101 lines
3.1 KiB
C
101 lines
3.1 KiB
C
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// Part of dump1090, a Mode S message decoder for RTLSDR devices.
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//
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// mode_s.h: Mode S message decoding (prototypes)
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//
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// Copyright (c) 2017 FlightAware, LLC
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// Copyright (c) 2017 Oliver Jowett <oliver@mutability.co.uk>
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//
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// This file is free software: you may copy, redistribute and/or modify it
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// under the terms of the GNU General Public License as published by the
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// Free Software Foundation, either version 2 of the License, or (at your
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// option) any later version.
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//
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// This file is distributed in the hope that it will be useful, but
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// WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// 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. If not, see <http://www.gnu.org/licenses/>.
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#ifndef MODE_S_H
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#define MODE_S_H
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#include <assert.h>
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//
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// Functions exported from mode_s.c
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//
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int modesMessageLenByType(int type);
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int scoreModesMessage(unsigned char *msg, int validbits);
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int decodeModesMessage (struct modesMessage *mm, unsigned char *msg);
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void displayModesMessage(struct modesMessage *mm);
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void useModesMessage (struct modesMessage *mm);
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// datafield extraction helpers
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// The first bit (MSB of the first byte) is numbered 1, for consistency
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// with how the specs number them.
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// Extract one bit from a message.
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static inline __attribute__((always_inline)) unsigned getbit(unsigned char *data, unsigned bitnum)
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{
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unsigned bi = bitnum - 1;
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unsigned by = bi >> 3;
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unsigned mask = 1 << (7 - (bi & 7));
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return (data[by] & mask) != 0;
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}
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// Extract some bits (firstbit .. lastbit inclusive) from a message.
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static inline __attribute__((always_inline)) unsigned getbits(unsigned char *data, unsigned firstbit, unsigned lastbit)
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{
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unsigned fbi = firstbit - 1;
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unsigned lbi = lastbit - 1;
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unsigned nbi = (lastbit - firstbit + 1);
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unsigned fby = fbi >> 3;
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unsigned lby = lbi >> 3;
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unsigned nby = (lby - fby) + 1;
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unsigned shift = 7 - (lbi & 7);
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unsigned topmask = 0xFF >> (fbi & 7);
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assert (fbi <= lbi);
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assert (nbi <= 32);
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assert (nby <= 5);
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if (nby == 5) {
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return
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((data[fby] & topmask) << (32 - shift)) |
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(data[fby + 1] << (24 - shift)) |
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(data[fby + 2] << (16 - shift)) |
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(data[fby + 3] << (8 - shift)) |
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(data[fby + 4] >> shift);
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} else if (nby == 4) {
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return
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((data[fby] & topmask) << (24 - shift)) |
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(data[fby + 1] << (16 - shift)) |
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(data[fby + 2] << (8 - shift)) |
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(data[fby + 3] >> shift);
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} else if (nby == 3) {
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return
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((data[fby] & topmask) << (16 - shift)) |
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(data[fby + 1] << (8 - shift)) |
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(data[fby + 2] >> shift);
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} else if (nby == 2) {
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return
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((data[fby] & topmask) << (8 - shift)) |
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(data[fby + 1] >> shift);
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} else if (nby == 1) {
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return
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(data[fby] & topmask) >> shift;
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} else {
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return 0;
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}
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}
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extern char ais_charset[64];
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#endif
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