portapack-mayhem/firmware/application/de_bruijn.cpp
furrtek 42439d1885 Started writing (copying...) AFSK RX
Encoders: removed bit duration setting (frame duration is more useful)
2017-08-29 09:42:04 +01:00

105 lines
2.8 KiB
C++

/*
* Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
* Copyright (C) 2017 Furrtek
*
* This file is part of PortaPack.
*
* This program 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 2, or (at your option)
* any later version.
*
* This program 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 this program; see the file COPYING. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street,
* Boston, MA 02110-1301, USA.
*/
#include "de_bruijn.hpp"
/* How this works:
* B(2,4) means:
* Alphabet size = 2 (binary)
* Code length = 4
* The length of the bitstream is (2 ^ 4) + (4 - 1) = 19 bits
* The primitive polynomials come from the de_bruijn_polys[] array (one for each code length)
* The shift register is init with 1 and shifted left each step
* The polynomial is kept on the right, and used as a AND mask applied on the corresponding shift register bits
* The resulting bits are XORed together to produce the new bit pushed in the shift register
*
* 0001 (init)
* AND 1001 (polynomial)
* 0001 XOR'd -> 1
*
* 00011 (shift left)
* AND 1001
* 0001 XOR'd -> 1
*
* 000111 (shift left)
* AND 1001
* 0001 XOR'd -> 1
*
* 0001111 (shift left)
* AND 1001
* 1001 XOR'd -> 0
*
* 00011110 (shift left)
* AND 1001
* 1000 XOR'd -> 1
* ...
* After 16 steps: (0+) 000111101011001000
* Each of the 16 possible values appears in the sequence:
* -0000-111101011001000 0
* 0-0001-11101011001000 1
* 0000111101011-0010-00 2
* 00-0011-1101011001000 3
* 00001111010110-0100-0 4
* 00001111-0101-1001000 5
* 0000111101-0110-01000 6
* 000-0111-101011001000 7
* 000011110101100-1000- 8
* 000011110101-1001-000 9
* 0000111-1010-11001000 10
* 000011110-1011-001000 11
* 00001111010-1100-1000 12
* 000011-1101-011001000 13
* 00001-1110-1011001000 14
* 0000-1111-01011001000 15
*/
size_t de_bruijn::init(const uint32_t n) {
// Cap
if ((n < 3) || (n > 16))
length = 3;
else
length = n;
poly = de_bruijn_polys[length - 3];
shift_register = 1;
return (1U << length) + (length - 1);
}
uint32_t de_bruijn::compute(const uint32_t steps) {
uint32_t step, bits, masked;
uint8_t new_bit;
for (step = 0; step < steps; step++) {
masked = shift_register & poly;
new_bit = 0;
for (bits = 0; bits < length; bits++) {
new_bit ^= (masked & 1);
masked >>= 1;
}
shift_register <<= 1;
shift_register |= new_bit;
}
return shift_register;
}