
44 * x^6 + x^5 + x^0 = 0x61
46 * seed - the initialization vector placed into the register
47 * durring initialization. Low order bit corresponds
48 * to x^0 coefficient -- the first to be shifted as output.
50 * reg_len - specifies the length of the feedback shift register
51 * to be used. Durring each iteration, the register
52 * is rightshifted one and the new bit is placed in bit reg_len.
53 * reg_len should generally be at least order(mask) + 1
56 * see http://en.wikipedia.org/wiki/Linear_feedback_shift_register
57 * for more explanation.
61 * next_bit() - Standard LFSR operation
63 * Perform one cycle of the LFSR. The output bit is taken from
64 * the shift register LSB. The shift register MSB is assigned from
65 * the modulo 2 sum of the masked shift register.
67 * next_bit_scramble(unsigned char input) - Scramble an input stream
69 * Perform one cycle of the LFSR. The output bit is taken from
70 * the shift register LSB. The shift register MSB is assigned from
71 * the modulo 2 sum of the masked shift register and the input LSB.
73 * next_bit_descramble(unsigned char input) - Descramble an input stream
75 * Perform one cycle of the LFSR. The output bit is taken from
76 * the modulo 2 sum of the masked shift register and the input LSB.
77 * The shift register MSB is assigned from the LSB of the input.
79 * See http://en.wikipedia.org/wiki/Scrambler for operation of these
80 * last two functions (see multiplicative scrambler.)
87 uint32_t d_shift_register;
90 uint32_t d_shift_register_length; // less than 32
95 uint32_t r = x - ((x >> 1) & 033333333333)
96 - ((x >> 2) & 011111111111);
97 return ((r + (r >> 3)) & 030707070707) % 63;
102 gri_lfsr(uint32_t mask, uint32_t seed, uint32_t reg_len)
103 : d_shift_register(seed),
106 d_shift_register_length(reg_len)
109 throw std::invalid_argument("reg_len must be <= 31");
112 unsigned char next_bit() {
113 unsigned char output = d_shift_register & 1;
114 unsigned char newbit = popCount( d_shift_register & d_mask )%2;
115 d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
119 unsigned char next_bit_scramble(unsigned char input) {
120 unsigned char output = d_shift_register & 1;
121 unsigned char newbit = (popCount( d_shift_register & d_mask )%2)^(input & 1);
122 d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
126 unsigned char next_bit_descramble(unsigned char input) {
127 unsigned char output = (popCount( d_shift_register & d_mask )%2)^(input & 1);
128 unsigned char newbit = input & 1;
129 d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
134 * Reset shift register to initial seed value
136 void reset() { d_shift_register = d_seed; }
139 * Rotate the register through x number of bits
140 * where we are just throwing away the results to get queued up correctly
142 void pre_shift(int num){
143 for(int i=0; i<num; i++){
148 int mask() const { return d_mask; }
151 #endif /* INCLUDED_GRI_LFSR_H */