Add a --no-fix-df option

Usually, all message candidates are speculatively corrected as if
they were DF11/17/18, even if the DF field has a different value. This
allows correction of messages when there is correctable damage to the
DF field.

However, running every message candidate through a couple of CRC checks
is expensive, CPU-wise, when decoding messages off the air, as there are
a large number of message candidates that are actually just junk data, and
computing CRCs for all of those adds up. The --no-fix-df option allows
disabling this sort of correction. The tradeoff is that messages with
damage to the DF field will not be corrected.

dump1090.c

@@ -113,6 +113,7 @@ static void modesInitConfig(void) {
     Modes.gain                    = MODES_MAX_GAIN;
     Modes.freq                    = MODES_DEFAULT_FREQ;
     Modes.check_crc               = 1;
+    Modes.fix_df                  = 1;
     Modes.net_heartbeat_interval  = MODES_NET_HEARTBEAT_INTERVAL;
     Modes.interactive_display_ttl = MODES_INTERACTIVE_DISPLAY_TTL;
     Modes.json_interval           = 1000;
@@ -334,6 +335,7 @@ static void showHelp(void)
 "--fix                    Enable single-bit error correction using CRC\n"
 "--fix-2bit               Enable two-bit error correction using CRC (use with caution)\n"
 "--no-fix                 Disable error correction using CRC\n"
+"--no-fix-df              Disable error correction of the DF message field (reduces CPU requirements)\n"
 "--no-crc-check           Disable messages with broken CRC (discouraged)\n"
 "--mlat                   display raw messages in Beast ascii mode\n"
 "--stats                  With --ifile print stats at exit. No other output\n"
@@ -547,6 +549,8 @@ int main(int argc, char **argv) {
             Modes.nfix_crc = 2;
         } else if (!strcmp(argv[j],"--no-fix")) {
             Modes.nfix_crc = 0;
+        } else if (!strcmp(argv[j],"--no-fix-df")) {
+            Modes.fix_df = 0;
         } else if (!strcmp(argv[j],"--no-crc-check")) {
             Modes.check_crc = 0;
         } else if (!strcmp(argv[j],"--phase-enhance")) {

dump1090.h

@@ -324,6 +324,7 @@ struct _Modes {                             // Internal state
     sdr_type_t sdr_type;             // where are we getting data from?
     int   nfix_crc;                  // Number of crc bit error(s) to correct
     int   check_crc;                 // Only display messages with good CRC
+    int   fix_df;                    // Try to correct damage to the DF field, as well as the main message body
     int   raw;                       // Raw output format
     int   mode_ac;                   // Enable decoding of SSR Modes A & C
     int   mode_ac_auto;              // allow toggling of A/C by Beast commands

faup1090.c

@@ -71,6 +71,7 @@ static void faupInitConfig(void) {
     // Now initialise things that should not be 0/NULL to their defaults
     Modes.nfix_crc                = 1;
     Modes.check_crc               = 1;
+    Modes.fix_df                  = 1;
     Modes.net                     = 1;
     Modes.net_heartbeat_interval  = MODES_NET_HEARTBEAT_INTERVAL;
     Modes.maxRange                = 1852 * 360; // 360NM default max range; this also disables receiver-relative positions

mode_s.c

@@ -268,8 +268,13 @@ static int correctMessage(const unsigned char *in, unsigned char *out)
     const unsigned uncorrected_df = getbits(in, 1, 5);
     const uint32_t df_bit = 1 << uncorrected_df;
 
+    // Select the right bitset based on the maximum number of bit errors in the DF field that we could correct.
+    // nb: strictly speaking, --no-fix-df doesn't _entirely_ disable correction of the DF field when nfix_crc == 2
+    // (DF17 could be corrected to DF18 or vice versa), but it does disable the CPU hungry part of it.
+    const unsigned fix_df_bits = (Modes.fix_df ? Modes.nfix_crc : 0);
+
     struct errorinfo *long_ei = NULL;
-    if (df_correctable_long[Modes.nfix_crc] & df_bit) {
+    if (df_correctable_long[fix_df_bits] & df_bit) {
         uint32_t long_syndrome = modesChecksum(in, MODES_LONG_MSG_BITS);
         if (isLongPIMessage(in) && long_syndrome == 0) {
             // DF17/18 message with correct checksum
@@ -281,7 +286,7 @@ static int correctMessage(const unsigned char *in, unsigned char *out)
     }
 
     struct errorinfo *short_ei = NULL;
-    if (df_correctable_short[Modes.nfix_crc] & df_bit) {
+    if (df_correctable_short[fix_df_bits] & df_bit) {
         uint32_t short_syndrome = modesChecksum(in, MODES_SHORT_MSG_BITS);
         if (isShortPIMessage(in) && (short_syndrome & 0xFFFF80) == 0) {
             // DF11 message with correct checksum

view1090.c

@@ -57,6 +57,7 @@ static void view1090InitConfig(void) {
 
     // Now initialise things that should not be 0/NULL to their defaults
     Modes.check_crc               = 1;
+    Modes.fix_df                  = 1;
     Modes.interactive_display_ttl = MODES_INTERACTIVE_DISPLAY_TTL;
     Modes.interactive             = 1;
     Modes.maxRange                = 1852 * 300; // 300NM default max range