WIP: More Comm-B & ADS-B decoding.

mode_s.h

@@ -0,0 +1,100 @@
+// Part of dump1090, a Mode S message decoder for RTLSDR devices.
+//
+// mode_s.h: Mode S message decoding (prototypes)
+//
+// Copyright (c) 2017 FlightAware, LLC
+// Copyright (c) 2017 Oliver Jowett <oliver@mutability.co.uk>
+//
+// This file is free software: you may copy, redistribute and/or modify it
+// under the terms of the GNU General Public License as published by the
+// Free Software Foundation, either version 2 of the License, or (at your
+// option) 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;
+    }
+}
+
+extern char ais_charset[64];
+
+#endif