105 lines
2.8 KiB
C++
105 lines
2.8 KiB
C++
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/*
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* Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
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* Copyright (C) 2017 Furrtek
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*
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* This file is part of PortaPack.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2, or (at your option)
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* any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; see the file COPYING. If not, write to
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* the Free Software Foundation, Inc., 51 Franklin Street,
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* Boston, MA 02110-1301, USA.
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*/
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#include "de_bruijn.hpp"
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/* How this works:
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* B(2,4) means:
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* Alphabet size = 2 (binary)
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* Code length = 4
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* The length of the bitstream is (2 ^ 4) + (4 - 1) = 19 bits
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* The primitive polynomials come from the de_bruijn_polys[] array (one for each code length)
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* The shift register is init with 1 and shifted left each step
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* The polynomial is kept on the right, and used as a AND mask applied on the corresponding shift register bits
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* The resulting bits are XORed together to produce the new bit pushed in the shift register
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*
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* 0001 (init)
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* AND 1001 (polynomial)
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* 0001 XOR'd -> 1
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*
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* 00011 (shift left)
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* AND 1001
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* 0001 XOR'd -> 1
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*
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* 000111 (shift left)
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* AND 1001
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* 0001 XOR'd -> 1
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*
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* 0001111 (shift left)
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* AND 1001
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* 1001 XOR'd -> 0
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*
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* 00011110 (shift left)
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* AND 1001
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* 1000 XOR'd -> 1
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* ...
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* After 16 steps: (0+) 000111101011001000
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* Each of the 16 possible values appears in the sequence:
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* -0000-111101011001000 0
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* 0-0001-11101011001000 1
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* 0000111101011-0010-00 2
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* 00-0011-1101011001000 3
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* 00001111010110-0100-0 4
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* 00001111-0101-1001000 5
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* 0000111101-0110-01000 6
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* 000-0111-101011001000 7
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* 000011110101100-1000- 8
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* 000011110101-1001-000 9
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* 0000111-1010-11001000 10
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* 000011110-1011-001000 11
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* 00001111010-1100-1000 12
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* 000011-1101-011001000 13
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* 00001-1110-1011001000 14
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* 0000-1111-01011001000 15
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*/
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size_t de_bruijn::init(const uint32_t n) {
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// Cap
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if ((n < 3) || (n > 16))
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length = 3;
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else
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length = n;
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poly = de_bruijn_polys[length - 3];
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shift_register = 1;
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return (1U << length) + (length - 1);
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}
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uint32_t de_bruijn::compute(const uint32_t steps) {
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uint32_t step, bits, masked;
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uint8_t new_bit;
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for (step = 0; step < steps; step++) {
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masked = shift_register & poly;
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new_bit = 0;
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for (bits = 0; bits < length; bits++) {
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new_bit ^= (masked & 1);
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masked >>= 1;
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}
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shift_register <<= 1;
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shift_register |= new_bit;
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}
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return shift_register;
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}
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