AFG/dump1090
3
0
Fork 0
Code Issues Pull requests Packages Projects Releases Wiki Activity

Improve DF field error correction speed

Browse source 
Use bitmasks to speed up the detection of valid and invalid ICAO DF
field types.
This commit is contained in:
Malcolm Robb 2013-04-30 10:35:52 +01:00
parent 8d65abd317
commit c81498a0d0
1 changed files with 17 additions and 10 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

25
dump1090.c
View file

@ -1977,18 +1977,25 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
if (msglen > i) {msglen = i;} if (msglen > i) {msglen = i;}
else if (msglen < i) {msglen = 0;} else if (msglen < i) {msglen = 0;}
//
// If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible.
// Do this by looking to see if the original guess results in the DF type being one of the ICAO defined
// message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined.
// if the new value is ICAO defined, then update it in our message.
if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) { if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) {
// We guessed at one of the message type bits. See if our guess is "likely" // We guessed at one (and only one) of the message type bits. See if our guess is "likely"
// to be correct by comparing the DF against a list of known good DF's // to be correct by comparing the DF against a list of known good DF's
int DF = ((theByte = msg[0]) >> 3) & 0x1f; int thisDF = ((theByte = msg[0]) >> 3) & 0x1f;
if ( (DF != 0) && (DF != 4) && (DF != 5) && (DF != 11) uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24
&& (DF != 16) && (DF != 17) && (DF != 18) && (DF != 19) && (DF != 20) && (DF != 21) && (DF != 22) && (DF != 24) ) { uint32_t thisDFbit = (1 << thisDF);
// Other DF values are probably errors. Toggle the bit we guessed at and see if the resultant DF is more likely if (0 == (validDFbits & thisDFbit)) {
// The current DF is not ICAO defined, so is probably an errors.
// Toggle the bit we guessed at and see if the resultant DF is more likely
theByte ^= theErrs; theByte ^= theErrs;
DF = (theByte >> 3) & 0x1f; thisDF = (theByte >> 3) & 0x1f;
// if this DF any more likely?? thisDFbit = (1 << thisDF);
if ( (DF == 0) || (DF == 4) || (DF == 5) || (DF == 11) // if this DF any more likely?
|| (DF == 16) || (DF == 17) || (DF == 18) || (DF == 19) || (DF == 20) || (DF == 21) || (DF == 22) || (DF == 24) ) { if (validDFbits & thisDFbit) {
// Yep, more likely, so update the main message // Yep, more likely, so update the main message
msg[0] = theByte; msg[0] = theByte;
Modes.stat_DF_Type_Corrected++; Modes.stat_DF_Type_Corrected++;