mode_s.h

// Part of readsb, a Mode-S/ADSB/TIS message decoder.
//
// mode_s.h: Mode S message decoding (header)
//
// Copyright (c) 2020 Michael Wolf <michael@mictronics.de>
//
// This code is based on a detached fork of dump1090-fa.
//
// Copyright (c) 2017 FlightAware, LLC
// Copyright (c) 2017 Oliver Jowett <oliver@mutability.co.uk>
//
// This file 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 3 of the License, or
// any later version.
//
// This file 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.  If not, see <http://www.gnu.org/licenses/>.

#ifndef MODE_S_H
#define MODE_S_H

#include <assert.h>

//
// Functions exported from mode_s.c
//
int modesMessageLenByType(int type);
int scoreModesMessage(unsigned char *msg, int validbits);
int decodeModesMessage(struct modesMessage *mm, unsigned char *msg);
void displayModesMessage(struct modesMessage *mm);
void useModesMessage(struct modesMessage *mm);

// datafield extraction helpers

// The first bit (MSB of the first byte) is numbered 1, for consistency
// with how the specs number them.

// Extract one bit from a message.

static inline __attribute__ ((always_inline)) unsigned
getbit(unsigned char *data, unsigned bitnum) {
    unsigned bi = bitnum - 1;
    unsigned by = bi >> 3;
    unsigned mask = 1 << (7 - (bi & 7));

    return (data[by] & mask) != 0;
}

// Extract some bits (firstbit .. lastbit inclusive) from a message.

static inline __attribute__ ((always_inline)) unsigned
getbits(unsigned char *data, unsigned firstbit, unsigned lastbit) {
    unsigned fbi = firstbit - 1;
    unsigned lbi = lastbit - 1;
    unsigned nbi = (lastbit - firstbit + 1);

    unsigned fby = fbi >> 3;
    unsigned lby = lbi >> 3;
    unsigned nby = (lby - fby) + 1;

    unsigned shift = 7 - (lbi & 7);
    unsigned topmask = 0xFF >> (fbi & 7);

    assert(fbi <= lbi);
    assert(nbi <= 32);
    assert(nby <= 5);

    if (nby == 5) {
        return
        ((data[fby] & topmask) << (32 - shift)) |
                (data[fby + 1] << (24 - shift)) |
                (data[fby + 2] << (16 - shift)) |
                (data[fby + 3] << (8 - shift)) |
                (data[fby + 4] >> shift);
    } else if (nby == 4) {
        return
        ((data[fby] & topmask) << (24 - shift)) |
                (data[fby + 1] << (16 - shift)) |
                (data[fby + 2] << (8 - shift)) |
                (data[fby + 3] >> shift);
    } else if (nby == 3) {
        return
        ((data[fby] & topmask) << (16 - shift)) |
                (data[fby + 1] << (8 - shift)) |
                (data[fby + 2] >> shift);
    } else if (nby == 2) {
        return
        ((data[fby] & topmask) << (8 - shift)) |
                (data[fby + 1] >> shift);
    } else if (nby == 1) {
        return
        (data[fby] & topmask) >> shift;
    } else {
        return 0;
    }
}

#endif